fix(cnc): stop double-counting first incremental rapid in rapid display

RapidEnumerator primed the walk position at the first pierce point, then
the skipped first rapid advanced it again. Raw programs start with a zero
rapid so this was invisible, but lead-in programs start with a real
incremental offset to the pierce, which shifted every later rapid by that
delta and drew rapids off the sheet. Start the walk at the program origin.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -217,3 +217,7 @@ docs/superpowers/
 
 # Local test config (contains user-specific paths to proprietary test assets)
 OpenNest.Tests/test-config.json
+
+# Vendor programming manuals: keep reference copies outside source control.
+CINCINNATI LASER PROGRAMMING MANUAL.pdf
+TF5200_programming_manual_en.pdf

AGENTS.md

@@ -111,7 +111,8 @@ MCP server for Claude Code integration. Exposes nesting operations as MCP tools
 The UI application with MDI interface.
 
 - **Auto Nest engine routing**: when the selected engine is not a built-in fill strategy (`EngineSelection.IsFillStrategy` is false, i.e. StockLadder or an `Engines/` plug-in), `MainForm.RunJobEngineAsync` solves the whole job through `INestingEngine.Solve`. `JobEngineNest` builds the `NestJob` from the auto-nest items and either the plate options or the current plate, and it converts `NestJobProgress` for `NestProgressForm`: an engine's `LegacyProgress` passes through, and otherwise the stage and committed counts become the description. It then binds the result poses back onto the nest's own drawings. Whole-job engines throw on cancel, so the progress form hides Accept (`AllowAccept = false`) and Stop discards the run. Built-in strategies keep the existing per-plate fill path.
-- **Forms/**: `MainForm` (MDI parent), `EditNestForm` (MDI child per nest), `SplitDrawingForm` (split oversized drawings into smaller pieces, launched from CadConverterForm), plus dialogs for plate editing, auto-nesting, DXF conversion, cut parameters, etc.
+- **Nest defaults**: new nests load plate defaults (units, size, quadrant, part/edge spacing) from `%APPDATA%\OpenNest\defaults.json` via `OpenNest.Data.NestDefaults` — a single JSON file edited via Tools > Nest Defaults or captured from the active plate via Tools > Save Current Plate as Defaults. Loading never throws: missing/corrupt/invalid fields fall back individually (units then come from the legacy `DefaultUnit` setting). This replaces the `.nstdot` nest-template file; a set `NestTemplatePath` setting is converted to `defaults.json` once at startup and cleared. Console/Training `--template <nest>` flags are unrelated explicit inputs and remain.
+- **Forms/**: `MainForm` (MDI parent), `EditNestForm` (MDI child per nest), `SplitDrawingForm` (split oversized drawings into smaller pieces, launched from CadConverterForm), `NestDefaultsForm` (persisted new-nest defaults), plus dialogs for plate editing, auto-nesting, DXF conversion, cut parameters, etc.
 - **Controls/**: `PlateView` (2D plate renderer with zoom/pan, supports temporary preview parts), `DrawingListBox`, `DrawControl`, `QuadrantSelect`.
 - **Actions/**: User interaction modes — `ActionSelect`, `ActionClone`, `ActionFillArea`, `ActionSelectArea`, `ActionZoomWindow`, `ActionSetSequence`, `ActionCutOff`.
 - **Post-processing**: `IPostProcessor` plugin interface loaded from DLLs in a `Posts/` directory at runtime. Plugin sources live in the repository's `Posts/` folder (the solution's `PostProcessors` folder).
@@ -137,6 +138,8 @@ Always keep `README.md` and `AGENTS.md` up to date when making changes that affe
 
 **Do not commit** design specs, implementation plans, or other temporary planning documents (`docs/superpowers/` etc.) to the repository. These are working documents only — keep them local and untracked.
 
+Keep vendor programming manuals and full-text extracts outside source control unless redistribution permission has been established. Maintain project-written post behavior references instead: [Cincinnati CL](docs/cincinnati-post-output.md) and [Cincinnati CI Fiber](docs/cincinnati-ci-fiber-post-output.md). Cite the manual edition and relevant sections, distinguish controller rules from machine-specific macros, and document unconfirmed behavior without copying vendor text.
+
 ## Key Patterns
 
 - OpenNest.Core uses multiple namespaces: `OpenNest` (root domain), `OpenNest.CNC`, `OpenNest.Geometry`, `OpenNest.Converters`, `OpenNest.Math`, `OpenNest.Collections`.

OpenNest.Core/CNC/RapidEnumerator.cs

@@ -12,13 +12,14 @@ namespace OpenNest.CNC
             var results = new List<Segment>();
 
             // Draw the rapid from the previous tool position to the program's first
-            // pierce point. This also primes pos so the interior walk interprets
-            // Incremental deltas from the correct absolute location (basePos), which
-            // matters for raw pre-lead-in programs that are emitted Incremental.
+            // pierce point. The walk then starts at the program origin (basePos), not
+            // the pierce: the skipped first rapid still advances pos, so starting at
+            // the pierce would apply a nonzero Incremental first delta twice (as in
+            // lead-in programs) and shift every later rapid by it.
             var firstPierce = FirstPiercePoint(pgm, basePos);
             results.Add(new Segment(startPos, firstPierce));
 
-            var pos = firstPierce;
+            var pos = basePos;
             Walk(pgm, basePos, ref pos, skipFirst: true, results);
             return results;
         }

OpenNest.Core/Geometry/Clearance.cs

@@ -0,0 +1,336 @@
+using System;
+using System.Collections.Generic;
+using OpenNest.Math;
+
+namespace OpenNest.Geometry
+{
+    /// <summary>
+    /// Signed clearance between two closed polygons, plus the unit direction that
+    /// increases it by moving the first polygon.
+    /// </summary>
+    public struct ClearanceResult
+    {
+        /// <summary>
+        /// &gt; 0: minimum boundary distance. 0: touching. &lt; 0: penetration depth
+        /// (the translation of <c>a</c> along <see cref="Direction"/> needed to end
+        /// contact).
+        /// </summary>
+        public double Distance;
+
+        /// <summary>
+        /// Unit direction for translating <c>a</c> away from <c>b</c>. For penetration
+        /// this is the minimum-translation direction. Never zero-length; degenerate
+        /// (coincident-centroid) penetration resolves to a deterministic axis.
+        /// </summary>
+        public Vector Direction;
+
+        public ClearanceResult(double distance, Vector direction)
+        {
+            Distance = distance;
+            Direction = direction;
+        }
+    }
+
+    /// <summary>
+    /// Omnidirectional clearance between two closed, lines-only polygons.
+    /// Complements <see cref="SpatialQuery.DirectionalDistance"/> (movement along a
+    /// fixed ray) with the all-directions minimum distance and separating direction,
+    /// and <see cref="Collision"/> (boolean overlap) with depth and direction.
+    /// <para>
+    /// Reference quality, not hot-loop quality: separation is a brute-force
+    /// segment-pair minimum with a bounding-box reject, penetration is a
+    /// separating-axis sweep over both polygons' edge normals. The overlap verdict
+    /// defers to <see cref="Collision.HasOverlap(Polygon, Polygon, List{Polygon}, List{Polygon})"/>
+    /// so callers that validate with Collision never see a disagreeing kernel.
+    /// Rings with holes are handled by the caller: pass every ring pair (a part's
+    /// material boundary is its outer ring plus its hole rings).
+    /// </para>
+    /// </summary>
+    public static class Clearance
+    {
+        public static ClearanceResult Between(Polygon a, Polygon b)
+        {
+            var linesA = a.ToLines();
+            var linesB = b.ToLines();
+
+            if (linesA.Count == 0 || linesB.Count == 0)
+                return new ClearanceResult(0, new Vector(1, 0));
+
+            if (Collision.HasOverlap(a, b))
+                return Penetration(linesA, linesB);
+
+            return Separation(linesA, linesB);
+        }
+
+        /// <summary>
+        /// Non-negative minimum boundary (edge-to-edge) distance between two rings
+        /// and the direction that translates <paramref name="a"/> away from
+        /// <paramref name="b"/> at the closest contact. Never tests overlap: a ring
+        /// contained in another (a part inside a cutout ring) still reports its true
+        /// gap. For signed material clearance use <see cref="Between"/>.
+        /// </summary>
+        public static ClearanceResult BoundaryDistance(Polygon a, Polygon b)
+        {
+            var linesA = a.ToLines();
+            var linesB = b.ToLines();
+
+            if (linesA.Count == 0 || linesB.Count == 0)
+                return new ClearanceResult(0, new Vector(1, 0));
+
+            return Separation(linesA, linesB);
+        }
+
+        /// <summary>
+        /// Minimum boundary distance between two non-overlapping rings and the
+        /// direction that translates <paramref name="linesA"/> away from
+        /// <paramref name="linesB"/> at the closest contact.
+        /// </summary>
+        private static ClearanceResult Separation(List<Line> linesA, List<Line> linesB)
+        {
+            var minDist = double.MaxValue;
+            var pa = Vector.Zero;
+            var pb = Vector.Zero;
+
+            var boxes = new Box[linesB.Count];
+            for (var i = 0; i < linesB.Count; i++)
+                boxes[i] = SegmentBox(linesB[i]);
+
+            foreach (var la in linesA)
+            {
+                var boxA = SegmentBox(la);
+
+                for (var i = 0; i < linesB.Count; i++)
+                {
+                    if (!BoxesWithin(boxA, boxes[i], minDist))
+                        continue;
+
+                    var d = SegmentDistance(la, linesB[i], out var qa, out var qb);
+                    if (d < minDist)
+                    {
+                        minDist = d;
+                        pa = qa;
+                        pb = qb;
+                    }
+                }
+            }
+
+            var dir = pa - pb;
+            var len = Magnitude(dir);
+
+            if (len <= Tolerance.Epsilon)
+                dir = CentroidAway(linesA, linesB);
+            else
+                dir = dir / len;
+
+            return new ClearanceResult(minDist, dir);
+        }
+
+        /// <summary>
+        /// Penetration depth and minimum-translation direction along the separating-
+        /// axis candidates of both rings. Per candidate axis the true translation
+        /// depth is used (exit distance to the far side), so containment reports the
+        /// depth that actually ends contact, not the interval-intersection length.
+        /// Depth is reported as a negative clearance.
+        /// </summary>
+        private static ClearanceResult Penetration(List<Line> linesA, List<Line> linesB)
+        {
+            var ca = Centroid(linesA);
+            var cb = Centroid(linesB);
+
+            var bestDepth = double.MaxValue;
+            var bestDir = new Vector(1, 0);
+
+            var bestAxis = -1;
+
+            for (var axis = 0; axis < 2; axis++)
+            {
+                var lines = axis == 0 ? linesA : linesB;
+
+                foreach (var line in lines)
+                {
+                    var edge = line.pt2 - line.pt1;
+                    var n = new Vector(edge.Y, -edge.X);
+                    var len = Magnitude(n);
+                    if (len <= Tolerance.Epsilon)
+                        continue;
+                    n = n / len;
+
+                    var (minA, maxA) = Project(linesA, n);
+                    var (minB, maxB) = Project(linesB, n);
+
+                    if (maxA <= minB || maxB <= minA)
+                        continue; // separating axis found
+
+                    // Depth pushing a away from b along ±n.
+                    var forward = maxB - minA; // move a in +n until minA >= maxB
+                    var backward = maxA - minB; // move a in -n until maxA <= minB
+
+                    double depth;
+                    Vector dir;
+                    if (forward <= backward)
+                    {
+                        depth = forward;
+                        dir = n;
+                    }
+                    else
+                    {
+                        depth = backward;
+                        dir = -n;
+                    }
+
+                    if (depth < bestDepth - Tolerance.Epsilon || bestAxis < 0)
+                    {
+                        bestDepth = depth;
+                        bestDir = dir;
+                        bestAxis = axis;
+                    }
+                }
+            }
+
+            if (bestAxis < 0)
+            {
+                // No candidate axis (degenerate rings): deterministic fallback.
+                var away = ca - cb;
+                var len = Magnitude(away);
+                bestDir = len > Tolerance.Epsilon ? away / len : new Vector(1, 0);
+                bestDepth = 0;
+            }
+
+            return new ClearanceResult(-bestDepth, bestDir);
+        }
+
+        private static Vector CentroidAway(List<Line> linesA, List<Line> linesB)
+        {
+            var away = Centroid(linesA) - Centroid(linesB);
+            var len = Magnitude(away);
+            return len > Tolerance.Epsilon ? away / len : new Vector(1, 0);
+        }
+
+        private static Vector Centroid(List<Line> lines)
+        {
+            var sum = Vector.Zero;
+            foreach (var line in lines)
+            {
+                sum += line.pt1;
+                sum += line.pt2;
+            }
+            return sum / (2 * lines.Count);
+        }
+
+        private static (double Min, double Max) Project(List<Line> lines, Vector n)
+        {
+            var min = double.MaxValue;
+            var max = double.MinValue;
+
+            foreach (var line in lines)
+            {
+                var d1 = line.pt1.DotProduct(n);
+                var d2 = line.pt2.DotProduct(n);
+                if (d1 < min)
+                    min = d1;
+                if (d1 > max)
+                    max = d1;
+                if (d2 < min)
+                    min = d2;
+                if (d2 > max)
+                    max = d2;
+            }
+
+            return (min, max);
+        }
+
+        /// <summary>
+        /// Minimum distance between two segments with the closest points.
+        /// Non-parallel segments use the classic clamped closest-point solve;
+        /// (near-)parallel segments fall back to the four endpoint-to-segment
+        /// distances, which is where the minimum always lies.
+        /// </summary>
+        private static double SegmentDistance(Line a, Line b, out Vector pa, out Vector pb)
+        {
+            var p = a.pt1;
+            var r = a.pt2 - a.pt1;
+            var q = b.pt1;
+            var s = b.pt2 - b.pt1;
+
+            var rxr = r.DotProduct(r);
+            var sxs = s.DotProduct(s);
+            var rxs = r.DotProduct(s);
+
+            const double eps = 1e-12;
+
+            var denom = rxr * sxs - rxs * rxs;
+            if (denom > eps && rxr > eps && sxs > eps)
+            {
+                // Minimize |(p + r t) - (q + s u)|^2; setting both partials to
+                // zero and solving (Cramer) with d0 = p - q:
+                //   t = ((r.s)(d0.s) - (d0.r)(s.s)) / (rr.ss - (r.s)^2)
+                //   u = ((r.r)(d0.s) - (r.s)(d0.r)) / (rr.ss - (r.s)^2)
+                var d0 = p - q;
+                var d0r = d0.DotProduct(r);
+                var d0s = d0.DotProduct(s);
+
+                var t = Clamp((rxs * d0s - d0r * sxs) / denom, 0, 1);
+                var u = Clamp((rxs * t + d0s) / sxs, 0, 1); // nearest u on b for clamped t
+                t = Clamp((rxs * u - d0r) / rxr, 0, 1); // re-solve t for clamped u
+
+                pa = p + r * t;
+                pb = q + s * u;
+                return pa.DistanceTo(pb);
+            }
+
+            // Degenerate or parallel: the minimum is attained at an endpoint.
+            var bestPa = p;
+            var bestPb = q;
+            var best = double.MaxValue;
+
+            void Consider(Vector pt, Line seg, bool ptOnA)
+            {
+                var d = seg.pt2 - seg.pt1;
+                var len2 = d.DotProduct(d);
+                var u = len2 <= eps ? 0 : Clamp((pt - seg.pt1).DotProduct(d) / len2, 0, 1);
+                var on = seg.pt1 + d * u;
+                var dist = pt.DistanceTo(on);
+                if (dist < best)
+                {
+                    best = dist;
+                    bestPa = ptOnA ? pt : on;
+                    bestPb = ptOnA ? on : pt;
+                }
+            }
+
+            Consider(p, b, true);
+            Consider(a.pt2, b, true);
+            Consider(q, a, false);
+            Consider(b.pt2, a, false);
+
+            pa = bestPa;
+            pb = bestPb;
+            return best;
+        }
+
+        private static double Clamp(double v, double lo, double hi) =>
+            v < lo ? lo : (v > hi ? hi : v);
+
+        private static double Magnitude(Vector v) => System.Math.Sqrt(v.X * v.X + v.Y * v.Y);
+
+        private static Box SegmentBox(Line line)
+        {
+            return new Box(
+                System.Math.Min(line.pt1.X, line.pt2.X),
+                System.Math.Min(line.pt1.Y, line.pt2.Y),
+                System.Math.Abs(line.pt2.X - line.pt1.X),
+                System.Math.Abs(line.pt2.Y - line.pt1.Y)
+            );
+        }
+
+        private static bool BoxesWithin(Box a, Box b, double distance)
+        {
+            return !(
+                a.Right + distance < b.Left
+                || b.Right + distance < a.Left
+                || a.Top + distance < b.Bottom
+                || b.Top + distance < a.Bottom
+            );
+        }
+    }
+}

OpenNest.Core/Geometry/SlideContact.cs

@@ -0,0 +1,831 @@
+using System.Collections.Generic;
+using OpenNest.Math;
+
+namespace OpenNest.Geometry
+{
+    internal enum ContactSide
+    {
+        /// <summary>The boundary could not be decomposed into closed loops.</summary>
+        Unresolved,
+
+        /// <summary>The point is not on the boundary: a tolerance near-miss, not a contact.</summary>
+        Off,
+
+        /// <summary>Several boundary runs meet here, or the corner is a cusp or spike.</summary>
+        Ambiguous,
+
+        /// <summary>The material sector is known.</summary>
+        Sector,
+    }
+
+    /// <summary>
+    /// Closed boundary loops of one entity list, prepared so a directional slide can tell
+    /// which side of each boundary point is material. Immutable after
+    /// <see cref="Prepare"/>, so one instance may be shared by concurrent queries.
+    /// </summary>
+    /// <remarks>
+    /// Loops are recovered from contiguous runs whose end points chain back to their start
+    /// (the order produced by <see cref="ShapeBuilder"/> and the offset helpers). Nesting
+    /// depth decides holes: material is inside even-depth loops and outside odd-depth ones.
+    /// When the list cannot be decomposed that way, every contact query is unresolved.
+    /// </remarks>
+    public sealed class SlideContactGeometry
+    {
+        // Contact points are computed from unsnapped ray parameters, so a genuine contact is
+        // on both boundaries to floating-point accuracy. This also bounds the overlap sliver a
+        // tangential classification can admit, so keep it far below spacing tolerances.
+        internal const double IncidenceTolerance = 1e-7;
+
+        private readonly List<Entity> entities;
+        private readonly int[] loopOf;
+        private readonly int[] previous;
+        private readonly int[] following;
+        private readonly bool[] materialLeft;
+
+        private SlideContactGeometry(
+            List<Entity> entities,
+            int[] loopOf,
+            int[] previous,
+            int[] following,
+            bool[] materialLeft
+        )
+        {
+            this.entities = entities;
+            this.loopOf = loopOf;
+            this.previous = previous;
+            this.following = following;
+            this.materialLeft = materialLeft;
+        }
+
+        /// <summary>True when every entity belongs to a closed loop with a known material side.</summary>
+        public bool IsResolved => materialLeft != null;
+
+        public static SlideContactGeometry Prepare(List<Entity> entities)
+        {
+            var count = entities.Count;
+            var loopOf = new int[count];
+            var previous = new int[count];
+            var following = new int[count];
+            var loops = new List<(int First, int Last)>();
+
+            var i = 0;
+            while (i < count)
+            {
+                var first = i;
+                if (entities[i] is Circle)
+                {
+                    i++;
+                }
+                else
+                {
+                    if (!TryEndpoints(entities[i], out var start, out _))
+                        return Unresolved(entities);
+
+                    var closed = false;
+                    while (i < count && TryEndpoints(entities[i], out _, out var end))
+                    {
+                        // A lone closed arc is a loop; a lone line cannot be, even when it
+                        // has zero length and so ends where it starts.
+                        if (Near(end, start) && (i > first || entities[i] is Arc))
+                        {
+                            closed = true;
+                            i++;
+                            break;
+                        }
+
+                        if (
+                            i + 1 >= count
+                            || !TryEndpoints(entities[i + 1], out var nextStart, out _)
+                            || !Near(nextStart, end)
+                        )
+                            break;
+
+                        i++;
+                    }
+
+                    if (!closed)
+                        return Unresolved(entities);
+                }
+
+                var loop = loops.Count;
+                loops.Add((first, i - 1));
+                for (var k = first; k < i; k++)
+                {
+                    loopOf[k] = loop;
+                    previous[k] = k == first ? i - 1 : k - 1;
+                    following[k] = k == i - 1 ? first : k + 1;
+                }
+            }
+
+            var materialLeft = new bool[loops.Count];
+
+            for (var loop = 0; loop < loops.Count; loop++)
+            {
+                var area = SignedArea(entities, loops[loop].First, loops[loop].Last);
+                if (System.Math.Abs(area) <= Tolerance.Epsilon)
+                    return Unresolved(entities);
+
+                var depth = 0;
+                if (loops.Count > 1)
+                {
+                    var sample = SamplePoint(entities[loops[loop].First]);
+                    for (var other = 0; other < loops.Count; other++)
+                    {
+                        if (other == loop)
+                            continue;
+                        if (Contains(entities, loops[other].First, loops[other].Last, sample))
+                            depth++;
+                    }
+                }
+
+                materialLeft[loop] = (area > 0) == (depth % 2 == 0);
+            }
+
+            return new SlideContactGeometry(entities, loopOf, previous, following, materialLeft);
+        }
+
+        private static SlideContactGeometry Unresolved(List<Entity> entities) =>
+            new SlideContactGeometry(entities, null, null, null, null);
+
+        /// <summary>
+        /// Material directions at a boundary point: an angular sector starting at
+        /// <paramref name="start"/> and sweeping CCW by <paramref name="width"/>.
+        /// Concavity is recorded separately at each sector ray: only the supporting
+        /// curve, not an unrelated curve at that corner, can block a tangential slide.
+        /// Entities wholly inside the incidence tolerance are treated as part of the corner.
+        /// </summary>
+        internal ContactSide GetMaterialSector(
+            Vector point,
+            out double start,
+            out double width,
+            out bool startConcave,
+            out bool endConcave
+        )
+        {
+            start = width = 0;
+            startConcave = endConcave = false;
+            if (materialLeft == null)
+                return ContactSide.Unresolved;
+
+            var best = -1;
+            var bestDistance = double.MaxValue;
+            for (var i = 0; i < entities.Count; i++)
+            {
+                var distance = DistanceTo(entities[i], point);
+                if (distance < bestDistance)
+                {
+                    bestDistance = distance;
+                    best = i;
+                }
+            }
+
+            if (best < 0 || bestDistance > IncidenceTolerance)
+                return ContactSide.Off;
+
+            // Walk to the entities that enter and leave the tolerance disc.
+            var loopLength = LoopLength(best);
+            var incoming = best;
+            var steps = 0;
+            var smoothLoop = loopLength == 1 && (entities[best] is Circle
+                || entities[best] is Arc fullArc && fullArc.IsFullCircle());
+            while (!smoothLoop && StartsNear(incoming, point))
+            {
+                incoming = previous[incoming];
+                if (++steps >= loopLength)
+                    return ContactSide.Ambiguous;
+            }
+
+            var outgoing = best;
+            steps = 0;
+            while (!smoothLoop && EndsNear(outgoing, point))
+            {
+                outgoing = following[outgoing];
+                if (++steps >= loopLength)
+                    return ContactSide.Ambiguous;
+            }
+
+            // Anything else touching this point (another loop, a spike, a self-crossing)
+            // makes the local material side ambiguous.
+            for (var i = 0; i < entities.Count; i++)
+            {
+                if (InRun(i, incoming, outgoing))
+                    continue;
+                if (DistanceTo(entities[i], point) <= IncidenceTolerance)
+                    return ContactSide.Ambiguous;
+            }
+
+            var interior = incoming == best && outgoing == best && !EndsNear(best, point);
+            var inTangent = interior ? TangentAt(entities[best], point) : EndTangent(entities[incoming]);
+            var outTangent = interior
+                ? inTangent
+                : StartTangent(entities[outgoing]);
+
+            // A circle has no endpoints, so its point is always interior.
+            if (smoothLoop)
+                inTangent = outTangent = TangentAt(entities[best], point);
+
+            if (IsZero(inTangent) || IsZero(outTangent))
+                return ContactSide.Ambiguous;
+
+            var outAngle = System.Math.Atan2(outTangent.Y, outTangent.X);
+            var inAngle = System.Math.Atan2(-inTangent.Y, -inTangent.X);
+            var left = materialLeft[loopOf[best]];
+
+            start = left ? outAngle : inAngle;
+            width = Angle.NormalizeRad((left ? inAngle : outAngle) - start);
+
+            startConcave = IsConcave(entities[left ? outgoing : incoming], left);
+            endConcave = IsConcave(entities[left ? incoming : outgoing], left);
+
+            return
+                width > SlideContact.AngleTolerance
+                && width < Angle.TwoPI - 2 * SlideContact.SplitOverlap
+                ? ContactSide.Sector
+                : ContactSide.Ambiguous;
+        }
+
+        private int LoopLength(int index)
+        {
+            var length = 1;
+            for (var i = following[index]; i != index; i = following[i])
+                length++;
+            return length;
+        }
+
+        private bool StartsNear(int index, Vector point) =>
+            TryEndpoints(entities[index], out var start, out _)
+            && start.DistanceTo(point) <= IncidenceTolerance;
+
+        private bool EndsNear(int index, Vector point) =>
+            TryEndpoints(entities[index], out _, out var end)
+            && end.DistanceTo(point) <= IncidenceTolerance;
+
+        private bool InRun(int index, int first, int last)
+        {
+            for (var i = first; ; i = following[i])
+            {
+                if (i == index)
+                    return true;
+                if (i == last)
+                    return false;
+            }
+        }
+
+        private static bool IsZero(Vector v) => v.X == 0 && v.Y == 0;
+
+        private static bool IsConcave(Entity entity, bool materialLeft)
+        {
+            // A CCW curve has its center on its left; that center is on the free side
+            // (a concave boundary) exactly when material is on the right.
+            return entity switch
+            {
+                Arc arc => materialLeft == arc.IsReversed,
+                Circle circle => materialLeft == (circle.Rotation == RotationType.CW),
+                _ => false,
+            };
+        }
+
+        private static Vector StartTangent(Entity entity) =>
+            entity switch
+            {
+                Line line => Direction(line.pt1, line.pt2),
+                Arc arc => ArcTangent(arc.StartAngle, arc.IsReversed),
+                _ => new Vector(),
+            };
+
+        private static Vector EndTangent(Entity entity) =>
+            entity switch
+            {
+                Line line => Direction(line.pt1, line.pt2),
+                Arc arc => ArcTangent(arc.EndAngle, arc.IsReversed),
+                _ => new Vector(),
+            };
+
+        private static Vector TangentAt(Entity entity, Vector point) =>
+            entity switch
+            {
+                Line line => Direction(line.pt1, line.pt2),
+                Arc arc => ArcTangent(arc.Center.AngleTo(point), arc.IsReversed),
+                Circle circle => ArcTangent(
+                    circle.Center.AngleTo(point),
+                    circle.Rotation == RotationType.CW
+                ),
+                _ => new Vector(),
+            };
+
+        private static Vector ArcTangent(double angle, bool clockwise)
+        {
+            var sign = clockwise ? -1.0 : 1.0;
+            return new Vector(-System.Math.Sin(angle) * sign, System.Math.Cos(angle) * sign);
+        }
+
+        private static Vector Direction(Vector from, Vector to)
+        {
+            var dx = to.X - from.X;
+            var dy = to.Y - from.Y;
+            var length = System.Math.Sqrt(dx * dx + dy * dy);
+            return length > 0 ? new Vector(dx / length, dy / length) : new Vector();
+        }
+
+        private static double DistanceTo(Entity entity, Vector point)
+        {
+            switch (entity)
+            {
+                case Line line:
+                    return point.DistanceTo(line.ClosestPointTo(point));
+                case Arc arc:
+                    {
+                        var angle = arc.Center.AngleTo(point);
+                        if (Angle.IsBetweenRad(angle, arc.StartAngle, arc.EndAngle, arc.IsReversed))
+                            return System.Math.Abs(arc.Center.DistanceTo(point) - arc.Radius);
+                        return System.Math.Min(
+                            point.DistanceTo(arc.StartPoint()),
+                            point.DistanceTo(arc.EndPoint())
+                        );
+                    }
+                case Circle circle:
+                    return System.Math.Abs(circle.Center.DistanceTo(point) - circle.Radius);
+                default:
+                    return double.MaxValue;
+            }
+        }
+
+        private static bool TryEndpoints(Entity entity, out Vector start, out Vector end)
+        {
+            switch (entity)
+            {
+                case Line line:
+                    start = line.pt1;
+                    end = line.pt2;
+                    return true;
+                case Arc arc:
+                    start = arc.StartPoint();
+                    end = arc.EndPoint();
+                    return true;
+                default:
+                    start = end = new Vector();
+                    return false;
+            }
+        }
+
+        private static bool Near(Vector a, Vector b) => a.DistanceTo(b) <= IncidenceTolerance;
+
+        private static double SignedArea(List<Entity> entities, int first, int last)
+        {
+            var area = 0.0;
+            for (var i = first; i <= last; i++)
+            {
+                switch (entities[i])
+                {
+                    case Circle circle:
+                        var sign = circle.Rotation == RotationType.CW ? -1 : 1;
+                        area += sign * System.Math.PI * circle.Radius * circle.Radius;
+                        break;
+                    case Line line:
+                        area += Cross(line.pt1, line.pt2) / 2;
+                        break;
+                    case Arc arc:
+                        var sweep = arc.IsReversed ? -arc.SweepAngle() : arc.SweepAngle();
+                        var r = arc.Radius;
+                        area += Cross(arc.StartPoint(), arc.EndPoint()) / 2;
+                        area += r * r / 2 * (sweep - System.Math.Sin(sweep));
+                        break;
+                }
+            }
+            return area;
+        }
+
+        private static double Cross(Vector a, Vector b) => a.X * b.Y - b.X * a.Y;
+
+        private static Vector SamplePoint(Entity entity) =>
+            entity switch
+            {
+                Circle circle => new Vector(circle.Center.X + circle.Radius, circle.Center.Y),
+                Arc arc => arc.StartPoint(),
+                Line line => line.pt1,
+                _ => new Vector(),
+            };
+
+        // Exact horizontal-ray parity. Split arcs at Y extrema so every piece is
+        // monotone; the same half-open endpoint rule as lines avoids seam double counts.
+        // A coarse inscribed polygon can misclassify thin rings as solid material.
+        private static bool Contains(List<Entity> entities, int first, int last, Vector point)
+        {
+            var inside = false;
+            for (var i = first; i <= last; i++)
+            {
+                if (entities[i] is Circle circle)
+                    return circle.Center.DistanceTo(point) < circle.Radius;
+                if (entities[i] is Line line)
+                {
+                    var a = line.pt1;
+                    var b = line.pt2;
+                    if ((a.Y > point.Y) != (b.Y > point.Y)
+                        && point.X < (b.X - a.X) * (point.Y - a.Y) / (b.Y - a.Y) + a.X)
+                        inside = !inside;
+                }
+                else if (entities[i] is Arc arc)
+                {
+                    var sweep = arc.SweepAngle();
+                    var sign = arc.IsReversed ? -1.0 : 1.0;
+                    var cuts = new List<double> { 0, sweep };
+                    foreach (var extreme in new[] { Angle.HalfPI, 3 * Angle.HalfPI })
+                    {
+                        var t = Angle.NormalizeRad(sign * (extreme - arc.StartAngle));
+                        if (t > 0 && t < sweep)
+                            cuts.Add(t);
+                    }
+                    cuts.Sort();
+                    for (var k = 1; k < cuts.Count; k++)
+                    {
+                        var a = arc.StartAngle + sign * cuts[k - 1];
+                        var b = arc.StartAngle + sign * cuts[k];
+                        var y1 = arc.Center.Y + arc.Radius * System.Math.Sin(a);
+                        var y2 = arc.Center.Y + arc.Radius * System.Math.Sin(b);
+                        if ((y1 > point.Y) == (y2 > point.Y))
+                            continue;
+                        var dy = point.Y - arc.Center.Y;
+                        var dx = System.Math.Sqrt(System.Math.Max(0, arc.Radius * arc.Radius - dy * dy));
+                        var x = arc.Center.X + (System.Math.Cos((a + b) / 2) >= 0 ? dx : -dx);
+                        if (point.X < x)
+                            inside = !inside;
+                    }
+                }
+            }
+            return inside;
+        }
+    }
+
+    /// <summary>
+    /// Contact classifier for one moving/stationary pair of boundaries. Geometry is prepared
+    /// on first use, so a slide whose nearest contact is never classified pays nothing; call
+    /// <see cref="Prepare"/> before sharing one instance across threads. Each boundary is
+    /// given in its own frame; the origins place those frames in the world coordinates used
+    /// by slide events.
+    /// </summary>
+    public sealed class SlideContactClassifier
+    {
+        private readonly System.Func<List<Entity>> movingSource;
+        private readonly System.Func<List<Entity>> stationarySource;
+        private SlideContactGeometry moving;
+        private SlideContactGeometry stationary;
+
+        public SlideContactClassifier(List<Entity> movingEntities, List<Entity> stationaryEntities)
+            : this(movingEntities, Vector.Zero, stationaryEntities, Vector.Zero) { }
+
+        public SlideContactClassifier(
+            List<Entity> movingEntities,
+            Vector movingOrigin,
+            List<Entity> stationaryEntities,
+            Vector stationaryOrigin
+        )
+            : this(() => movingEntities, movingOrigin, () => stationaryEntities, stationaryOrigin)
+        { }
+
+        public SlideContactClassifier(
+            SlideContactGeometry moving,
+            Vector movingOrigin,
+            SlideContactGeometry stationary,
+            Vector stationaryOrigin
+        )
+        {
+            this.moving = moving;
+            this.stationary = stationary;
+            MovingOrigin = movingOrigin;
+            StationaryOrigin = stationaryOrigin;
+        }
+
+        private SlideContactClassifier(
+            System.Func<List<Entity>> movingSource,
+            Vector movingOrigin,
+            System.Func<List<Entity>> stationarySource,
+            Vector stationaryOrigin
+        )
+        {
+            this.movingSource = movingSource;
+            this.stationarySource = stationarySource;
+            MovingOrigin = movingOrigin;
+            StationaryOrigin = stationaryOrigin;
+        }
+
+        public Vector MovingOrigin { get; }
+
+        public Vector StationaryOrigin { get; }
+
+        public static SlideContactClassifier FromLines(
+            List<Line> movingLines,
+            Vector movingOrigin,
+            List<Line> stationaryLines,
+            Vector stationaryOrigin
+        ) =>
+            new SlideContactClassifier(
+                () => new List<Entity>(movingLines),
+                movingOrigin,
+                () => new List<Entity>(stationaryLines),
+                stationaryOrigin
+            );
+
+        public static SlideContactClassifier FromEdges(
+            (Vector start, Vector end)[] movingEdges,
+            Vector movingOrigin,
+            (Vector start, Vector end)[] stationaryEdges,
+            Vector stationaryOrigin
+        )
+        {
+            // The kernel sorts edge arrays in place, so snapshot the chain order now.
+            var moving = ((Vector start, Vector end)[])movingEdges.Clone();
+            var stationary = ((Vector start, Vector end)[])stationaryEdges.Clone();
+            return new SlideContactClassifier(
+                () => ToLines(moving),
+                movingOrigin,
+                () => ToLines(stationary),
+                stationaryOrigin
+            );
+        }
+
+        private static List<Entity> ToLines((Vector start, Vector end)[] edges)
+        {
+            var lines = new List<Entity>(edges.Length);
+            foreach (var (start, end) in edges)
+                lines.Add(new Line(start, end));
+            // Public edge arrays are sorted in place by previous queries. Recover their
+            // chains on private line objects; never reverse or reorder caller geometry.
+            var ordered = new List<Entity>(lines.Count);
+            foreach (var shape in ShapeBuilder.GetShapes(lines))
+                ordered.AddRange(shape.Entities);
+            return ordered;
+        }
+
+        public SlideContactClassifier Prepare()
+        {
+            moving ??= SlideContactGeometry.Prepare(movingSource?.Invoke() ?? new List<Entity>());
+            stationary ??= SlideContactGeometry.Prepare(
+                stationarySource?.Invoke() ?? new List<Entity>()
+            );
+            return this;
+        }
+
+        /// <summary>The same prepared boundaries placed at other origins.</summary>
+        public SlideContactClassifier At(Vector movingOrigin, Vector stationaryOrigin)
+        {
+            Prepare();
+            return new SlideContactClassifier(moving, movingOrigin, stationary, stationaryOrigin);
+        }
+
+        /// <summary>
+        /// True when moving along (dirX, dirY) from this world-space contact would push
+        /// material into material, or the contact cannot be classified.
+        /// </summary>
+        public bool Blocks(Vector movingPoint, Vector stationaryPoint, double dirX, double dirY)
+        {
+            Prepare();
+            return SlideContact.Blocks(
+                moving,
+                movingPoint - MovingOrigin,
+                stationary,
+                stationaryPoint - StationaryOrigin,
+                dirX,
+                dirY
+            );
+        }
+    }
+
+    /// <summary>Receives candidate contact events from a directional slide query.</summary>
+    public interface ISlideEventSink
+    {
+        /// <summary>True once further events cannot change this sink's result.</summary>
+        bool IsDone { get; }
+
+        /// <param name="distance">Travel to the contact, snapped to zero within Tolerance.Epsilon.</param>
+        /// <param name="movingPoint">Contact on the moving boundary, at its start position.</param>
+        /// <param name="stationaryPoint">Contact on the stationary boundary.</param>
+        void Add(double distance, Vector movingPoint, Vector stationaryPoint);
+    }
+
+    /// <summary>
+    /// Enumerates every candidate contact of one slide. Must yield the same events each
+    /// time it is enumerated.
+    /// </summary>
+    public interface ISlideEventSource
+    {
+        void Enumerate<TSink>(ref TSink sink)
+            where TSink : struct, ISlideEventSink;
+    }
+
+    /// <summary>Keeps the nearest event; stops at a contact that is already touching.</summary>
+    public struct NearestSlideEvent : ISlideEventSink
+    {
+        public bool Found;
+        public double Distance;
+        public Vector MovingPoint;
+        public Vector StationaryPoint;
+
+        public bool IsDone => Found && Distance <= 0;
+
+        public void Add(double distance, Vector movingPoint, Vector stationaryPoint)
+        {
+            if (Found && distance >= Distance)
+                return;
+
+            Found = true;
+            Distance = distance;
+            MovingPoint = movingPoint;
+            StationaryPoint = stationaryPoint;
+        }
+    }
+
+    internal struct SlideEventList : ISlideEventSink
+    {
+        public List<(double Distance, Vector MovingPoint, Vector StationaryPoint)> Events;
+
+        public bool IsDone => false;
+
+        public void Add(double distance, Vector movingPoint, Vector stationaryPoint) =>
+            Events.Add((distance, movingPoint, stationaryPoint));
+    }
+
+    public static class SlideResolver
+    {
+        /// <summary>
+        /// Travel to the first contact that blocks the slide, or double.MaxValue. When the
+        /// nearest contact blocks (every contact, for unresolved boundaries), the result is
+        /// exactly the nearest event distance and the events are enumerated once.
+        /// </summary>
+        public static double FirstBlocking<TSource>(
+            ref TSource source,
+            SlideContactClassifier contacts,
+            double dirX,
+            double dirY
+        )
+            where TSource : struct, ISlideEventSource
+        {
+            var nearest = new NearestSlideEvent();
+            source.Enumerate(ref nearest);
+
+            if (!nearest.Found)
+                return double.MaxValue;
+
+            if (contacts.Blocks(nearest.MovingPoint, nearest.StationaryPoint, dirX, dirY))
+                return nearest.Distance;
+
+            var all = new SlideEventList
+            {
+                Events = new List<(double, Vector, Vector)>(),
+            };
+            source.Enumerate(ref all);
+            all.Events.Sort((a, b) => a.Distance.CompareTo(b.Distance));
+
+            foreach (var (distance, movingPoint, stationaryPoint) in all.Events)
+            {
+                if (contacts.Blocks(movingPoint, stationaryPoint, dirX, dirY))
+                    return distance;
+            }
+
+            return double.MaxValue;
+        }
+    }
+
+    /// <summary>
+    /// Decides whether a first-contact event found by a directional slide stops the slide.
+    /// </summary>
+    /// <remarks>
+    /// Parts that already touch may slide along each other or apart. Only a direction that
+    /// would create positive-area overlap blocks: with S the stationary material sector and
+    /// M the moving one at the contact point, that is the open Minkowski cone S ⊕ −M.
+    /// A direction on that cone's boundary is a tangential slide; it blocks only when an
+    /// incident curve is concave, because the second-order bend then closes the gap.
+    /// Unresolved or ambiguous topology blocks, which is the previous behavior for every
+    /// contact.
+    /// </remarks>
+    public static class SlideContact
+    {
+        internal const double AngleTolerance = 1e-7;
+
+        // Reflex sectors are split into two overlapping convex halves; the overlap keeps
+        // the split ray in the interior of the union.
+        internal const double SplitOverlap = 1e-3;
+
+        /// <summary>
+        /// True when moving along (dirX, dirY) from this contact would push material into
+        /// material, or when the contact cannot be classified. False for a near-miss whose
+        /// point is not on both boundaries.
+        /// </summary>
+        /// <param name="movingPoint">Contact point in the moving entities' own frame.</param>
+        /// <param name="stationaryPoint">The same contact in the stationary frame.</param>
+        public static bool Blocks(
+            SlideContactGeometry moving,
+            Vector movingPoint,
+            SlideContactGeometry stationary,
+            Vector stationaryPoint,
+            double dirX,
+            double dirY
+        )
+        {
+            if (moving == null || stationary == null)
+                return true;
+
+            var stationarySide = stationary.GetMaterialSector(
+                stationaryPoint,
+                out var stationaryStart,
+                out var stationaryWidth,
+                out var stationaryStartConcave,
+                out var stationaryEndConcave
+            );
+            var movingSide = moving.GetMaterialSector(
+                movingPoint,
+                out var movingStart,
+                out var movingWidth,
+                out var movingStartConcave,
+                out var movingEndConcave
+            );
+
+            if (stationarySide == ContactSide.Unresolved || movingSide == ContactSide.Unresolved)
+                return true;
+
+            // Ray tolerances report hits slightly beyond an entity's end; such a point is
+            // not on the other boundary, so the parts pass without touching there.
+            if (stationarySide == ContactSide.Off || movingSide == ContactSide.Off)
+                return false;
+
+            if (stationarySide == ContactSide.Ambiguous || movingSide == ContactSide.Ambiguous)
+                return true;
+
+            var direction = System.Math.Atan2(dirY, dirX);
+            var stationaryPieces = Split(stationaryStart, stationaryWidth);
+            var movingPieces = Split(movingStart + System.Math.PI, movingWidth);
+            var onBoundary = false;
+
+            foreach (var s in stationaryPieces)
+            {
+                foreach (var m in movingPieces)
+                {
+                    if (!TryHull(s, m, out var hullStart, out var hullWidth))
+                        return true;
+
+                    var offset = Angle.NormalizeRad(direction - hullStart);
+                    if (offset > AngleTolerance && offset < hullWidth - AngleTolerance)
+                        return true;
+
+                    if (
+                        offset <= AngleTolerance
+                        || offset >= Angle.TwoPI - AngleTolerance
+                        || System.Math.Abs(offset - hullWidth) <= AngleTolerance
+                    )
+                        onBoundary = true;
+                }
+            }
+
+            return onBoundary && (
+                stationaryStartConcave && SameRay(direction, stationaryStart)
+                || stationaryEndConcave && SameRay(direction, stationaryStart + stationaryWidth)
+                || movingStartConcave && SameRay(direction, movingStart + System.Math.PI)
+                || movingEndConcave && SameRay(direction, movingStart + movingWidth + System.Math.PI));
+        }
+
+        private static bool SameRay(double a, double b)
+        {
+            var offset = Angle.NormalizeRad(a - b);
+            return offset <= AngleTolerance || offset >= Angle.TwoPI - AngleTolerance;
+        }
+
+        private static (double Start, double Width)[] Split(double start, double width)
+        {
+            if (width <= System.Math.PI + AngleTolerance)
+                return new[] { (start, width) };
+
+            var half = width / 2;
+            return new[]
+            {
+                (start, half + SplitOverlap),
+                (start + half - SplitOverlap, half + SplitOverlap),
+            };
+        }
+
+        /// <summary>
+        /// Convex cone generated by two convex sectors. False when it is the whole plane.
+        /// </summary>
+        private static bool TryHull(
+            (double Start, double Width) a,
+            (double Start, double Width) b,
+            out double start,
+            out double width
+        )
+        {
+            var fromA = System.Math.Max(a.Width, Angle.NormalizeRad(b.Start - a.Start) + b.Width);
+            var fromB = System.Math.Max(b.Width, Angle.NormalizeRad(a.Start - b.Start) + a.Width);
+
+            if (fromA <= fromB)
+            {
+                start = a.Start;
+                width = fromA;
+            }
+            else
+            {
+                start = b.Start;
+                width = fromB;
+            }
+
+            return width <= System.Math.PI + AngleTolerance;
+        }
+    }
+}

OpenNest.Core/Geometry/SlideEvents.cs

@@ -0,0 +1,806 @@
+using System.Collections.Generic;
+using OpenNest.Math;
+
+namespace OpenNest.Geometry
+{
+    /// <summary>
+    /// Candidate contact events of directional slides. Each emitter reports every forward
+    /// hit its distance kernel considers, with the distance snapped exactly as that kernel
+    /// snaps it, so the nearest event equals the kernel's historical minimum.
+    /// </summary>
+    internal static class SlideEvents
+    {
+        private const double Eps = Tolerance.Epsilon;
+
+        private static double Snap(double t) => t > Eps ? t : 0;
+
+        /// <summary>
+        /// Ray from a vertex against one entity. When <paramref name="vertexMoves"/> is true
+        /// the vertex belongs to the moving boundary and the ray follows the push direction;
+        /// otherwise it is a stationary vertex and the ray runs opposite to the push.
+        /// </summary>
+        public static void Ray<TSink>(
+            ref TSink sink,
+            double vx,
+            double vy,
+            Entity entity,
+            double entityDx,
+            double entityDy,
+            double rayX,
+            double rayY,
+            bool vertexMoves
+        )
+            where TSink : struct, ISlideEventSink
+        {
+            switch (entity)
+            {
+                case Line line:
+                    RayLine(
+                        ref sink,
+                        vx,
+                        vy,
+                        line.pt1.X + entityDx,
+                        line.pt1.Y + entityDy,
+                        line.pt2.X + entityDx,
+                        line.pt2.Y + entityDy,
+                        rayX,
+                        rayY,
+                        vertexMoves
+                    );
+                    break;
+
+                case Arc arc:
+                    {
+                        var cx = arc.Center.X + entityDx;
+                        var cy = arc.Center.Y + entityDy;
+                        if (!SolveRayCircle(vx, vy, cx, cy, arc.Radius, rayX, rayY, out var t1, out var t2))
+                            return;
+
+                        for (var k = 0; k < 2; k++)
+                        {
+                            var t = k == 0 ? t1 : t2;
+                            if (t <= -Eps)
+                                continue;
+
+                            var hitAngle = Angle.NormalizeRad(
+                                System.Math.Atan2(vy + t * rayY - cy, vx + t * rayX - cx)
+                            );
+                            if (!Angle.IsBetweenRad(hitAngle, arc.StartAngle, arc.EndAngle, arc.IsReversed))
+                                continue;
+
+                            Emit(ref sink, vx, vy, t, rayX, rayY, vertexMoves);
+                            if (sink.IsDone)
+                                return;
+                        }
+                        break;
+                    }
+
+                case Circle circle:
+                    {
+                        if (
+                            !SolveRayCircle(
+                                vx,
+                                vy,
+                                circle.Center.X + entityDx,
+                                circle.Center.Y + entityDy,
+                                circle.Radius,
+                                rayX,
+                                rayY,
+                                out var t1,
+                                out var t2
+                            )
+                        )
+                            return;
+
+                        for (var k = 0; k < 2; k++)
+                        {
+                            var t = k == 0 ? t1 : t2;
+                            if (t < -Eps)
+                                continue;
+
+                            Emit(ref sink, vx, vy, t, rayX, rayY, vertexMoves);
+                            if (sink.IsDone)
+                                return;
+                        }
+                        break;
+                    }
+            }
+        }
+
+        /// <summary>Same hit rule as <see cref="SpatialQuery.RayEdgeDistance(double, double, double, double, double, double, double, double)"/>.</summary>
+        public static void RayLine<TSink>(
+            ref TSink sink,
+            double vx,
+            double vy,
+            double p1x,
+            double p1y,
+            double p2x,
+            double p2y,
+            double rayX,
+            double rayY,
+            bool vertexMoves
+        )
+            where TSink : struct, ISlideEventSink
+        {
+            var ex = p2x - p1x;
+            var ey = p2y - p1y;
+
+            var det = ex * rayY - ey * rayX;
+            if (System.Math.Abs(det) < Eps)
+                return;
+
+            var dvx = p1x - vx;
+            var dvy = p1y - vy;
+
+            var t = (ex * dvy - ey * dvx) / det;
+            if (t < -Eps)
+                return;
+
+            var s = (rayX * dvy - rayY * dvx) / det;
+            if (s < -Eps || s > 1.0 + Eps)
+                return;
+
+            Emit(ref sink, vx, vy, t, rayX, rayY, vertexMoves);
+        }
+
+        /// <summary>
+        /// Axis-aligned ray against a segment, with the same hit rule as the
+        /// <see cref="PushDirection"/> kernel.
+        /// </summary>
+        public static void AxisRayLine<TSink>(
+            ref TSink sink,
+            double vx,
+            double vy,
+            double p1x,
+            double p1y,
+            double p2x,
+            double p2y,
+            PushDirection rayDirection,
+            bool vertexMoves
+        )
+            where TSink : struct, ISlideEventSink
+        {
+            double dist,
+                hx,
+                hy;
+
+            switch (rayDirection)
+            {
+                case PushDirection.Left:
+                case PushDirection.Right:
+                    {
+                        var dy = p2y - p1y;
+                        if (System.Math.Abs(dy) < Eps)
+                            return;
+
+                        var t = (vy - p1y) / dy;
+                        if (t < -Eps || t > 1.0 + Eps)
+                            return;
+
+                        hx = p1x + t * (p2x - p1x);
+                        hy = vy;
+                        dist = rayDirection == PushDirection.Left ? vx - hx : hx - vx;
+                        break;
+                    }
+
+                case PushDirection.Down:
+                case PushDirection.Up:
+                    {
+                        var dx = p2x - p1x;
+                        if (System.Math.Abs(dx) < Eps)
+                            return;
+
+                        var t = (vx - p1x) / dx;
+                        if (t < -Eps || t > 1.0 + Eps)
+                            return;
+
+                        hx = vx;
+                        hy = p1y + t * (p2y - p1y);
+                        dist = rayDirection == PushDirection.Down ? vy - hy : hy - vy;
+                        break;
+                    }
+
+                default:
+                    return;
+            }
+
+            if (dist < -Eps)
+                return;
+
+            var vertex = new Vector(vx, vy);
+            var hit = new Vector(hx, hy);
+            if (vertexMoves)
+                sink.Add(Snap(dist), vertex, hit);
+            else
+                sink.Add(Snap(dist), hit, vertex);
+        }
+
+        /// <summary>
+        /// Closest-approach points of arcs against lines, which vertex sampling can miss.
+        /// </summary>
+        public static void ArcToLine<TSink>(
+            ref TSink sink,
+            List<Entity> arcEntities,
+            double arcDx,
+            double arcDy,
+            List<Entity> lineEntities,
+            double lineDx,
+            double lineDy,
+            double rayX,
+            double rayY,
+            bool arcMoves
+        )
+            where TSink : struct, ISlideEventSink
+        {
+            for (var i = 0; i < arcEntities.Count; i++)
+            {
+                if (!TryGetCurve(arcEntities[i], out var localCx, out var localCy, out var r))
+                    continue;
+
+                var arc = arcEntities[i] as Arc;
+                var cx = localCx + arcDx;
+                var cy = localCy + arcDy;
+
+                for (var j = 0; j < lineEntities.Count; j++)
+                {
+                    if (lineEntities[j] is not Line line)
+                        continue;
+
+                    var p1x = line.pt1.X + lineDx;
+                    var p1y = line.pt1.Y + lineDy;
+                    var p2x = line.pt2.X + lineDx;
+                    var p2y = line.pt2.Y + lineDy;
+                    var ex = p2x - p1x;
+                    var ey = p2y - p1y;
+
+                    var det = ex * rayY - ey * rayX;
+                    if (System.Math.Abs(det) < Eps)
+                        continue;
+
+                    // The directional distance from an arc point at angle θ to the
+                    // line is t(θ) = [A + r·(ey·cosθ − ex·sinθ)] / det.
+                    // dt/dθ = 0 at θ = atan2(−ex, ey) and θ + π.
+                    var theta1 = Angle.NormalizeRad(System.Math.Atan2(-ex, ey));
+                    var theta2 = Angle.NormalizeRad(theta1 + System.Math.PI);
+
+                    for (var k = 0; k < 2; k++)
+                    {
+                        var theta = k == 0 ? theta1 : theta2;
+
+                        if (arc != null && !Angle.IsBetweenRad(theta, arc.StartAngle, arc.EndAngle, arc.IsReversed))
+                            continue;
+
+                        var qx = cx + r * System.Math.Cos(theta);
+                        var qy = cy + r * System.Math.Sin(theta);
+
+                        RayLine(ref sink, qx, qy, p1x, p1y, p2x, p2y, rayX, rayY, arcMoves);
+                        if (sink.IsDone)
+                            return;
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// External and internal tangencies of two curves along a unit direction. Radii must
+        /// be nonnegative; a null arc is a full circle.
+        /// </summary>
+        public static void CurveTangency<TSink>(
+            ref TSink sink,
+            double movingCx,
+            double movingCy,
+            double movingRadius,
+            Arc movingArc,
+            double stationaryCx,
+            double stationaryCy,
+            double stationaryRadius,
+            Arc stationaryArc,
+            double dirX,
+            double dirY
+        )
+            where TSink : struct, ISlideEventSink
+        {
+            for (var kind = 0; kind < 2; kind++)
+            {
+                var internalContact = kind == 1;
+                var radius = internalContact
+                    ? System.Math.Abs(movingRadius - stationaryRadius)
+                    : movingRadius + stationaryRadius;
+
+                // Equal-radius internal contact has coincident centers, not a unique
+                // tangent point. Endpoints detect any overlap of those angular spans.
+                if (radius == 0)
+                    continue;
+
+                if (
+                    !SolveRayCircle(
+                        movingCx,
+                        movingCy,
+                        stationaryCx,
+                        stationaryCy,
+                        radius,
+                        dirX,
+                        dirY,
+                        out var t1,
+                        out var t2
+                    )
+                )
+                    continue;
+
+                // The nearer center-circle root can be outside an arc while the farther
+                // root is its first contact. Check the actual tangent point at BOTH roots.
+                for (var root = 0; root < 2; root++)
+                {
+                    var t = root == 0 ? t1 : t2;
+                    if (t < -Eps)
+                        continue;
+
+                    var toX = stationaryCx - (movingCx + t * dirX);
+                    var toY = stationaryCy - (movingCy + t * dirY);
+                    var movingSign = internalContact && movingRadius < stationaryRadius ? -1 : 1;
+                    var stationarySign = internalContact ? movingSign : -1;
+                    if (
+                        !ContainsContactAngle(
+                            movingArc,
+                            movingRadius,
+                            movingSign * toX,
+                            movingSign * toY
+                        )
+                        || !ContainsContactAngle(
+                            stationaryArc,
+                            stationaryRadius,
+                            stationarySign * toX,
+                            stationarySign * toY
+                        )
+                    )
+                        continue;
+
+                    var length = System.Math.Sqrt(toX * toX + toY * toY);
+                    var ux = length > 0 ? toX / length : 0;
+                    var uy = length > 0 ? toY / length : 0;
+                    var movingPoint = new Vector(
+                        movingCx + movingSign * movingRadius * ux,
+                        movingCy + movingSign * movingRadius * uy
+                    );
+                    var stationaryPoint = new Vector(
+                        stationaryCx + stationarySign * stationaryRadius * ux,
+                        stationaryCy + stationarySign * stationaryRadius * uy
+                    );
+
+                    sink.Add(Snap(t), movingPoint, stationaryPoint);
+                    if (sink.IsDone)
+                        return;
+                }
+            }
+        }
+
+        public static bool TryGetCurve(Entity entity, out double cx, out double cy, out double r)
+        {
+            switch (entity)
+            {
+                case Circle circle:
+                    cx = circle.Center.X;
+                    cy = circle.Center.Y;
+                    r = circle.Radius;
+                    return true;
+                case Arc arc:
+                    cx = arc.Center.X;
+                    cy = arc.Center.Y;
+                    r = arc.Radius;
+                    return true;
+                default:
+                    cx = cy = r = 0;
+                    return false;
+            }
+        }
+
+        private static void Emit<TSink>(
+            ref TSink sink,
+            double vx,
+            double vy,
+            double t,
+            double rayX,
+            double rayY,
+            bool vertexMoves
+        )
+            where TSink : struct, ISlideEventSink
+        {
+            var vertex = new Vector(vx, vy);
+            var hit = new Vector(vx + t * rayX, vy + t * rayY);
+            if (vertexMoves)
+                sink.Add(Snap(t), vertex, hit);
+            else
+                sink.Add(Snap(t), hit, vertex);
+        }
+
+        private static bool ContainsContactAngle(Arc arc, double radius, double x, double y)
+        {
+            // A zero-radius curve is a point: its angular range has no geometric meaning.
+            if (arc == null || radius == 0)
+                return true;
+            var angle = Angle.NormalizeRad(System.Math.Atan2(y, x));
+            return Angle.IsBetweenRad(angle, arc.StartAngle, arc.EndAngle, arc.IsReversed);
+        }
+
+        internal static bool SolveRayCircle(
+            double vx,
+            double vy,
+            double cx,
+            double cy,
+            double r,
+            double dirX,
+            double dirY,
+            out double t1,
+            out double t2
+        )
+        {
+            var ox = vx - cx;
+            var oy = vy - cy;
+
+            var a = dirX * dirX + dirY * dirY;
+            var b = 2.0 * (ox * dirX + oy * dirY);
+            var c = ox * ox + oy * oy - r * r;
+
+            var discriminant = b * b - 4.0 * a * c;
+            if (discriminant < 0)
+            {
+                t1 = t2 = double.MaxValue;
+                return false;
+            }
+
+            var sqrtD = System.Math.Sqrt(discriminant);
+            var inv2a = 1.0 / (2.0 * a);
+            t1 = (-b - sqrtD) * inv2a;
+            t2 = (-b + sqrtD) * inv2a;
+            return true;
+        }
+    }
+
+    /// <summary>
+    /// Slide events between native Line/Arc/Circle boundaries. The moving entities and
+    /// vertices are translated by (movingDx, movingDy); vertex arrays may be subsets.
+    /// </summary>
+    public struct EntitySlideEvents : ISlideEventSource
+    {
+        private readonly List<Entity> moving;
+        private readonly Vector[] movingVertices;
+        private readonly double movingDx;
+        private readonly double movingDy;
+        private readonly List<Entity> stationary;
+        private readonly Vector[] stationaryVertices;
+        private readonly double dirX;
+        private readonly double dirY;
+        private readonly bool arcToLine;
+
+        public EntitySlideEvents(
+            List<Entity> moving,
+            Vector[] movingVertices,
+            double movingDx,
+            double movingDy,
+            List<Entity> stationary,
+            Vector[] stationaryVertices,
+            double dirX,
+            double dirY,
+            bool arcToLine
+        )
+        {
+            this.moving = moving;
+            this.movingVertices = movingVertices;
+            this.movingDx = movingDx;
+            this.movingDy = movingDy;
+            this.stationary = stationary;
+            this.stationaryVertices = stationaryVertices;
+            this.dirX = dirX;
+            this.dirY = dirY;
+            this.arcToLine = arcToLine;
+        }
+
+        public void Enumerate<TSink>(ref TSink sink)
+            where TSink : struct, ISlideEventSink
+        {
+            // Phase 1: moving vertices along the push against stationary entities.
+            for (var v = 0; v < movingVertices.Length; v++)
+            {
+                var vx = movingVertices[v].X + movingDx;
+                var vy = movingVertices[v].Y + movingDy;
+
+                for (var j = 0; j < stationary.Count; j++)
+                {
+                    SlideEvents.Ray(ref sink, vx, vy, stationary[j], 0, 0, dirX, dirY, true);
+                    if (sink.IsDone)
+                        return;
+                }
+            }
+
+            // Phase 2: stationary vertices against the push onto moving entities.
+            for (var v = 0; v < stationaryVertices.Length; v++)
+            {
+                var vx = stationaryVertices[v].X;
+                var vy = stationaryVertices[v].Y;
+
+                for (var j = 0; j < moving.Count; j++)
+                {
+                    SlideEvents.Ray(
+                        ref sink,
+                        vx,
+                        vy,
+                        moving[j],
+                        movingDx,
+                        movingDy,
+                        -dirX,
+                        -dirY,
+                        false
+                    );
+                    if (sink.IsDone)
+                        return;
+                }
+            }
+
+            // Phase 3: arc-to-line closest points, which vertex sampling can miss.
+            if (arcToLine)
+            {
+                SlideEvents.ArcToLine(
+                    ref sink,
+                    moving,
+                    movingDx,
+                    movingDy,
+                    stationary,
+                    0,
+                    0,
+                    dirX,
+                    dirY,
+                    true
+                );
+                if (sink.IsDone)
+                    return;
+                SlideEvents.ArcToLine(
+                    ref sink,
+                    stationary,
+                    0,
+                    0,
+                    moving,
+                    movingDx,
+                    movingDy,
+                    -dirX,
+                    -dirY,
+                    false
+                );
+                if (sink.IsDone)
+                    return;
+            }
+
+            // Phase 4: native curve tangency, including a convex corner inside a concave arc.
+            for (var i = 0; i < moving.Count; i++)
+            {
+                if (!SlideEvents.TryGetCurve(moving[i], out var mcx, out var mcy, out var mr))
+                    continue;
+
+                for (var j = 0; j < stationary.Count; j++)
+                {
+                    if (!SlideEvents.TryGetCurve(stationary[j], out var scx, out var scy, out var sr))
+                        continue;
+
+                    SlideEvents.CurveTangency(
+                        ref sink,
+                        mcx + movingDx,
+                        mcy + movingDy,
+                        mr,
+                        moving[i] as Arc,
+                        scx,
+                        scy,
+                        sr,
+                        stationary[j] as Arc,
+                        dirX,
+                        dirY
+                    );
+                    if (sink.IsDone)
+                        return;
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Slide events between line boundaries along an arbitrary unit direction. The moving
+    /// lines and vertices are translated by (movingDx, movingDy); vertex arrays may be subsets.
+    /// </summary>
+    public struct LineSlideEvents : ISlideEventSource
+    {
+        private readonly List<Line> moving;
+        private readonly Vector[] movingVertices;
+        private readonly double movingDx;
+        private readonly double movingDy;
+        private readonly List<Line> stationary;
+        private readonly Vector[] stationaryVertices;
+        private readonly double dirX;
+        private readonly double dirY;
+
+        public LineSlideEvents(
+            List<Line> moving,
+            Vector[] movingVertices,
+            double movingDx,
+            double movingDy,
+            List<Line> stationary,
+            Vector[] stationaryVertices,
+            double dirX,
+            double dirY
+        )
+        {
+            this.moving = moving;
+            this.movingVertices = movingVertices;
+            this.movingDx = movingDx;
+            this.movingDy = movingDy;
+            this.stationary = stationary;
+            this.stationaryVertices = stationaryVertices;
+            this.dirX = dirX;
+            this.dirY = dirY;
+        }
+
+        public void Enumerate<TSink>(ref TSink sink)
+            where TSink : struct, ISlideEventSink
+        {
+            for (var v = 0; v < movingVertices.Length; v++)
+            {
+                var vx = movingVertices[v].X + movingDx;
+                var vy = movingVertices[v].Y + movingDy;
+
+                for (var j = 0; j < stationary.Count; j++)
+                {
+                    var e = stationary[j];
+                    SlideEvents.RayLine(
+                        ref sink,
+                        vx,
+                        vy,
+                        e.pt1.X,
+                        e.pt1.Y,
+                        e.pt2.X,
+                        e.pt2.Y,
+                        dirX,
+                        dirY,
+                        true
+                    );
+                    if (sink.IsDone)
+                        return;
+                }
+            }
+
+            for (var v = 0; v < stationaryVertices.Length; v++)
+            {
+                var vx = stationaryVertices[v].X;
+                var vy = stationaryVertices[v].Y;
+
+                for (var j = 0; j < moving.Count; j++)
+                {
+                    var e = moving[j];
+                    SlideEvents.RayLine(
+                        ref sink,
+                        vx,
+                        vy,
+                        e.pt1.X + movingDx,
+                        e.pt1.Y + movingDy,
+                        e.pt2.X + movingDx,
+                        e.pt2.Y + movingDy,
+                        -dirX,
+                        -dirY,
+                        false
+                    );
+                    if (sink.IsDone)
+                        return;
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Axis-aligned slide events between edge arrays sorted for pruning, as used by the
+    /// <see cref="PushDirection"/> kernel. Offsets translate each side into world space.
+    /// </summary>
+    public struct AxisSlideEvents : ISlideEventSource
+    {
+        private readonly (Vector start, Vector end)[] movingEdges;
+        private readonly Vector movingOffset;
+        private readonly Vector[] movingVertices;
+        private readonly (Vector start, Vector end)[] stationaryEdges;
+        private readonly Vector stationaryOffset;
+        private readonly Vector[] stationaryVertices;
+        private readonly PushDirection direction;
+
+        /// <param name="movingVertices">World-space moving vertices.</param>
+        /// <param name="stationaryVertices">World-space stationary vertices.</param>
+        public AxisSlideEvents(
+            (Vector start, Vector end)[] movingEdges,
+            Vector movingOffset,
+            Vector[] movingVertices,
+            (Vector start, Vector end)[] stationaryEdges,
+            Vector stationaryOffset,
+            Vector[] stationaryVertices,
+            PushDirection direction
+        )
+        {
+            this.movingEdges = movingEdges;
+            this.movingOffset = movingOffset;
+            this.movingVertices = movingVertices;
+            this.stationaryEdges = stationaryEdges;
+            this.stationaryOffset = stationaryOffset;
+            this.stationaryVertices = stationaryVertices;
+            this.direction = direction;
+        }
+
+        public void Enumerate<TSink>(ref TSink sink)
+            where TSink : struct, ISlideEventSink
+        {
+            for (var v = 0; v < movingVertices.Length; v++)
+            {
+                OneWay(ref sink, movingVertices[v], stationaryEdges, stationaryOffset, direction, true);
+                if (sink.IsDone)
+                    return;
+            }
+
+            var opposite = SpatialQuery.OppositeDirection(direction);
+            for (var v = 0; v < stationaryVertices.Length; v++)
+            {
+                OneWay(ref sink, stationaryVertices[v], movingEdges, movingOffset, opposite, false);
+                if (sink.IsDone)
+                    return;
+            }
+        }
+
+        private static void OneWay<TSink>(
+            ref TSink sink,
+            Vector vertex,
+            (Vector start, Vector end)[] edges,
+            Vector edgeOffset,
+            PushDirection rayDirection,
+            bool vertexMoves
+        )
+            where TSink : struct, ISlideEventSink
+        {
+            var vx = vertex.X;
+            var vy = vertex.Y;
+            var horizontal = SpatialQuery.IsHorizontalDirection(rayDirection);
+
+            // Edges are sorted by their perpendicular min-coordinate.
+            for (var i = 0; i < edges.Length; i++)
+            {
+                var e1 = edges[i].start + edgeOffset;
+                var e2 = edges[i].end + edgeOffset;
+
+                double perpValue,
+                    edgeMin,
+                    edgeMax;
+                if (horizontal)
+                {
+                    perpValue = vy;
+                    edgeMin = e1.Y < e2.Y ? e1.Y : e2.Y;
+                    edgeMax = e1.Y > e2.Y ? e1.Y : e2.Y;
+                }
+                else
+                {
+                    perpValue = vx;
+                    edgeMin = e1.X < e2.X ? e1.X : e2.X;
+                    edgeMax = e1.X > e2.X ? e1.X : e2.X;
+                }
+
+                if (perpValue < edgeMin - Tolerance.Epsilon)
+                    break;
+
+                if (perpValue > edgeMax + Tolerance.Epsilon)
+                    continue;
+
+                SlideEvents.AxisRayLine(
+                    ref sink,
+                    vx,
+                    vy,
+                    e1.X,
+                    e1.Y,
+                    e2.X,
+                    e2.Y,
+                    rayDirection,
+                    vertexMoves
+                );
+                if (sink.IsDone)
+                    return;
+            }
+        }
+    }
+}

OpenNest.Core/Geometry/SpatialQuery.cs

@@ -320,8 +320,9 @@ namespace OpenNest.Geometry
         }
 
         /// <summary>
-        /// Computes the minimum translation distance along a push direction before
-        /// any edge of movingLines contacts any edge of stationaryLines.
+        /// Computes the translation distance along a push direction before any edge of
+        /// movingLines first blocks against an edge of stationaryLines. A contact that
+        /// the push slides along or leaves does not block (see <see cref="SlideContact"/>).
         /// Returns double.MaxValue if no collision path exists.
         /// </summary>
         public static double DirectionalDistance(
@@ -334,7 +335,7 @@ namespace OpenNest.Geometry
         }
 
         /// <summary>
-        /// Computes the minimum directional distance with the moving lines translated
+        /// Computes the directional distance with the moving lines translated
         /// by (movingDx, movingDy) without creating new Line objects.
         /// </summary>
         public static double DirectionalDistance(
@@ -345,37 +346,57 @@ namespace OpenNest.Geometry
             PushDirection direction
         )
         {
-            var minDist = double.MaxValue;
-            var movingOffset = new Vector(movingDx, movingDy);
+            return DirectionalDistance(
+                movingLines,
+                movingDx,
+                movingDy,
+                stationaryLines,
+                direction,
+                SlideContactClassifier.FromLines(
+                    movingLines,
+                    new Vector(movingDx, movingDy),
+                    stationaryLines,
+                    Vector.Zero
+                )
+            );
+        }
 
-            // Case 1: Each moving vertex -> each stationary edge
-            var movingVertices = CollectVertices(movingLines, movingOffset);
+        /// <summary>
+        /// <see cref="DirectionalDistance(List{Line}, double, double, List{Line}, PushDirection)"/>
+        /// with caller-supplied contact topology, for inputs that are not complete closed
+        /// boundaries (for example direction-filtered edges).
+        /// </summary>
+        public static double DirectionalDistance(
+            List<Line> movingLines,
+            double movingDx,
+            double movingDy,
+            List<Line> stationaryLines,
+            PushDirection direction,
+            SlideContactClassifier contacts
+        )
+        {
+            var movingOffset = new Vector(movingDx, movingDy);
+            var movingVertices = CollectVertices(movingLines, movingOffset).ToArray();
 
             var stationaryEdges = ToEdgeArray(stationaryLines);
             SortEdgesForPruning(stationaryEdges, direction);
 
-            foreach (var mv in movingVertices)
-            {
-                var d = OneWayDistance(mv, stationaryEdges, Vector.Zero, direction);
-                if (d < minDist)
-                    minDist = d;
-            }
-
-            // Case 2: Each stationary vertex -> each moving edge (opposite direction)
-            var opposite = OppositeDirection(direction);
-            var stationaryVertices = CollectVertices(stationaryLines, Vector.Zero);
+            var stationaryVertices = CollectVertices(stationaryLines, Vector.Zero).ToArray();
 
             var movingEdges = ToEdgeArray(movingLines);
-            SortEdgesForPruning(movingEdges, opposite);
+            SortEdgesForPruning(movingEdges, OppositeDirection(direction));
 
-            foreach (var sv in stationaryVertices)
-            {
-                var d = OneWayDistance(sv, movingEdges, movingOffset, opposite);
-                if (d < minDist)
-                    minDist = d;
-            }
-
-            return minDist;
+            var source = new AxisSlideEvents(
+                movingEdges,
+                movingOffset,
+                movingVertices,
+                stationaryEdges,
+                Vector.Zero,
+                stationaryVertices,
+                direction
+            );
+            var unit = DirectionToOffset(direction, 1.0);
+            return SlideResolver.FirstBlocking(ref source, contacts, unit.X, unit.Y);
         }
 
         /// <summary>
@@ -396,8 +417,8 @@ namespace OpenNest.Geometry
         }
 
         /// <summary>
-        /// Computes the minimum directional distance using raw edge arrays and location offsets
-        /// to avoid all intermediate object allocations.
+        /// Computes the blocking directional distance using raw edge arrays and location
+        /// offsets. Sorts both edge arrays in place for pruning.
         /// </summary>
         public static double DirectionalDistance(
             (Vector start, Vector end)[] movingEdges,
@@ -407,36 +428,58 @@ namespace OpenNest.Geometry
             PushDirection direction
         )
         {
-            var minDist = double.MaxValue;
-
-            SortEdgesForPruning(stationaryEdges, direction);
-
-            // Case 1: Each moving vertex -> each stationary edge
-            var movingVertices = CollectVertices(movingEdges, movingOffset);
-
-            foreach (var mv in movingVertices)
-            {
-                var d = OneWayDistance(mv, stationaryEdges, stationaryOffset, direction);
-                if (d < minDist)
-                    minDist = d;
-            }
-
-            // Case 2: Each stationary vertex -> each moving edge (opposite direction)
-            var opposite = OppositeDirection(direction);
-            SortEdgesForPruning(movingEdges, opposite);
-
-            var stationaryVertices = CollectVertices(stationaryEdges, stationaryOffset);
-
-            foreach (var sv in stationaryVertices)
-            {
-                var d = OneWayDistance(sv, movingEdges, movingOffset, opposite);
-                if (d < minDist)
-                    minDist = d;
-            }
-
-            return minDist;
+            return DirectionalDistance(
+                movingEdges,
+                movingOffset,
+                stationaryEdges,
+                stationaryOffset,
+                direction,
+                SlideContactClassifier.FromEdges(
+                    movingEdges,
+                    movingOffset,
+                    stationaryEdges,
+                    stationaryOffset
+                )
+            );
         }
 
+        /// <summary>
+        /// Edge-array overload with caller-supplied contact topology. The classifier's
+        /// origins must match <paramref name="movingOffset"/> and
+        /// <paramref name="stationaryOffset"/> in the frame of its boundaries.
+        /// </summary>
+        public static double DirectionalDistance(
+            (Vector start, Vector end)[] movingEdges,
+            Vector movingOffset,
+            (Vector start, Vector end)[] stationaryEdges,
+            Vector stationaryOffset,
+            PushDirection direction,
+            SlideContactClassifier contacts
+        )
+        {
+            SortEdgesForPruning(stationaryEdges, direction);
+            var movingVertices = CollectVertices(movingEdges, movingOffset).ToArray();
+
+            SortEdgesForPruning(movingEdges, OppositeDirection(direction));
+            var stationaryVertices = CollectVertices(stationaryEdges, stationaryOffset).ToArray();
+
+            var source = new AxisSlideEvents(
+                movingEdges,
+                movingOffset,
+                movingVertices,
+                stationaryEdges,
+                stationaryOffset,
+                stationaryVertices,
+                direction
+            );
+            var unit = DirectionToOffset(direction, 1.0);
+            return SlideResolver.FirstBlocking(ref source, contacts, unit.X, unit.Y);
+        }
+
+        /// <summary>
+        /// Nearest raw hit from one vertex along a push direction against edges sorted for
+        /// pruning. This is a first-touch primitive; it does not classify sliding contacts.
+        /// </summary>
         public static double OneWayDistance(
             Vector vertex,
             (Vector start, Vector end)[] edges,
@@ -628,8 +671,8 @@ namespace OpenNest.Geometry
         }
 
         /// <summary>
-        /// Computes the minimum translation distance along an arbitrary unit direction
-        /// before any edge of movingLines contacts any edge of stationaryLines.
+        /// Computes the translation distance along an arbitrary unit direction before any
+        /// edge of movingLines first blocks against an edge of stationaryLines.
         /// </summary>
         public static double DirectionalDistance(
             List<Line> movingLines,
@@ -637,58 +680,41 @@ namespace OpenNest.Geometry
             Vector direction
         )
         {
-            var minDist = double.MaxValue;
-            var dirX = direction.X;
-            var dirY = direction.Y;
+            return DirectionalDistance(
+                movingLines,
+                stationaryLines,
+                direction,
+                SlideContactClassifier.FromLines(
+                    movingLines,
+                    Vector.Zero,
+                    stationaryLines,
+                    Vector.Zero
+                )
+            );
+        }
 
-            var movingVertices = CollectVertices(movingLines, Vector.Zero);
-
-            foreach (var mv in movingVertices)
-            {
-                for (var i = 0; i < stationaryLines.Count; i++)
-                {
-                    var e = stationaryLines[i];
-                    var d = RayEdgeDistance(
-                        mv.X,
-                        mv.Y,
-                        e.pt1.X,
-                        e.pt1.Y,
-                        e.pt2.X,
-                        e.pt2.Y,
-                        dirX,
-                        dirY
-                    );
-                    if (d < minDist)
-                        minDist = d;
-                }
-            }
-
-            var oppX = -dirX;
-            var oppY = -dirY;
-
-            var stationaryVertices = CollectVertices(stationaryLines, Vector.Zero);
-
-            foreach (var sv in stationaryVertices)
-            {
-                for (var i = 0; i < movingLines.Count; i++)
-                {
-                    var e = movingLines[i];
-                    var d = RayEdgeDistance(
-                        sv.X,
-                        sv.Y,
-                        e.pt1.X,
-                        e.pt1.Y,
-                        e.pt2.X,
-                        e.pt2.Y,
-                        oppX,
-                        oppY
-                    );
-                    if (d < minDist)
-                        minDist = d;
-                }
-            }
-
-            return minDist;
+        /// <summary>
+        /// <see cref="DirectionalDistance(List{Line}, List{Line}, Vector)"/> with
+        /// caller-supplied contact topology.
+        /// </summary>
+        public static double DirectionalDistance(
+            List<Line> movingLines,
+            List<Line> stationaryLines,
+            Vector direction,
+            SlideContactClassifier contacts
+        )
+        {
+            var source = new LineSlideEvents(
+                movingLines,
+                CollectVertices(movingLines, Vector.Zero).ToArray(),
+                0,
+                0,
+                stationaryLines,
+                CollectVertices(stationaryLines, Vector.Zero).ToArray(),
+                direction.X,
+                direction.Y
+            );
+            return SlideResolver.FirstBlocking(ref source, contacts, direction.X, direction.Y);
         }
 
         /// <summary>
@@ -710,10 +736,10 @@ namespace OpenNest.Geometry
         }
 
         /// <summary>
-        /// Computes the minimum translation distance along an arbitrary unit direction
-        /// before any vertex/edge of movingEntities contacts any vertex/edge of
-        /// stationaryEntities. Works with native Line, Arc, and Circle entities
-        /// without tessellation.
+        /// Computes the translation distance along an arbitrary unit direction before any
+        /// vertex/edge of movingEntities first blocks against stationaryEntities. Works with
+        /// native Line, Arc, and Circle entities without tessellation. A contact that the
+        /// push slides along or leaves does not block (see <see cref="SlideContact"/>).
         /// </summary>
         public static double DirectionalDistance(
             List<Entity> movingEntities,
@@ -721,228 +747,42 @@ namespace OpenNest.Geometry
             Vector direction
         )
         {
-            var minDist = double.MaxValue;
-            var dirX = direction.X;
-            var dirY = direction.Y;
-
-            var movingVertices = ExtractEntityVertices(movingEntities);
-
-            for (var v = 0; v < movingVertices.Length; v++)
-            {
-                var vx = movingVertices[v].X;
-                var vy = movingVertices[v].Y;
-
-                for (var j = 0; j < stationaryEntities.Count; j++)
-                {
-                    var d = RayEntityDistance(vx, vy, stationaryEntities[j], dirX, dirY);
-                    if (d < minDist)
-                    {
-                        minDist = d;
-                        if (d <= 0)
-                            return 0;
-                    }
-                }
-            }
-
-            var oppX = -dirX;
-            var oppY = -dirY;
-
-            var stationaryVertices = ExtractEntityVertices(stationaryEntities);
-
-            for (var v = 0; v < stationaryVertices.Length; v++)
-            {
-                var vx = stationaryVertices[v].X;
-                var vy = stationaryVertices[v].Y;
-
-                for (var j = 0; j < movingEntities.Count; j++)
-                {
-                    var d = RayEntityDistance(vx, vy, movingEntities[j], oppX, oppY);
-                    if (d < minDist)
-                    {
-                        minDist = d;
-                        if (d <= 0)
-                            return 0;
-                    }
-                }
-            }
-
-            // Phase 3: Arc-to-line closest-point check.
-            // Phases 1-2 sample arc endpoints and cardinal extremes, but the actual
-            // closest point on a small corner arc to a straight edge may lie between
-            // those samples. Use ClosestPointTo to find it and fire a ray from there.
-            minDist = ArcToLineClosestDistance(
+            return DirectionalDistance(
                 movingEntities,
                 stationaryEntities,
-                dirX,
-                dirY,
-                minDist
+                direction,
+                new SlideContactClassifier(movingEntities, stationaryEntities)
             );
-            if (minDist <= 0)
-                return 0;
-            minDist = ArcToLineClosestDistance(
-                stationaryEntities,
+        }
+
+        /// <summary>
+        /// <see cref="DirectionalDistance(List{Entity}, List{Entity}, Vector)"/> with
+        /// caller-supplied contact topology.
+        /// </summary>
+        public static double DirectionalDistance(
+            List<Entity> movingEntities,
+            List<Entity> stationaryEntities,
+            Vector direction,
+            SlideContactClassifier contacts
+        )
+        {
+            // Phases: vertex rays both ways, arc-to-line closest points (vertex sampling
+            // misses interior arc contact), then native curve tangency.
+            var source = new EntitySlideEvents(
                 movingEntities,
-                oppX,
-                oppY,
-                minDist
+                ExtractEntityVertices(movingEntities),
+                0,
+                0,
+                stationaryEntities,
+                ExtractEntityVertices(stationaryEntities),
+                direction.X,
+                direction.Y,
+                arcToLine: true
             );
-            if (minDist <= 0)
-                return 0;
-
-            // Phase 4: Native curve tangency, including a convex corner inside a concave arc.
-            for (var i = 0; i < movingEntities.Count; i++)
-            {
-                var me = movingEntities[i];
-                if (!TryGetCurveParams(me, out var mcx, out var mcy, out var mr))
-                    continue;
-
-                for (var j = 0; j < stationaryEntities.Count; j++)
-                {
-                    var se = stationaryEntities[j];
-                    if (!TryGetCurveParams(se, out var scx, out var scy, out var sr))
-                        continue;
-
-                    var d = CurveTangencyDistance(
-                        mcx, mcy, mr, me as Arc,
-                        scx, scy, sr, se as Arc, dirX, dirY);
-                    if (d >= minDist)
-                        continue;
-
-                    minDist = d;
-                    if (d <= 0)
-                        return 0;
-                }
-            }
-
-            return minDist;
+            return SlideResolver.FirstBlocking(ref source, contacts, direction.X, direction.Y);
         }
 
-        private static double ArcToLineClosestDistance(
-            List<Entity> arcEntities,
-            List<Entity> lineEntities,
-            double dirX,
-            double dirY,
-            double minDist
-        )
-        {
-            for (var i = 0; i < arcEntities.Count; i++)
-            {
-                if (arcEntities[i] is not Arc arc)
-                    continue;
-
-                var cx = arc.Center.X;
-                var cy = arc.Center.Y;
-                var r = arc.Radius;
-
-                for (var j = 0; j < lineEntities.Count; j++)
-                {
-                    if (lineEntities[j] is not Line line)
-                        continue;
-
-                    var p1x = line.pt1.X;
-                    var p1y = line.pt1.Y;
-                    var ex = line.pt2.X - p1x;
-                    var ey = line.pt2.Y - p1y;
-
-                    var det = ex * dirY - ey * dirX;
-                    if (System.Math.Abs(det) < Tolerance.Epsilon)
-                        continue;
-
-                    // The directional distance from an arc point at angle θ to the
-                    // line is t(θ) = [A + r·(ey·cosθ − ex·sinθ)] / det.
-                    // dt/dθ = 0 at θ = atan2(−ex, ey) and θ + π.
-                    var theta1 = Angle.NormalizeRad(System.Math.Atan2(-ex, ey));
-                    var theta2 = Angle.NormalizeRad(theta1 + System.Math.PI);
-
-                    for (var k = 0; k < 2; k++)
-                    {
-                        var theta = k == 0 ? theta1 : theta2;
-
-                        if (
-                            !Angle.IsBetweenRad(theta, arc.StartAngle, arc.EndAngle, arc.IsReversed)
-                        )
-                            continue;
-
-                        var qx = cx + r * System.Math.Cos(theta);
-                        var qy = cy + r * System.Math.Sin(theta);
-
-                        var d = RayEdgeDistance(
-                            qx,
-                            qy,
-                            p1x,
-                            p1y,
-                            line.pt2.X,
-                            line.pt2.Y,
-                            dirX,
-                            dirY
-                        );
-                        if (d < minDist)
-                        {
-                            minDist = d;
-                            if (d <= 0)
-                                return 0;
-                        }
-                    }
-                }
-            }
-            return minDist;
-        }
-
-        private static double RayEntityDistance(
-            double vx,
-            double vy,
-            Entity entity,
-            double dirX,
-            double dirY
-        )
-        {
-            if (entity is Line line)
-            {
-                return RayEdgeDistance(
-                    vx,
-                    vy,
-                    line.pt1.X,
-                    line.pt1.Y,
-                    line.pt2.X,
-                    line.pt2.Y,
-                    dirX,
-                    dirY
-                );
-            }
-
-            if (entity is Arc arc)
-            {
-                return RayArcDistance(
-                    vx,
-                    vy,
-                    arc.Center.X,
-                    arc.Center.Y,
-                    arc.Radius,
-                    arc.StartAngle,
-                    arc.EndAngle,
-                    arc.IsReversed,
-                    dirX,
-                    dirY
-                );
-            }
-
-            if (entity is Circle circle)
-            {
-                return RayCircleDistance(
-                    vx,
-                    vy,
-                    circle.Center.X,
-                    circle.Center.Y,
-                    circle.Radius,
-                    dirX,
-                    dirY
-                );
-            }
-
-            return double.MaxValue;
-        }
-
-        private static Vector[] ExtractEntityVertices(List<Entity> entities)
+        public static Vector[] ExtractEntityVertices(List<Entity> entities)
         {
             var vertices = new HashSet<Vector>();
 
@@ -1041,31 +881,6 @@ namespace OpenNest.Geometry
                 );
         }
 
-        private static bool TryGetCurveParams(
-            Entity entity,
-            out double cx,
-            out double cy,
-            out double r
-        )
-        {
-            if (entity is Circle circle)
-            {
-                cx = circle.Center.X;
-                cy = circle.Center.Y;
-                r = circle.Radius;
-                return true;
-            }
-            if (entity is Arc arc)
-            {
-                cx = arc.Center.X;
-                cy = arc.Center.Y;
-                r = arc.Radius;
-                return true;
-            }
-            cx = cy = r = 0;
-            return false;
-        }
-
         private static double BoxProjectionMin(Box box, double dx, double dy)
         {
             var x = dx >= 0 ? box.Left : box.Right;

OpenNest.Data/NestDefaults.cs

@@ -0,0 +1,270 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using OpenNest.Geometry;
+
+namespace OpenNest.Data;
+
+/// <summary>Outcome of <see cref="NestDefaults.Load(string, out NestDefaultsStatus)"/>.</summary>
+public enum NestDefaultsStatus
+{
+    /// <summary>Defaults were read from the file (invalid fields still fall back individually).</summary>
+    Ok,
+
+    /// <summary>No file exists at the path; built-in fallback values were used.</summary>
+    Missing,
+
+    /// <summary>The file exists but could not be read or parsed; fallback values were used.</summary>
+    Invalid,
+}
+
+/// <summary>
+/// Plate/nest defaults persisted to a single JSON file
+/// (by default %APPDATA%\OpenNest\defaults.json), replacing the
+/// .nstdot nest-template mechanism. Loading never throws: a missing,
+/// corrupt, or partially valid file degrades field-by-field to
+/// <see cref="Fallback"/> values so creating a new nest is never blocked.
+/// </summary>
+public sealed class NestDefaults
+{
+    public const int CurrentVersion = 1;
+
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        PropertyNameCaseInsensitive = true,
+        Converters = { new JsonStringEnumConverter(JsonNamingPolicy.CamelCase) },
+    };
+
+    public Units Units { get; set; } = Units.Inches;
+
+    public Size Size { get; set; } = new(100, 100);
+
+    public int Quadrant { get; set; } = 1;
+
+    public double PartSpacing { get; set; } = 1;
+
+    public Spacing EdgeSpacing { get; set; } = new(1, 1, 1, 1);
+
+    /// <summary>
+    /// The built-in defaults used when no file exists and for every field
+    /// that is missing or invalid. Matches the historical
+    /// MainForm.CreateDefaultNest values (units default to Inches; callers
+    /// may override from their own settings).
+    /// </summary>
+    public static NestDefaults Fallback => new();
+
+    /// <summary>%APPDATA%\OpenNest\defaults.json.</summary>
+    public static string DefaultPath =>
+        Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+            "OpenNest",
+            "defaults.json"
+        );
+
+    /// <summary>
+    /// Loads defaults from <paramref name="path"/>, falling back field by
+    /// field for a missing file, invalid JSON, or invalid values.
+    /// </summary>
+    public static NestDefaults Load(string path) => Load(path, out _);
+
+    /// <summary>
+    /// Loads defaults and reports whether the file was missing, loaded, or
+    /// present but unreadable/invalid, so callers can warn about a corrupt
+    /// file while still returning usable values.
+    /// </summary>
+    public static NestDefaults Load(string path, out NestDefaultsStatus status)
+    {
+        var defaults = Fallback;
+        if (string.IsNullOrWhiteSpace(path) || !File.Exists(path))
+        {
+            status = NestDefaultsStatus.Missing;
+            return defaults;
+        }
+
+        NestDefaultsDto? dto;
+        try
+        {
+            var json = File.ReadAllText(path);
+            dto = JsonSerializer.Deserialize<NestDefaultsDto>(json, JsonOptions);
+        }
+        catch (JsonException)
+        {
+            status = NestDefaultsStatus.Invalid;
+            return defaults;
+        }
+        catch (IOException)
+        {
+            status = NestDefaultsStatus.Invalid;
+            return defaults;
+        }
+
+        if (dto is null)
+        {
+            status = NestDefaultsStatus.Invalid;
+            return defaults;
+        }
+
+        status = NestDefaultsStatus.Ok;
+
+        if (
+            dto.Units is not null
+            && Enum.TryParse<Units>(dto.Units, ignoreCase: true, out var units)
+        )
+            defaults.Units = units;
+
+        if (
+            dto.Size?.Width is { } width
+            && dto.Size.Length is { } length
+            && IsValidSize(width, length)
+        )
+            defaults.Size = new Size(width, length);
+
+        if (dto.Quadrant is { } quadrant && quadrant is >= 1 and <= 4)
+            defaults.Quadrant = quadrant;
+
+        if (dto.PartSpacing is { } partSpacing && IsValidSpacing(partSpacing))
+            defaults.PartSpacing = partSpacing;
+
+        if (
+            dto.EdgeSpacing?.Left is { } left
+            && dto.EdgeSpacing.Bottom is { } bottom
+            && dto.EdgeSpacing.Right is { } right
+            && dto.EdgeSpacing.Top is { } top
+            && IsValidSpacing(left)
+            && IsValidSpacing(bottom)
+            && IsValidSpacing(right)
+            && IsValidSpacing(top)
+        )
+            defaults.EdgeSpacing = new Spacing(left, bottom, right, top);
+
+        return defaults;
+    }
+
+    /// <summary>
+    /// Captures the current units and plate defaults from a nest.
+    /// </summary>
+    public static NestDefaults FromNest(Nest nest)
+    {
+        ArgumentNullException.ThrowIfNull(nest);
+        var plate = nest.PlateDefaults;
+        return new NestDefaults
+        {
+            Units = nest.Units,
+            Size = plate.Size,
+            Quadrant = plate.Quadrant,
+            PartSpacing = plate.PartSpacing,
+            EdgeSpacing = plate.EdgeSpacing,
+        };
+    }
+
+    /// <summary>
+    /// Captures defaults from an existing plate (a copy of its size,
+    /// quadrant, and spacing), e.g. the active plate in the desktop app.
+    /// </summary>
+    public static NestDefaults FromPlate(Units units, Plate plate)
+    {
+        ArgumentNullException.ThrowIfNull(plate);
+        return new NestDefaults
+        {
+            Units = units,
+            Size = plate.Size,
+            Quadrant = plate.Quadrant,
+            PartSpacing = plate.PartSpacing,
+            EdgeSpacing = plate.EdgeSpacing,
+        };
+    }
+
+    public void ApplyTo(Nest nest)
+    {
+        ArgumentNullException.ThrowIfNull(nest);
+        nest.Units = Units;
+        var plate = nest.PlateDefaults;
+        plate.Size = Size;
+        plate.Quadrant = Quadrant;
+        plate.PartSpacing = PartSpacing;
+        plate.EdgeSpacing = EdgeSpacing;
+    }
+
+    /// <summary>
+    /// Writes the file (creating the parent directory), retrying briefly on
+    /// IO collisions the same way <see cref="LocalJsonProvider"/> does.
+    /// </summary>
+    public void Save(string path, int maxRetries = 3)
+    {
+        var dto = new NestDefaultsDto
+        {
+            Version = CurrentVersion,
+            Units = Units.ToString().ToLowerInvariant(),
+            Size = new SizeDto { Width = Size.Width, Length = Size.Length },
+            Quadrant = Quadrant,
+            PartSpacing = PartSpacing,
+            EdgeSpacing = new SpacingDto
+            {
+                Left = EdgeSpacing.Left,
+                Bottom = EdgeSpacing.Bottom,
+                Right = EdgeSpacing.Right,
+                Top = EdgeSpacing.Top,
+            },
+        };
+
+        var json = JsonSerializer.Serialize(dto, JsonOptions);
+
+        var directory = Path.GetDirectoryName(Path.GetFullPath(path));
+        if (!string.IsNullOrEmpty(directory))
+            Directory.CreateDirectory(directory);
+
+        for (var attempt = 0; attempt < maxRetries; attempt++)
+        {
+            try
+            {
+                File.WriteAllText(path, json);
+                return;
+            }
+            catch (IOException) when (attempt < maxRetries - 1)
+            {
+                Thread.Sleep(100);
+            }
+        }
+    }
+
+    private static bool IsValidSize(double width, double length) =>
+        !double.IsNaN(width)
+            && !double.IsNaN(length)
+            && !double.IsInfinity(width)
+            && !double.IsInfinity(length)
+            && width > 0
+            && length > 0;
+
+    private static bool IsValidSpacing(double value) =>
+        !double.IsNaN(value) && !double.IsInfinity(value) && value >= 0;
+
+    /// <summary>
+    /// Wire format. Every field is nullable so a partial file merges over
+    /// the fallback field by field; unknown fields (including a future
+    /// higher <c>version</c>) are ignored rather than rejected.
+    /// </summary>
+    private sealed record NestDefaultsDto
+    {
+        public int? Version { get; init; } = CurrentVersion;
+        public string? Units { get; init; }
+        public SizeDto? Size { get; init; }
+        public int? Quadrant { get; init; }
+        public double? PartSpacing { get; init; }
+        public SpacingDto? EdgeSpacing { get; init; }
+    }
+
+    private sealed record SizeDto
+    {
+        public double? Width { get; init; }
+        public double? Length { get; init; }
+    }
+
+    private sealed record SpacingDto
+    {
+        public double? Left { get; init; }
+        public double? Bottom { get; init; }
+        public double? Right { get; init; }
+        public double? Top { get; init; }
+    }
+}

OpenNest.Engine.Tests/Fill/ExpanderTests.cs

@@ -0,0 +1,347 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using OpenNest.CNC;
+using OpenNest.Engine.Fill;
+using OpenNest.Geometry;
+using Xunit;
+
+namespace OpenNest.Engine.Tests.Fill;
+
+/// <summary>
+/// PlateView spacing expander: grows part-to-part spacing with the work area
+/// and non-selected parts as hard boundaries.
+/// </summary>
+public class ExpanderTests
+{
+    private static Program Rectangle(double width = 4, double length = 4)
+    {
+        var program = new Program();
+        program.MoveTo(0, 0);
+        program.LineTo(width, 0);
+        program.LineTo(width, length);
+        program.LineTo(0, length);
+        program.LineTo(0, 0);
+        return program;
+    }
+
+    private static Part AddSquare(Plate plate, double x, double y, double size = 4)
+    {
+        var part = new Part(new Drawing($"sq{plate.Parts.Count}", Rectangle(size, size)), new Vector(x, y));
+        plate.Parts.Add(part);
+        return part;
+    }
+
+    private static Plate MakePlate(double lengthX, double widthY, double edge = 0.5)
+    {
+        var plate = new Plate(new Size(widthY, lengthX));
+        plate.EdgeSpacing = new Spacing(edge, edge);
+        return plate;
+    }
+
+    /// <summary>Independent clearance oracle: naive vertex/segment min distance over raw part lines.</summary>
+    private static double BruteClearance(Part a, Part b)
+    {
+        var linesA = PartGeometry.GetPartLines(a);
+        var linesB = PartGeometry.GetPartLines(b);
+
+        double min = double.MaxValue;
+        foreach (var la in linesA)
+            foreach (var lb in linesB)
+            {
+                min = System.Math.Min(min, PointSegment(a, la.StartPoint, lb));
+                min = System.Math.Min(min, PointSegment(a, la.EndPoint, lb));
+                min = System.Math.Min(min, PointSegment(b, lb.StartPoint, la));
+                min = System.Math.Min(min, PointSegment(b, lb.EndPoint, la));
+            }
+        return min;
+    }
+
+    private static double PointSegment(Part owner, Vector pt, Line seg)
+    {
+        var d = seg.EndPoint - seg.StartPoint;
+        var len2 = d.DotProduct(d);
+        var t = len2 <= 1e-12 ? 0 : System.Math.Clamp((pt - seg.StartPoint).DotProduct(d) / len2, 0, 1);
+        return pt.DistanceTo(seg.StartPoint + d * t);
+    }
+
+    private static void AssertNoOverlaps(Plate plate)
+    {
+        for (var i = 0; i < plate.Parts.Count; i++)
+            for (var j = i + 1; j < plate.Parts.Count; j++)
+                Assert.False(
+                    plate.Parts[i].Intersects(plate.Parts[j], out _),
+                    $"{plate.Parts[i].BaseDrawing.Name} overlaps {plate.Parts[j].BaseDrawing.Name}"
+                );
+    }
+
+    [Fact]
+    public void Expand_TwoSquares_GrowUntilEdgeFloor_AndAnchorStaysPut()
+    {
+        var plate = MakePlate(24, 24);
+        var a = AddSquare(plate, 6, 10);
+        var b = AddSquare(plate, 14, 10);
+
+        var result = Expander.Expand(new List<Part> { a, b }, plate);
+
+        // Max gap: B flush against the right edge floor (23.5): 23.5 - 14 - 4 + gap base...
+        // A stays (anchor); B slides to x=19.5 -> gap 9.5.
+        Assert.Equal(6, a.Location.X, 6);
+        Assert.Equal(10, a.Location.Y, 6);
+        Assert.Equal(9.5, b.Location.X - (a.Location.X + 4), 1);
+        Assert.True(result.AchievedSpacing >= 9.4, $"achieved {result.AchievedSpacing}");
+        Assert.True(result.AchievedSpacing <= 9.6, $"achieved {result.AchievedSpacing}");
+        AssertNoOverlaps(plate);
+        Assert.True(b.BoundingBox.Right <= 23.5 + 1e-6);
+    }
+
+    [Fact]
+    public void Expand_SandwichedBetweenWalls_ConvergesOnlyToInitialGaps_AndKeepsWalls()
+    {
+        var plate = MakePlate(26, 10, edge: 0.0);
+        var wallL = AddSquare(plate, 0, 3);
+        var wallR = AddSquare(plate, 18, 3);
+        var a = AddSquare(plate, 6, 3);
+        var b = AddSquare(plate, 12, 3);
+
+        var result = Expander.Expand(new List<Part> { a, b }, plate);
+
+        // Every gap starts at exactly 2.0; straight separation moves cannot open
+        // the row (opening one gap costs another), so the run stays at ~2.0.
+        Assert.Equal(0, wallL.Location.X, 6);
+        Assert.Equal(18, wallR.Location.X, 6);
+        Assert.True(
+            result.AchievedSpacing >= 1.9 && result.AchievedSpacing <= 2.05,
+            $"achieved {result.AchievedSpacing}"
+        );
+        AssertNoOverlaps(plate);
+    }
+
+    [Fact]
+    public void Expand_OverlappingPair_SeparatesAndClearsOverlap()
+    {
+        var plate = MakePlate(30, 12);
+        var a = AddSquare(plate, 5, 4);
+        var b = AddSquare(plate, 7, 4); // 2.0 overlap in X
+
+        var result = Expander.Expand(new List<Part> { a, b }, plate, new Expander.Options
+        {
+            InitialStep = 0.5,
+            MaxSpacing = 3,
+        });
+
+        Assert.False(a.Intersects(b, out _));
+        var gap = b.Location.X - (a.Location.X + 4);
+        Assert.True(gap >= 2.99, $"gap {gap}");
+        Assert.True(result.AchievedSpacing >= 2.9);
+        AssertNoOverlaps(plate);
+    }
+
+    [Fact]
+    public void Separate_PinnedPart_ReportsViolationsWithoutOverlap()
+    {
+        var plate = MakePlate(20, 20, edge: 0.0);
+        var pinned = AddSquare(plate, 8, 8);
+        // Walls 0.2 clear on all four sides.
+        var left = AddSquare(plate, 3.8, 8);
+        var right = AddSquare(plate, 12.2, 8);
+        var bottom = AddSquare(plate, 8, 3.8);
+        var top = AddSquare(plate, 8, 12.2);
+
+        var (converged, positions, violations) = Expander.Separate(
+            new List<Part> { pinned },
+            plate,
+            spacing: 1.0
+        );
+
+        Assert.False(converged);
+        Assert.NotEmpty(violations);
+        // The pinned part may slide into the walls but never through them.
+        AssertNoOverlaps(plate);
+        Assert.Equal(3.8, left.Location.X, 6);
+        Assert.Equal(12.2, right.Location.X, 6);
+        Assert.Equal(3.8, bottom.Location.Y, 6);
+        Assert.Equal(12.2, top.Location.Y, 6);
+    }
+
+    [Fact]
+    public void Expand_CancelledBeforeRun_LeavesEverythingInPlace()
+    {
+        var plate = MakePlate(24, 24);
+        var a = AddSquare(plate, 6, 10);
+        var b = AddSquare(plate, 14, 10);
+        using var cts = new CancellationTokenSource();
+        cts.Cancel();
+
+        var result = Expander.Expand(
+            new List<Part> { a, b },
+            plate,
+            token: cts.Token
+        );
+
+        Assert.True(result.Cancelled);
+        Assert.Equal(6, a.Location.X, 6);
+        Assert.Equal(14, b.Location.X, 6);
+    }
+
+    [Fact]
+    public void Expand_ThreeInRow_FirstSelectedNeverMoves_AndOracleConfirmsSpacing()
+    {
+        var plate = MakePlate(60, 14);
+        var a = AddSquare(plate, 5, 5);
+        var b = AddSquare(plate, 10, 5);
+        var c = AddSquare(plate, 15, 5);
+
+        var result = Expander.Expand(
+            new List<Part> { a, b, c },
+            plate,
+            new Expander.Options { MaxSpacing = 8 }
+        );
+
+        Assert.Equal(5, a.Location.X, 6); // anchor: never the later index of any pair
+        Assert.True(result.AchievedSpacing >= 7.9);
+
+        // Independent oracle: every pair clears the reported spacing.
+        var parts = new List<Part> { a, b, c };
+        for (var i = 0; i < parts.Count; i++)
+            for (var j = i + 1; j < parts.Count; j++)
+            {
+                var clearance = BruteClearance(parts[i], parts[j]);
+                Assert.True(
+                    clearance >= result.AchievedSpacing - 0.01,
+                    $"{parts[i].BaseDrawing.Name}/{parts[j].BaseDrawing.Name}: oracle {clearance} < reported {result.AchievedSpacing}"
+                );
+            }
+        AssertNoOverlaps(plate);
+    }
+
+    private static Program RectangleWithHole(
+        double width,
+        double length,
+        double hx,
+        double hy,
+        double hw,
+        double hh
+    )
+    {
+        var program = Rectangle(width, length);
+        program.MoveTo(hx, hy);
+        program.LineTo(hx + hw, hy);
+        program.LineTo(hx + hw, hy + hh);
+        program.LineTo(hx, hy + hh);
+        program.LineTo(hx, hy);
+        return program;
+    }
+
+    /// <summary>
+    /// Hole-subtracting overlap check matching NestValidator semantics (a part in
+    /// a cutout is legal). Part.Intersects is perimeter-only, so it cannot
+    /// certify part-in-cutout layouts.
+    /// </summary>
+    private static bool MateriallyOverlaps(Part a, Part b)
+    {
+        var (outerA, holesA) = Rings(a);
+        var (outerB, holesB) = Rings(b);
+        return Collision.HasOverlap(outerA, outerB, holesA, holesB);
+    }
+
+    private static (Polygon Outer, List<Polygon> Holes) Rings(Part part)
+    {
+        var entities = OpenNest.Converters.ConvertProgram
+            .ToGeometry(part.Program)
+            .Where(e => SpecialLayers.IsMaterial(e.Layer))
+            .ToList();
+        var profile = new ShapeProfile(entities);
+
+        var outer = profile.Perimeter.ToPolygonWithTolerance(0.001);
+        outer.Offset(part.Location);
+
+        var holes = new List<Polygon>();
+        foreach (var cutout in profile.Cutouts)
+        {
+            var hole = cutout.ToPolygonWithTolerance(0.001);
+            hole.Offset(part.Location);
+            holes.Add(hole);
+        }
+
+        return (outer, holes.Count == 0 ? null : holes);
+    }
+
+    [Fact]
+    public void Expand_PartInsideCutout_KeepsLegalAndClearsHoleWalls()
+    {
+        var plate = MakePlate(40, 24);
+
+        // Wall part with a 10x10 cutout; a small selected part sits inside it.
+        var wall = new Part(
+            new Drawing(
+                "wall",
+                RectangleWithHole(20, 20, 5, 5, 10, 10)
+            ),
+            new Vector(0, 0)
+        );
+        plate.Parts.Add(wall);
+
+        var inside = new Part(new Drawing("inside", Rectangle(2, 2)), new Vector(9, 9));
+        var other = new Part(new Drawing("other", Rectangle(2, 2)), new Vector(30, 9));
+        plate.Parts.Add(inside);
+        plate.Parts.Add(other);
+
+        var result = Expander.Expand(
+            new List<Part> { inside, other },
+            plate,
+            new Expander.Options { MaxSpacing = 2 }
+        );
+
+        // Part-in-cutout is legal, never a material overlap.
+        Assert.False(MateriallyOverlaps(inside, wall));
+        Assert.False(MateriallyOverlaps(other, wall));
+        Assert.False(MateriallyOverlaps(inside, other));
+        Assert.True(result.AchievedSpacing >= 1.9);
+
+        // The part that started in the cutout must clear the hole walls too.
+        var holeLeft = 5;
+        var holeRight = 15;
+        var gapLeft = inside.Location.X - holeLeft;
+        var gapRight = holeRight - (inside.Location.X + 2);
+        var gapBottom = inside.Location.Y - holeLeft;
+        var gapTop = holeRight - (inside.Location.Y + 2);
+        var minGap = System.Math.Min(
+            System.Math.Min(gapLeft, gapRight),
+            System.Math.Min(gapBottom, gapTop)
+        );
+        Assert.True(minGap >= 1.9, $"closest hole-wall gap {minGap}");
+    }
+
+    [Fact]
+    public void Expand_DegenerateInputs_Throw()
+    {
+        var plate = MakePlate(10, 10);
+        var a = AddSquare(plate, 1, 1);
+        var stranger = new Part(new Drawing("stranger", Rectangle()), new Vector(50, 50));
+
+        Assert.Throws<ArgumentException>(() => Expander.Expand(new List<Part>(), plate));
+        Assert.Throws<ArgumentException>(() => Expander.Expand(new List<Part> { a }, plate));
+        Assert.Throws<ArgumentException>(() => Expander.Expand(new List<Part> { a, stranger }, plate));
+        Assert.Throws<ArgumentNullException>(() => Expander.Expand(new List<Part> { a, a }, null));
+    }
+
+    [Fact]
+    public void Expand_WallsAndUnselectedPairs_StayExactlyAtClearance()
+    {
+        // Selection must not be pushed to open gaps between parts it excludes.
+        var plate = MakePlate(40, 12);
+        var w1 = AddSquare(plate, 2, 4);
+        var w2 = AddSquare(plate, 6.5, 4); // 0.5 apart from w1, both unselected
+        var a = AddSquare(plate, 14, 4);
+        var b = AddSquare(plate, 20, 4);
+
+        Expander.Expand(new List<Part> { a, b }, plate);
+
+        Assert.Equal(2, w1.Location.X, 6);
+        Assert.Equal(6.5, w2.Location.X, 6);
+        Assert.True(a.Intersects(w1, out _) == false);
+        Assert.True(b.Intersects(w2, out _) == false);
+    }
+}

OpenNest.Engine/BestFit/CpuDistanceComputer.cs

@@ -1,7 +1,6 @@
 using System.Collections.Generic;
 using System.Linq;
 using OpenNest.Geometry;
-using OpenNest.Math;
 
 namespace OpenNest.Engine.BestFit
 {
@@ -13,117 +12,21 @@ namespace OpenNest.Engine.BestFit
             SlideOffset[] offsets
         )
         {
-            var count = offsets.Length;
-            var results = new double[count];
+            var results = new double[offsets.Length];
+            var movingVertices = Vertices(movingTemplateLines);
+            var stationaryVertices = Vertices(stationaryLines);
+            var contacts = SlideContactClassifier.FromLines(
+                movingTemplateLines, Vector.Zero, stationaryLines, Vector.Zero).Prepare();
 
-            var allMovingVerts = ExtractUniqueVertices(movingTemplateLines);
-            var allStationaryVerts = ExtractUniqueVertices(stationaryLines);
-
-            var vertexCache =
-                new Dictionary<(double, double), (Vector[] leading, Vector[] facing)>();
-
-            foreach (var offset in offsets)
+            System.Threading.Tasks.Parallel.For(0, offsets.Length, i =>
             {
-                var key = (offset.DirX, offset.DirY);
-                if (vertexCache.ContainsKey(key))
-                    continue;
-
-                var leading = FilterVerticesByProjection(
-                    allMovingVerts,
-                    offset.DirX,
-                    offset.DirY,
-                    keepHigh: true
-                );
-                var facing = FilterVerticesByProjection(
-                    allStationaryVerts,
-                    offset.DirX,
-                    offset.DirY,
-                    keepHigh: false
-                );
-                vertexCache[key] = (leading, facing);
-            }
-
-            System.Threading.Tasks.Parallel.For(
-                0,
-                count,
-                i =>
-                {
-                    var offset = offsets[i];
-                    var dirX = offset.DirX;
-                    var dirY = offset.DirY;
-                    var oppX = -dirX;
-                    var oppY = -dirY;
-
-                    var (leadingMoving, facingStationary) = vertexCache[(dirX, dirY)];
-
-                    var minDist = double.MaxValue;
-
-                    for (var v = 0; v < leadingMoving.Length; v++)
-                    {
-                        var vx = leadingMoving[v].X + offset.Dx;
-                        var vy = leadingMoving[v].Y + offset.Dy;
-
-                        for (var j = 0; j < stationaryLines.Count; j++)
-                        {
-                            var e = stationaryLines[j];
-                            var d = SpatialQuery.RayEdgeDistance(
-                                vx,
-                                vy,
-                                e.StartPoint.X,
-                                e.StartPoint.Y,
-                                e.EndPoint.X,
-                                e.EndPoint.Y,
-                                dirX,
-                                dirY
-                            );
-
-                            if (d < minDist)
-                            {
-                                minDist = d;
-                                if (d <= 0)
-                                {
-                                    results[i] = 0;
-                                    return;
-                                }
-                            }
-                        }
-                    }
-
-                    for (var v = 0; v < facingStationary.Length; v++)
-                    {
-                        var svx = facingStationary[v].X;
-                        var svy = facingStationary[v].Y;
-
-                        for (var j = 0; j < movingTemplateLines.Count; j++)
-                        {
-                            var e = movingTemplateLines[j];
-                            var d = SpatialQuery.RayEdgeDistance(
-                                svx,
-                                svy,
-                                e.StartPoint.X + offset.Dx,
-                                e.StartPoint.Y + offset.Dy,
-                                e.EndPoint.X + offset.Dx,
-                                e.EndPoint.Y + offset.Dy,
-                                oppX,
-                                oppY
-                            );
-
-                            if (d < minDist)
-                            {
-                                minDist = d;
-                                if (d <= 0)
-                                {
-                                    results[i] = 0;
-                                    return;
-                                }
-                            }
-                        }
-                    }
-
-                    results[i] = minDist;
-                }
-            );
-
+                var offset = offsets[i];
+                var source = new LineSlideEvents(
+                    movingTemplateLines, movingVertices, offset.Dx, offset.Dy,
+                    stationaryLines, stationaryVertices, offset.DirX, offset.DirY);
+                results[i] = SlideResolver.FirstBlocking(ref source,
+                    contacts.At(new Vector(offset.Dx, offset.Dy), Vector.Zero), offset.DirX, offset.DirY);
+            });
             return results;
         }
 
@@ -133,347 +36,26 @@ namespace OpenNest.Engine.BestFit
             SlideOffset[] offsets
         )
         {
-            var count = offsets.Length;
-            var results = new double[count];
+            var results = new double[offsets.Length];
+            var movingVertices = SpatialQuery.ExtractEntityVertices(movingEntities);
+            var stationaryVertices = SpatialQuery.ExtractEntityVertices(stationaryEntities);
+            var contacts = new SlideContactClassifier(movingEntities, stationaryEntities).Prepare();
 
-            var allMovingVerts = ExtractVerticesFromEntities(movingEntities);
-            var allStationaryVerts = ExtractVerticesFromEntities(stationaryEntities);
-
-            var movingCurves = ExtractCurveParams(movingEntities);
-            var stationaryCurves = ExtractCurveParams(stationaryEntities);
-
-            var vertexCache =
-                new Dictionary<(double, double), (Vector[] leading, Vector[] facing)>();
-
-            foreach (var offset in offsets)
+            // All vertices participate: a leading-half filter can miss the next contact
+            // after sliding past an initial touch on a concave boundary.
+            System.Threading.Tasks.Parallel.For(0, offsets.Length, i =>
             {
-                var key = (offset.DirX, offset.DirY);
-                if (vertexCache.ContainsKey(key))
-                    continue;
-
-                var leading = FilterVerticesByProjection(
-                    allMovingVerts,
-                    offset.DirX,
-                    offset.DirY,
-                    keepHigh: true
-                );
-                var facing = FilterVerticesByProjection(
-                    allStationaryVerts,
-                    offset.DirX,
-                    offset.DirY,
-                    keepHigh: false
-                );
-                vertexCache[key] = (leading, facing);
-            }
-
-            System.Threading.Tasks.Parallel.For(
-                0,
-                count,
-                i =>
-                {
-                    var offset = offsets[i];
-                    var dirX = offset.DirX;
-                    var dirY = offset.DirY;
-                    var oppX = -dirX;
-                    var oppY = -dirY;
-
-                    var (leadingMoving, facingStationary) = vertexCache[(dirX, dirY)];
-
-                    var minDist = double.MaxValue;
-
-                    // Case 1: Leading moving vertices → stationary entities
-                    for (var v = 0; v < leadingMoving.Length; v++)
-                    {
-                        var vx = leadingMoving[v].X + offset.Dx;
-                        var vy = leadingMoving[v].Y + offset.Dy;
-
-                        for (var j = 0; j < stationaryEntities.Count; j++)
-                        {
-                            var d = RayEntityDistance(
-                                vx,
-                                vy,
-                                stationaryEntities[j],
-                                0,
-                                0,
-                                dirX,
-                                dirY
-                            );
-
-                            if (d < minDist)
-                            {
-                                minDist = d;
-                                if (d <= 0)
-                                {
-                                    results[i] = 0;
-                                    return;
-                                }
-                            }
-                        }
-                    }
-
-                    // Case 2: Facing stationary vertices → moving entities (opposite direction)
-                    for (var v = 0; v < facingStationary.Length; v++)
-                    {
-                        var svx = facingStationary[v].X;
-                        var svy = facingStationary[v].Y;
-
-                        for (var j = 0; j < movingEntities.Count; j++)
-                        {
-                            var d = RayEntityDistance(
-                                svx,
-                                svy,
-                                movingEntities[j],
-                                offset.Dx,
-                                offset.Dy,
-                                oppX,
-                                oppY
-                            );
-
-                            if (d < minDist)
-                            {
-                                minDist = d;
-                                if (d <= 0)
-                                {
-                                    results[i] = 0;
-                                    return;
-                                }
-                            }
-                        }
-                    }
-
-                    // Phase 3: Curve-to-curve direct distance.
-                    // Vertex sampling misses the true contact between two curved entities
-                    // when the approach angle doesn't align with a sampled vertex.
-                    for (var m = 0; m < movingCurves.Length; m++)
-                    {
-                        var mc = movingCurves[m];
-                        var mcx = mc.Cx + offset.Dx;
-                        var mcy = mc.Cy + offset.Dy;
-
-                        for (var s = 0; s < stationaryCurves.Length; s++)
-                        {
-                            var sc = stationaryCurves[s];
-                            var d = SpatialQuery.CurveTangencyDistance(
-                                mcx, mcy, mc.Radius, mc.Entity as Arc,
-                                sc.Cx, sc.Cy, sc.Radius, sc.Entity as Arc, dirX, dirY);
-
-                            if (d >= minDist)
-                                continue;
-
-                            minDist = d;
-                            if (d <= 0)
-                            {
-                                results[i] = 0;
-                                return;
-                            }
-                        }
-                    }
-
-                    results[i] = minDist;
-                }
-            );
-
+                var offset = offsets[i];
+                var source = new EntitySlideEvents(
+                    movingEntities, movingVertices, offset.Dx, offset.Dy,
+                    stationaryEntities, stationaryVertices, offset.DirX, offset.DirY, arcToLine: true);
+                results[i] = SlideResolver.FirstBlocking(ref source,
+                    contacts.At(new Vector(offset.Dx, offset.Dy), Vector.Zero), offset.DirX, offset.DirY);
+            });
             return results;
         }
 
-        private readonly struct CurveParams
-        {
-            public readonly Entity Entity;
-            public readonly double Cx,
-                Cy,
-                Radius;
-
-            public CurveParams(Entity entity, double cx, double cy, double radius)
-            {
-                Entity = entity;
-                Cx = cx;
-                Cy = cy;
-                Radius = radius;
-            }
-        }
-
-        private static CurveParams[] ExtractCurveParams(List<Entity> entities)
-        {
-            var curves = new List<CurveParams>();
-            for (var i = 0; i < entities.Count; i++)
-            {
-                if (entities[i] is Circle circle)
-                    curves.Add(
-                        new CurveParams(circle, circle.Center.X, circle.Center.Y, circle.Radius)
-                    );
-                else if (entities[i] is Arc arc)
-                    curves.Add(new CurveParams(arc, arc.Center.X, arc.Center.Y, arc.Radius));
-            }
-            return curves.ToArray();
-        }
-
-        private static double RayEntityDistance(
-            double vx,
-            double vy,
-            Entity entity,
-            double entityOffsetX,
-            double entityOffsetY,
-            double dirX,
-            double dirY
-        )
-        {
-            if (entity is Line line)
-            {
-                return SpatialQuery.RayEdgeDistance(
-                    vx,
-                    vy,
-                    line.StartPoint.X + entityOffsetX,
-                    line.StartPoint.Y + entityOffsetY,
-                    line.EndPoint.X + entityOffsetX,
-                    line.EndPoint.Y + entityOffsetY,
-                    dirX,
-                    dirY
-                );
-            }
-
-            if (entity is Arc arc)
-            {
-                return SpatialQuery.RayArcDistance(
-                    vx,
-                    vy,
-                    arc.Center.X + entityOffsetX,
-                    arc.Center.Y + entityOffsetY,
-                    arc.Radius,
-                    arc.StartAngle,
-                    arc.EndAngle,
-                    arc.IsReversed,
-                    dirX,
-                    dirY
-                );
-            }
-
-            if (entity is Circle circle)
-            {
-                return SpatialQuery.RayCircleDistance(
-                    vx,
-                    vy,
-                    circle.Center.X + entityOffsetX,
-                    circle.Center.Y + entityOffsetY,
-                    circle.Radius,
-                    dirX,
-                    dirY
-                );
-            }
-
-            return double.MaxValue;
-        }
-
-        private static Vector[] ExtractVerticesFromEntities(List<Entity> entities)
-        {
-            var vertices = new HashSet<Vector>();
-
-            for (var i = 0; i < entities.Count; i++)
-            {
-                var entity = entities[i];
-
-                if (entity is Line line)
-                {
-                    vertices.Add(line.StartPoint);
-                    vertices.Add(line.EndPoint);
-                }
-                else if (entity is Arc arc)
-                {
-                    vertices.Add(arc.StartPoint());
-                    vertices.Add(arc.EndPoint());
-                    AddArcExtremes(vertices, arc);
-                }
-                else if (entity is Circle circle)
-                {
-                    // Four cardinal points
-                    vertices.Add(new Vector(circle.Center.X + circle.Radius, circle.Center.Y));
-                    vertices.Add(new Vector(circle.Center.X - circle.Radius, circle.Center.Y));
-                    vertices.Add(new Vector(circle.Center.X, circle.Center.Y + circle.Radius));
-                    vertices.Add(new Vector(circle.Center.X, circle.Center.Y - circle.Radius));
-                }
-            }
-
-            return vertices.ToArray();
-        }
-
-        private static void AddArcExtremes(HashSet<Vector> points, Arc arc)
-        {
-            var a1 = arc.StartAngle;
-            var a2 = arc.EndAngle;
-            var reversed = arc.IsReversed;
-
-            if (reversed)
-                Generic.Swap(ref a1, ref a2);
-
-            // Right (0°)
-            if (Angle.IsBetweenRad(Angle.TwoPI, a1, a2))
-                points.Add(new Vector(arc.Center.X + arc.Radius, arc.Center.Y));
-
-            // Top (90°)
-            if (Angle.IsBetweenRad(Angle.HalfPI, a1, a2))
-                points.Add(new Vector(arc.Center.X, arc.Center.Y + arc.Radius));
-
-            // Left (180°)
-            if (Angle.IsBetweenRad(System.Math.PI, a1, a2))
-                points.Add(new Vector(arc.Center.X - arc.Radius, arc.Center.Y));
-
-            // Bottom (270°)
-            if (Angle.IsBetweenRad(System.Math.PI * 1.5, a1, a2))
-                points.Add(new Vector(arc.Center.X, arc.Center.Y - arc.Radius));
-        }
-
-        private static Vector[] ExtractUniqueVertices(List<Line> lines)
-        {
-            var vertices = new HashSet<Vector>();
-            for (var i = 0; i < lines.Count; i++)
-            {
-                vertices.Add(lines[i].StartPoint);
-                vertices.Add(lines[i].EndPoint);
-            }
-            return vertices.ToArray();
-        }
-
-        private static Vector[] FilterVerticesByProjection(
-            Vector[] vertices,
-            double dirX,
-            double dirY,
-            bool keepHigh
-        )
-        {
-            if (vertices.Length == 0)
-                return vertices;
-
-            var projections = new double[vertices.Length];
-            var min = double.MaxValue;
-            var max = double.MinValue;
-
-            for (var i = 0; i < vertices.Length; i++)
-            {
-                projections[i] = vertices[i].X * dirX + vertices[i].Y * dirY;
-                if (projections[i] < min)
-                    min = projections[i];
-                if (projections[i] > max)
-                    max = projections[i];
-            }
-
-            var midpoint = (min + max) / 2;
-            var count = 0;
-
-            for (var i = 0; i < vertices.Length; i++)
-            {
-                if (keepHigh ? projections[i] >= midpoint : projections[i] <= midpoint)
-                    count++;
-            }
-
-            var result = new Vector[count];
-            var idx = 0;
-
-            for (var i = 0; i < vertices.Length; i++)
-            {
-                if (keepHigh ? projections[i] >= midpoint : projections[i] <= midpoint)
-                    result[idx++] = vertices[i];
-            }
-
-            return result;
-        }
+        private static Vector[] Vertices(List<Line> lines) =>
+            lines.SelectMany(line => new[] { line.StartPoint, line.EndPoint }).Distinct().ToArray();
     }
 }

OpenNest.Engine/BestFit/GpuDistanceComputer.cs

@@ -18,6 +18,15 @@ namespace OpenNest.Engine.BestFit
             SlideOffset[] offsets
         )
         {
+            // ISlideComputer is axis-only; do not quantize an arbitrary direction into
+            // an unrelated cardinal push. Native curves already use this same fallback.
+            foreach (var offset in offsets)
+            {
+                if (!((offset.DirX == 0 && System.Math.Abs(offset.DirY) == 1)
+                    || (offset.DirY == 0 && System.Math.Abs(offset.DirX) == 1)))
+                    return new CpuDistanceComputer().ComputeDistances(stationaryLines, movingTemplateLines, offsets);
+            }
+
             var stationarySegments = SpatialQuery.FlattenLines(stationaryLines);
             var movingSegments = SpatialQuery.FlattenLines(movingTemplateLines);
             var count = offsets.Length;
@@ -55,7 +64,7 @@ namespace OpenNest.Engine.BestFit
 
         /// <summary>
         /// Maps a unit direction vector to a PushDirection int for the GPU interface.
-        /// Left=0, Down=1, Right=2, Up=3.
+        /// Up=0, Down=1, Left=2, Right=3.
         /// </summary>
         private static int DirectionVectorToInt(double dirX, double dirY)
         {

OpenNest.Engine/BestFit/ISlideComputer.cs

@@ -9,7 +9,8 @@ namespace OpenNest.Engine.BestFit
     public interface ISlideComputer : IDisposable
     {
         /// <summary>
-        /// Computes the minimum directional distance for each offset position.
+        /// Computes the first blocking contact distance for each offset position.
+        /// Separating/tangential contacts on closed boundaries do not block.
         /// </summary>
         /// <param name="stationarySegments">Flat array [x1,y1,x2,y2, ...] for stationary edges.</param>
         /// <param name="stationaryCount">Number of line segments in stationarySegments.</param>
@@ -30,7 +31,7 @@ namespace OpenNest.Engine.BestFit
         );
 
         /// <summary>
-        /// Computes minimum directional distance for offsets with per-offset directions.
+        /// Computes first blocking contact distances with per-offset directions.
         /// Uploads segment data once for all offsets, reducing GPU round-trips.
         /// </summary>
         double[] ComputeBatchMultiDir(

OpenNest.Engine/Fill/Compactor.cs

@@ -127,24 +127,20 @@ namespace OpenNest.Engine.Fill
                                     : PartGeometry.GetPerimeterEntities(moving)
                             );
 
+                    // A moving part can be inside an obstacle's cutout. Omitting that
+                    // loop would let it cross the inner wall before seeing the perimeter.
                     obstacleEntities[i] ??=
                         halfSpacing > 0
-                            ? PartGeometry.GetOffsetPerimeterEntities(obstacleParts[i], halfSpacing)
-                            : PartGeometry.GetPerimeterEntities(obstacleParts[i]);
+                            ? PartGeometry.GetOffsetPartEntities(obstacleParts[i], halfSpacing)
+                            : PartGeometry.GetPartEntities(obstacleParts[i]);
 
+                    // Contacts left by a previous push only block directions that would
+                    // push material into material; the kernel classifies them.
                     var d = SpatialQuery.DirectionalDistance(
                         movingEntities,
                         obstacleEntities[i],
                         direction
                     );
-                    if (
-                        d <= Tolerance.Epsilon
-                        && partSpacing <= Tolerance.Epsilon
-                        && CanNudgeWithoutOverlap(moving, obstacleParts[i], direction)
-                    )
-                    {
-                        continue;
-                    }
 
                     if (d < distance)
                         distance = d;
@@ -176,27 +172,25 @@ namespace OpenNest.Engine.Fill
         {
             for (var i = 0; i < parts.Count; i++)
             {
-                if (candidate.Intersects(parts[i], out _))
+                if (!candidate.Intersects(parts[i], out _))
+                    continue;
+
+                // Part.Intersects compares outer perimeters only. A valid insert in a
+                // cutout must remain an obstacle, not be discarded as already overlapping.
+                var a = new ShapeProfile(PartGeometry.GetPartEntities(candidate));
+                var b = new ShapeProfile(PartGeometry.GetPartEntities(parts[i]));
+                if (a.Cutouts.Count == 0 && b.Cutouts.Count == 0)
+                    return true;
+                if (Collision.HasOverlap(
+                    a.Perimeter.ToPolygonWithTolerance(0.001),
+                    b.Perimeter.ToPolygonWithTolerance(0.001),
+                    a.Cutouts.Select(hole => hole.ToPolygonWithTolerance(0.001)).ToList(),
+                    b.Cutouts.Select(hole => hole.ToPolygonWithTolerance(0.001)).ToList()))
                     return true;
             }
             return false;
         }
 
-        private static bool CanNudgeWithoutOverlap(Part moving, Part obstacle, Vector direction)
-        {
-            var nudge = direction * (Tolerance.Epsilon * 10);
-
-            moving.Offset(nudge);
-            try
-            {
-                return !moving.Intersects(obstacle, out _);
-            }
-            finally
-            {
-                moving.Offset(-nudge);
-            }
-        }
-
         public static double Push(
             List<Part> movingParts,
             List<Part> obstacleParts,

OpenNest.Engine/Fill/Expander.cs

@@ -0,0 +1,771 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using OpenNest.Converters;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Fill
+{
+    /// <summary>
+    /// Pushes a group of selected parts apart until every constrained pair
+    /// (selected↔selected and selected↔obstacle) reaches a target part-to-part
+    /// spacing, with the plate work area and all non-selected parts as hard
+    /// boundaries. The inverse of <see cref="Compactor"/>: it grows gaps instead
+    /// of closing them.
+    /// <para>
+    /// Input with overlaps is accepted: an overlapping pair is a pair with
+    /// negative clearance and is separated along the minimum-translation
+    /// direction. Moves are straight-line only — a pair that could separate only
+    /// by routing around a blocker is reported as a violation instead. Final
+    /// positions are always overlap-free; nothing moves off the work area.
+    /// </para>
+    /// <para>
+    /// Anchor policy: within a violated pair the later-indexed selected part
+    /// moves; the counterpart moves only as a fallback when the anchor mover is
+    /// fully blocked by a work-area edge. Walls (non-selected parts) never move.
+    /// </para>
+    /// </summary>
+    public static class Expander
+    {
+        public sealed class Options
+        {
+            /// <summary>First spacing probed by the search; also the floor for the doubling step.</summary>
+            public double InitialStep = 1.0;
+
+            /// <summary>Bisection stops once the achievable spacing is known within this tolerance.</summary>
+            public double Tolerance = 0.01;
+
+            /// <summary>Relaxation iteration cap per separation run.</summary>
+            public int MaxIterations = 100;
+
+            /// <summary>Upper bound for the spacing search. 0 = auto (work-area diagonal).</summary>
+            public double MaxSpacing = 0;
+        }
+
+        public sealed class Violation
+        {
+            public Part A;
+            public Part B;
+
+            /// <summary>Clearance actually reached (may be negative for unresolvable overlaps).</summary>
+            public double Achieved;
+
+            /// <summary>True when a work-area edge, not a part, blocked the last needed move.</summary>
+            public bool BlockedByEdge;
+        }
+
+        public sealed class Result
+        {
+            /// <summary>
+            /// Minimum part-to-part clearance the returned layout satisfies,
+            /// never below zero. 0 with violations means the layout was only
+            /// cleaned up as far as possible, not opened up.
+            /// </summary>
+            public double AchievedSpacing;
+
+            /// <summary>True when the run was cancelled; no positions were changed.</summary>
+            public bool Cancelled;
+
+            /// <summary>Pairs that could not reach <see cref="AchievedSpacing"/>.</summary>
+            public List<Violation> Violations = new();
+        }
+
+        /// <summary>
+        /// Raises the part-to-part spacing of the selection as far as the plate
+        /// and its other parts allow, applying the best spacing found. Plate
+        /// PartSpacing/EdgeSpacing are not modified; edges keep their own
+        /// EdgeSpacing floor while only part-to-part clearance chases the target.
+        /// Mutates <paramref name="selected"/> locations; probing never touches
+        /// them, so a failed or cancelled run leaves the layout unchanged.
+        /// </summary>
+        public static Result Expand(
+            List<Part> selected,
+            Plate plate,
+            Options options = null,
+            CancellationToken token = default
+        )
+        {
+            if (plate == null)
+                throw new ArgumentNullException(nameof(plate));
+            if (selected == null || selected.Count < 2)
+                throw new ArgumentException(
+                    "Expand requires at least two selected parts.",
+                    nameof(selected)
+                );
+            if (selected.Any(p => !plate.Parts.Contains(p)))
+                throw new ArgumentException(
+                    "All selected parts must belong to the plate.",
+                    nameof(selected)
+                );
+
+            var opt = options ?? new Options();
+            var context = SeparationContext.Prepare(selected, plate);
+            var entry = context.Positions(); // all parts; movers are [0, count)
+            var result = new Result();
+
+            var sMin = MinimumPairClearance(context, entry);
+            result.AchievedSpacing = System.Math.Max(0, sMin);
+
+            if (token.IsCancellationRequested)
+            {
+                result.Cancelled = true;
+                return result;
+            }
+
+            var sGood = sMin;
+            var cap = opt.MaxSpacing > 0 ? opt.MaxSpacing : context.SpacingCap;
+            double hi; // first spacing that failed; double.NaN = none yet
+
+            (bool Converged, List<Vector> Positions) Probe(double spacing)
+            {
+                var attempt = Separate(context, entry, spacing, opt.MaxIterations, token);
+                return (attempt.Converged, attempt.Positions);
+            }
+
+            // Doubling phase: commit every spacing that converges. The cap gets
+            // its own probe even when the step jumps past it, and no spacing is
+            // probed twice.
+            var s = System.Math.Max(opt.InitialStep, 2 * System.Math.Max(0, sGood));
+            hi = double.NaN;
+
+            while (true)
+            {
+                if (s > cap)
+                {
+                    if (cap > sGood + opt.Tolerance)
+                        s = cap;
+                    else
+                        break;
+                }
+
+                if (s <= sGood + opt.Tolerance)
+                    break;
+
+                if (token.IsCancellationRequested)
+                {
+                    result.Cancelled = true;
+                    return result;
+                }
+
+                var probe = Probe(s);
+
+                if (probe.Converged)
+                {
+                    sGood = s;
+                    if (sGood >= cap)
+                        break;
+                    s = System.Math.Max(s * 2, sGood + opt.Tolerance);
+                }
+                else
+                {
+                    hi = s;
+                    break;
+                }
+            }
+
+            // Bisection between the last spacing that converged and the first that failed.
+            if (!double.IsNaN(hi))
+            {
+                var lo = System.Math.Max(0, sGood);
+                while (hi - lo > opt.Tolerance)
+                {
+                    if (token.IsCancellationRequested)
+                    {
+                        result.Cancelled = true;
+                        return result;
+                    }
+
+                    var mid = (lo + hi) / 2;
+                    if (Probe(mid).Converged)
+                        lo = mid;
+                    else
+                        hi = mid;
+                }
+                sGood = lo;
+            }
+
+            // Finalize: re-separate at the applied spacing from the entry state.
+            // Deterministic, so this reproduces any committed probe exactly; when
+            // sGood was never probed (overlapping entry) it still performs the
+            // best-effort cleanup and yields the violation report.
+            var final = Separate(context, entry, System.Math.Max(0, sGood), opt.MaxIterations, token);
+
+            if (token.IsCancellationRequested)
+            {
+                result.Cancelled = true;
+                return result;
+            }
+
+            var positions = final.Positions;
+
+            Apply(selected, entry, positions);
+
+            var measured = MinimumPairClearance(context, positions);
+            result.AchievedSpacing = System.Math.Max(0, System.Math.Min(System.Math.Max(0, sGood), measured));
+            result.Violations = final.Violations;
+            return result;
+        }
+
+        /// <summary>
+        /// Relaxes the given parts apart to a fixed target spacing against the
+        /// plate. Works on scratch positions; the caller applies them. Exposed
+        /// for testing and for callers that manage their own spacing search.
+        /// </summary>
+        public static (bool Converged, List<Vector> Positions, List<Violation> Violations) Separate(
+            List<Part> selected,
+            Plate plate,
+            double spacing,
+            int maxIterations = 100,
+            CancellationToken token = default
+        )
+        {
+            var context = SeparationContext.Prepare(selected, plate);
+            var positions = context.Positions();
+            return Separate(context, positions, spacing, maxIterations, token);
+        }
+
+        private static void Apply(List<Part> selected, List<Vector> from, List<Vector> to)
+        {
+            // Only movers occupy [0, selected.Count); walls never move.
+            for (var i = 0; i < selected.Count; i++)
+            {
+                var delta = to[i] - from[i];
+                if (delta.X != 0 || delta.Y != 0)
+                    selected[i].Offset(delta);
+            }
+        }
+
+        private static double MinimumPairClearance(SeparationContext context, List<Vector> positions)
+        {
+            var min = double.MaxValue;
+
+            foreach (var pair in context.Pairs)
+            {
+                var (distance, _) = context.PairClearance(pair.IndexA, pair.IndexB, positions);
+                if (distance < min)
+                    min = distance;
+            }
+
+            return min == double.MaxValue ? 0 : min;
+        }
+
+        private static (bool Converged, List<Vector> Positions, List<Violation> Violations) Separate(
+            SeparationContext context,
+            List<Vector> start,
+            double spacing,
+            int maxIterations,
+            CancellationToken token
+        )
+        {
+            var positions = new List<Vector>(start);
+            var stuck = new HashSet<int>();
+            var violations = new List<Violation>();
+            var epsMove = 1e-4;
+
+            // Internal margin absorbs the clearance kernel's tessellation error so
+            // the applied spacing holds against the production validators.
+            var target = spacing + 0.002;
+
+            var iterationLimit = maxIterations < 1 ? 1 : maxIterations;
+
+            for (var iteration = 0; iteration < iterationLimit; iteration++)
+            {
+                if (token.IsCancellationRequested)
+                    return (false, start, violations);
+
+                var moved = 0.0;
+                var stuckChanged = false;
+
+                foreach (var pair in context.Pairs)
+                {
+                    if (stuck.Contains(pair.Id))
+                        continue;
+
+                    var (distance, directionA) = context.PairClearance(
+                        pair.IndexA,
+                        pair.IndexB,
+                        positions
+                    );
+
+                    // Trigger at the user spacing, not the internal margin: a
+                    // pair already at the requested spacing must not be nudged,
+                    // or feasible layouts at the ceiling (every pair exactly at
+                    // spacing) would oscillate forever. The margin only sets how
+                    // far past the trigger a push carries, absorbing tessellation
+                    // error in the measurement.
+                    if (distance >= spacing)
+                        continue;
+
+                    var need = target - distance;
+
+                    // Anchor policy (decided): only the later-indexed selected
+                    // part of a violated pair moves. Counterparts and walls
+                    // never do — a pair whose anchor mover cannot reach the
+                    // target is a violation, not an invitation to drift the
+                    // anchor.
+                    var moverIndex = pair.Mover;
+
+                    // Clearance direction translates A away from B; a mover on
+                    // the B side travels the opposite way.
+                    var direction = moverIndex == pair.IndexA ? directionA : -directionA;
+
+                    var room = context.ClipToWorkArea(
+                        moverIndex,
+                        positions[moverIndex],
+                        direction,
+                        need
+                    );
+
+                    // Take the largest valid step up to `room`: partial moves let
+                    // a blocked mover advance again once its own blockers move
+                    // away in later iterations (a wave separates a chain).
+                    var applied = 0.0;
+                    var blockedByEdge = false;
+
+                    if (room > 0)
+                    {
+                        var trial = positions[moverIndex] + direction * room;
+
+                        if (context.MaintainsValidity(moverIndex, trial, positions))
+                        {
+                            positions[moverIndex] = trial;
+                            applied = room;
+                        }
+                        else
+                        {
+                            var lo = 0.0;
+                            var hi2 = room;
+                            for (var bisect = 0; bisect < 24 && hi2 - lo > 1e-6; bisect++)
+                            {
+                                var mid = (lo + hi2) / 2;
+                                if (
+                                    context.MaintainsValidity(
+                                        moverIndex,
+                                        positions[moverIndex] + direction * mid,
+                                        positions
+                                    )
+                                )
+                                    lo = mid;
+                                else
+                                    hi2 = mid;
+                            }
+
+                            if (lo > epsMove)
+                            {
+                                positions[moverIndex] = positions[moverIndex] + direction * lo;
+                                applied = lo;
+                            }
+                        }
+
+                        blockedByEdge = applied < need - epsMove;
+                        moved += applied;
+                    }
+                    else
+                    {
+                        blockedByEdge = true;
+                    }
+
+                    // A pair fully separated to the user target (the internal margin
+                    // absorbs tessellation slack) is satisfied even if not to target.
+                    if (applied > 0)
+                    {
+                        var (finalDistance, _) = context.PairClearance(
+                            pair.IndexA,
+                            pair.IndexB,
+                            positions
+                        );
+                        if (finalDistance >= spacing)
+                            continue;
+                        // Partial progress: keep the pair live — its blockers may
+                        // move away in later iterations and unblock the rest.
+                        continue;
+                    }
+
+                    // Zero progress twice in a row parks the pair; the final sweep
+                    // re-measures everything, so mid-loop bookkeeping never lies.
+                    if (stuck.Contains(pair.Id))
+                        continue;
+
+                    stuck.Add(pair.Id);
+                    stuckChanged = true;
+                }
+
+                if (moved < epsMove && !stuckChanged)
+                    break;
+            }
+
+            // Honest verdict: stuck bookkeeping and the internal margin can both
+            // let a pair read as satisfied mid-loop while a later pair move
+            // un-does it (oscillation). Re-measure every constrained pair at the
+            // final positions once; the violations this sweep finds are the
+            // report, and any violation makes the run non-converged.
+            violations.Clear();
+            foreach (var pair in context.Pairs)
+            {
+                var (distance, _) = context.PairClearance(
+                    pair.IndexA,
+                    pair.IndexB,
+                    positions
+                );
+
+                if (distance >= spacing)
+                    continue;
+
+                violations.Add(
+                    new Violation
+                    {
+                        A = context.PartOf(pair.IndexA),
+                        B = context.PartOf(pair.IndexB),
+                        Achieved = distance,
+                        BlockedByEdge = stuck.Contains(pair.Id),
+                    }
+                );
+            }
+
+            return (violations.Count == 0, positions, violations);
+        }
+
+        /// <summary>One constrained part↔part pair with its anchor mover.</summary>
+        private sealed class Pair
+        {
+            public int Id;
+            public int IndexA;
+            public int IndexB;
+            public int Mover;
+        }
+
+        /// <summary>
+        /// Per-run prepared geometry. Rings are local-frame polygons (world = local
+        /// + scratch position), prepared once per distinct Program by reference,
+        /// mirroring <see cref="PartOverlapChecker"/>'s caching but translatable.
+        /// </summary>
+        private sealed class SeparationContext
+        {
+            private readonly List<Part> parts; // movers [0, moverCount) then walls
+            private readonly List<RingSet> shapes; // per part
+            private readonly Box[] localBoxes; // per part, local frame
+            private readonly int moverCount;
+            private readonly Box workArea;
+
+            public readonly List<Pair> Pairs = new();
+            public readonly double SpacingCap;
+
+            private sealed class RingSet
+            {
+                public Polygon Outer;
+                public List<Polygon> Rings = new(); // outer + cutout rings, local frame
+                public List<Polygon> Holes = new();
+            }
+
+            private SeparationContext(
+                List<Part> parts,
+                List<RingSet> shapes,
+                Box[] localBoxes,
+                int moverCount,
+                Box workArea,
+                double spacingCap
+            )
+            {
+                this.parts = parts;
+                this.shapes = shapes;
+                this.localBoxes = localBoxes;
+                this.moverCount = moverCount;
+                this.workArea = workArea;
+                SpacingCap = spacingCap;
+            }
+
+            public static SeparationContext Prepare(List<Part> selected, Plate plate)
+            {
+                var movers = new List<Part>(selected);
+                var walls = plate.Parts.Where(p => !movers.Contains(p)).ToList();
+
+                var parts = new List<Part>(movers.Count + walls.Count);
+                parts.AddRange(movers);
+                parts.AddRange(walls);
+
+                var programs = new Dictionary<CNC.Program, RingSet>(
+                    ReferenceEqualityComparer.Instance
+                );
+                var shapes = new List<RingSet>(parts.Count);
+                var localBoxes = new Box[parts.Count];
+
+                for (var i = 0; i < parts.Count; i++)
+                {
+                    shapes.Add(PrepareProgram(programs, parts[i].Program));
+                    localBoxes[i] = LocalBox(parts[i]);
+                }
+
+                var workArea = plate.WorkArea();
+                var spacingCap = System.Math.Sqrt(
+                    workArea.Length * workArea.Length + workArea.Width * workArea.Width
+                );
+
+                var context = new SeparationContext(
+                    parts,
+                    shapes,
+                    localBoxes,
+                    movers.Count,
+                    workArea,
+                    spacingCap
+                );
+                context.BuildPairs();
+                return context;
+            }
+
+            private static RingSet PrepareProgram(
+                Dictionary<CNC.Program, RingSet> programs,
+                CNC.Program program
+            )
+            {
+                if (programs.TryGetValue(program, out var existing))
+                    return existing;
+
+                var prepared = new RingSet();
+                var entities = ConvertProgram
+                    .ToGeometry(program)
+                    .Where(e => SpecialLayers.IsMaterial(e.Layer))
+                    .ToList();
+
+                if (entities.Count > 0)
+                {
+                    var profile = new ShapeProfile(entities);
+
+                    if (profile.Perimeter != null)
+                    {
+                        prepared.Outer = profile.Perimeter.ToPolygonWithTolerance(0.001);
+                        prepared.Rings.Add(prepared.Outer);
+
+                        foreach (var cutout in profile.Cutouts)
+                        {
+                            var hole = cutout.ToPolygonWithTolerance(0.001);
+                            prepared.Rings.Add(hole);
+                            prepared.Holes.Add(hole);
+                        }
+                    }
+                }
+
+                programs.Add(program, prepared);
+                return prepared;
+            }
+
+            private static Box LocalBox(Part part)
+            {
+                var box = part.BoundingBox;
+                return new Box(
+                    box.Left - part.Location.X,
+                    box.Bottom - part.Location.Y,
+                    box.Length,
+                    box.Width
+                );
+            }
+
+            private void BuildPairs()
+            {
+                var id = 0;
+
+                for (var a = 0; a < parts.Count; a++)
+                {
+                    for (var b = a + 1; b < parts.Count; b++)
+                    {
+                        var aMover = a < moverCount;
+                        var bMover = b < moverCount;
+
+                        if (!aMover && !bMover)
+                            continue;
+
+                        Pairs.Add(
+                            new Pair
+                            {
+                                Id = id++,
+                                IndexA = a,
+                                IndexB = b,
+                                // Anchor policy: the later-index mover moves.
+                                Mover = bMover ? b : a,
+                            }
+                        );
+                    }
+                }
+            }
+
+            public bool IsMover(int index) => index < moverCount;
+
+            public Part PartOf(int index) => parts[index];
+
+            /// <summary>Current world positions of every part in pair-index order.</summary>
+            public List<Vector> Positions() => parts.Select(p => p.Location).ToList();
+
+            /// <summary>
+            /// Signed material clearance between two parts at the given scratch
+            /// positions. Material overlap (Collision oracle with hole subtraction)
+            /// reports negative penetration through the outer rings; otherwise the
+            /// clearance is the minimum boundary distance over all ring pairs, so a
+            /// part inside another's cutout measures its true gap to the hole ring
+            /// instead of a bogus outer-ring penetration.
+            /// </summary>
+            public (double Distance, Vector Direction) PairClearance(
+                int indexA,
+                int indexB,
+                List<Vector> positions
+            )
+            {
+                var setA = shapes[indexA];
+                var setB = shapes[indexB];
+
+                if (setA.Outer == null || setB.Outer == null)
+                    return (0, new Vector(1, 0));
+
+                var offsetA = positions[indexA];
+                var offsetB = positions[indexB];
+
+                var outerA = CloneAt(setA.Outer, offsetA);
+                var outerB = CloneAt(setB.Outer, offsetB);
+
+                var holesA = setA.Holes.Count == 0 ? null : CloneAll(setA.Holes, offsetA);
+                var holesB = setB.Holes.Count == 0 ? null : CloneAll(setB.Holes, offsetB);
+
+                if (Collision.HasOverlap(outerA, outerB, holesA, holesB))
+                {
+                    var penetration = Clearance.Between(outerA, outerB);
+                    if (penetration.Distance < 0)
+                        return (penetration.Distance, penetration.Direction);
+                    // Hole subtraction resolved what the outers overlap: touching.
+                    return (0, penetration.Direction);
+                }
+
+                double best = double.MaxValue;
+                var bestDir = new Vector(1, 0);
+
+                foreach (var ringA in setA.Rings)
+                {
+                    var worldA = CloneAt(ringA, offsetA);
+
+                    foreach (var ringB in setB.Rings)
+                    {
+                        var worldB = CloneAt(ringB, offsetB);
+                        var clearance = Clearance.BoundaryDistance(worldA, worldB);
+
+                        if (clearance.Distance < best)
+                        {
+                            best = clearance.Distance;
+                            bestDir = clearance.Direction;
+                        }
+                    }
+                }
+
+                return (best, bestDir);
+            }
+
+            /// <summary>
+            /// Largest α ≤ need such that translating the part by direction·α keeps
+            /// its AABB inside the work area.
+            /// </summary>
+            public double ClipToWorkArea(int index, Vector position, Vector direction, double need)
+            {
+                var box = localBoxes[index];
+                var left = position.X + box.Left - workArea.Left;
+                var right = workArea.Right - (position.X + box.Right);
+                var bottom = position.Y + box.Bottom - workArea.Bottom;
+                var top = workArea.Top - (position.Y + box.Top);
+
+                var max = need;
+                if (direction.X > 0)
+                    max = System.Math.Min(max, right / direction.X);
+                else if (direction.X < 0)
+                    max = System.Math.Min(max, left / -direction.X);
+
+                if (direction.Y > 0)
+                    max = System.Math.Min(max, top / direction.Y);
+                else if (direction.Y < 0)
+                    max = System.Math.Min(max, bottom / -direction.Y);
+
+                return max < 0 ? 0 : max;
+            }
+
+            /// <summary>
+            /// True when the part at <paramref name="trial"/> stays inside the work
+            /// area and keeps no material overlap with any other part (Collision
+            /// oracle with hole subtraction, so part-in-cutout stays legal).
+            /// </summary>
+            public bool MaintainsValidity(int index, Vector trial, List<Vector> positions)
+            {
+                var box = localBoxes[index];
+                var movedBox = box.Translate(trial);
+
+                if (
+                    movedBox.Left < workArea.Left - 1e-9
+                    || movedBox.Right > workArea.Right + 1e-9
+                    || movedBox.Bottom < workArea.Bottom - 1e-9
+                    || movedBox.Top > workArea.Top + 1e-9
+                )
+                    return false;
+
+                var outer = shapes[index].Outer;
+                if (outer == null)
+                    return true;
+
+                var worldOuter = CloneAt(outer, trial);
+                var worldHoles = shapes[index].Holes.Count == 0
+                    ? null
+                    : CloneAll(shapes[index].Holes, trial);
+
+                for (var i = 0; i < parts.Count; i++)
+                {
+                    if (i == index)
+                        continue;
+
+                    var otherOuter = shapes[i].Outer;
+                    if (otherOuter == null)
+                        continue;
+
+                    if (!BoxOverlap(movedBox, localBoxes[i].Translate(positions[i]), 0.002))
+                        continue;
+
+                    var worldOther = CloneAt(otherOuter, positions[i]);
+                    var worldOtherHoles = shapes[i].Holes.Count == 0
+                        ? null
+                        : CloneAll(shapes[i].Holes, positions[i]);
+
+                    if (
+                        Collision.HasOverlap(
+                            worldOuter,
+                            worldOther,
+                            worldHoles,
+                            worldOtherHoles
+                        )
+                    )
+                        return false;
+                }
+
+                return true;
+            }
+
+            private static List<Polygon> CloneAll(List<Polygon> polygons, Vector offset)
+            {
+                var list = new List<Polygon>(polygons.Count);
+                foreach (var polygon in polygons)
+                    list.Add(CloneAt(polygon, offset));
+                return list;
+            }
+
+            /// <summary>Clone with world bounds applied — prepared rings are never mutated.</summary>
+            private static Polygon CloneAt(Polygon polygon, Vector offset)
+            {
+                var clone = (Polygon)polygon.Clone();
+                clone.UpdateBounds();
+                clone.Offset(offset);
+                return clone;
+            }
+
+            private static bool BoxOverlap(Box a, Box b, double slack)
+            {
+                return !(
+                    a.Right + slack < b.Left
+                    || b.Right + slack < a.Left
+                    || a.Top + slack < b.Bottom
+                    || b.Top + slack < a.Bottom
+                );
+            }
+        }
+    }
+}

OpenNest.Engine/Fill/FillExtents.cs

@@ -83,8 +83,9 @@ namespace OpenNest.Engine.Fill
 
             // Slide uses locations, not cached bounds; Offset already translates the box.
             // Slide part2 left toward part1.
-            var movingLines = boundary2.GetLines(part2.Location, PushDirection.Left);
-            var stationaryLines = boundary1.GetLines(part1.Location, PushDirection.Right);
+            // Keep complete loops so the shared kernel can classify tangential contacts.
+            var movingLines = boundary2.GetLines(part2.Location);
+            var stationaryLines = boundary1.GetLines(part1.Location);
             var dist = SpatialQuery.DirectionalDistance(
                 movingLines,
                 stationaryLines,
@@ -234,15 +235,9 @@ namespace OpenNest.Engine.Fill
             PushDirection direction
         )
         {
-            var opposite = SpatialQuery.OppositeDirection(direction);
-            var movingEdges = movingBoundary.GetEdges(direction);
-            var stationaryEdges = stationaryBoundary.GetEdges(opposite);
-
             return SpatialQuery.DirectionalDistance(
-                movingEdges,
-                movingLocation,
-                stationaryEdges,
-                stationaryLocation,
+                movingBoundary.GetLines(movingLocation),
+                stationaryBoundary.GetLines(stationaryLocation),
                 direction
             );
         }

OpenNest.Gpu/GpuSlideComputer.cs

@@ -1,8 +1,13 @@
+#nullable enable
+
 using System;
+using System.Collections.Generic;
 using ILGPU;
 using ILGPU.Algorithms;
 using ILGPU.Runtime;
 using OpenNest.Engine.BestFit;
+using OpenNest.Geometry;
+using OpenNest.Math;
 
 namespace OpenNest.Gpu
 {
@@ -19,7 +24,7 @@ namespace OpenNest.Gpu
             ArrayView1D<double, Stride1D.Dense>, // stationaryPrep
             ArrayView1D<double, Stride1D.Dense>, // movingPrep
             ArrayView1D<double, Stride1D.Dense>, // offsets
-            ArrayView1D<double, Stride1D.Dense>, // results
+            ArrayView1D<ContactWitness, Stride1D.Dense>, // results
             int,
             int,
             int
@@ -30,7 +35,7 @@ namespace OpenNest.Gpu
             ArrayView1D<double, Stride1D.Dense>, // stationaryPrep
             ArrayView1D<double, Stride1D.Dense>, // movingPrep
             ArrayView1D<double, Stride1D.Dense>, // offsets
-            ArrayView1D<double, Stride1D.Dense>, // results
+            ArrayView1D<ContactWitness, Stride1D.Dense>, // results
             ArrayView1D<int, Stride1D.Dense>, // directions
             int,
             int
@@ -47,22 +52,24 @@ namespace OpenNest.Gpu
 
         private MemoryBuffer1D<double, Stride1D.Dense>? _gpuStationaryRaw;
         private MemoryBuffer1D<double, Stride1D.Dense>? _gpuStationaryPrep;
-        private double[]? _lastStationaryData; // Keep CPU copy/ref for content check
+        private double[]? _lastStationaryData; // Active segment snapshot used for upload and contact topology
 
         private MemoryBuffer1D<double, Stride1D.Dense>? _gpuMovingRaw;
         private MemoryBuffer1D<double, Stride1D.Dense>? _gpuMovingPrep;
-        private double[]? _lastMovingData; // Keep CPU copy/ref for content check
+        private double[]? _lastMovingData; // Active segment snapshot used for upload and contact topology
 
         private MemoryBuffer1D<double, Stride1D.Dense>? _gpuOffsets;
-        private MemoryBuffer1D<double, Stride1D.Dense>? _gpuResults;
+        private MemoryBuffer1D<ContactWitness, Stride1D.Dense>? _gpuResults;
         private MemoryBuffer1D<int, Stride1D.Dense>? _gpuDirs;
         private int _offsetCapacity;
 
-        public GpuSlideComputer()
+        public GpuSlideComputer() : this(preferCPU: false) { }
+
+        public GpuSlideComputer(bool preferCPU)
         {
             _context = Context.CreateDefault();
             _accelerator = _context
-                .GetPreferredDevice(preferCPU: false)
+                .GetPreferredDevice(preferCPU)
                 .CreateAccelerator(_context);
 
             _kernel = _accelerator.LoadAutoGroupedStreamKernel<
@@ -70,7 +77,7 @@ namespace OpenNest.Gpu
                 ArrayView1D<double, Stride1D.Dense>,
                 ArrayView1D<double, Stride1D.Dense>,
                 ArrayView1D<double, Stride1D.Dense>,
-                ArrayView1D<double, Stride1D.Dense>,
+                ArrayView1D<ContactWitness, Stride1D.Dense>,
                 int,
                 int,
                 int
@@ -81,7 +88,7 @@ namespace OpenNest.Gpu
                 ArrayView1D<double, Stride1D.Dense>,
                 ArrayView1D<double, Stride1D.Dense>,
                 ArrayView1D<double, Stride1D.Dense>,
-                ArrayView1D<double, Stride1D.Dense>,
+                ArrayView1D<ContactWitness, Stride1D.Dense>,
                 ArrayView1D<int, Stride1D.Dense>,
                 int,
                 int
@@ -118,21 +125,24 @@ namespace OpenNest.Gpu
                 EnsureMoving(movingTemplateSegments, movingCount);
                 EnsureOffsetBuffers(offsetCount);
 
-                _gpuOffsets!.View.SubView(0, offsetCount * 2).CopyFromCPU(offsets);
+                _gpuOffsets!.View.SubView(0, offsetCount * 2).CopyFromCPU(
+                    _accelerator.DefaultStream, (ReadOnlySpan<double>)offsets.AsSpan(0, offsetCount * 2));
 
                 _kernel(
                     offsetCount,
                     _gpuStationaryPrep!.View,
                     _gpuMovingPrep!.View,
-                    _gpuOffsets.View,
-                    _gpuResults!.View,
+                    _gpuOffsets.View.SubView(0, offsetCount * 2),
+                    _gpuResults!.View.SubView(0, offsetCount),
                     stationaryCount,
                     movingCount,
                     (int)direction
                 );
 
                 _accelerator.Synchronize();
-                _gpuResults.View.SubView(0, offsetCount).CopyToCPU(results);
+                var witnesses = new ContactWitness[offsetCount];
+                _gpuResults.View.SubView(0, offsetCount).CopyToCPU(witnesses);
+                ResolveContacts(witnesses, offsets, results, direction, null);
             }
 
             return results;
@@ -161,93 +171,138 @@ namespace OpenNest.Gpu
                 EnsureMoving(movingTemplateSegments, movingCount);
                 EnsureOffsetBuffers(offsetCount);
 
-                _gpuOffsets!.View.SubView(0, offsetCount * 2).CopyFromCPU(offsets);
-                _gpuDirs!.View.SubView(0, offsetCount).CopyFromCPU(directions);
+                _gpuOffsets!.View.SubView(0, offsetCount * 2).CopyFromCPU(
+                    _accelerator.DefaultStream, (ReadOnlySpan<double>)offsets.AsSpan(0, offsetCount * 2));
+                _gpuDirs!.View.SubView(0, offsetCount).CopyFromCPU(
+                    _accelerator.DefaultStream, (ReadOnlySpan<int>)directions.AsSpan(0, offsetCount));
 
                 _kernelMultiDir(
                     offsetCount,
                     _gpuStationaryPrep!.View,
                     _gpuMovingPrep!.View,
-                    _gpuOffsets.View,
-                    _gpuResults!.View,
-                    _gpuDirs.View,
+                    _gpuOffsets.View.SubView(0, offsetCount * 2),
+                    _gpuResults!.View.SubView(0, offsetCount),
+                    _gpuDirs.View.SubView(0, offsetCount),
                     stationaryCount,
                     movingCount
                 );
 
                 _accelerator.Synchronize();
-                _gpuResults.View.SubView(0, offsetCount).CopyToCPU(results);
+                var witnesses = new ContactWitness[offsetCount];
+                _gpuResults.View.SubView(0, offsetCount).CopyToCPU(witnesses);
+                ResolveContacts(witnesses, offsets, results, default, directions);
             }
 
             return results;
         }
 
-        public void InvalidateStationary() => _lastStationaryData = null;
+        public void InvalidateStationary()
+        {
+            lock (_lock)
+                _lastStationaryData = null;
+        }
 
-        public void InvalidateMoving() => _lastMovingData = null;
+        public void InvalidateMoving()
+        {
+            lock (_lock)
+                _lastMovingData = null;
+        }
+
+        private void ResolveContacts(
+            ContactWitness[] witnesses,
+            double[] offsets,
+            double[] results,
+            PushDirection direction,
+            int[]? directions
+        )
+        {
+            var moving = default(List<Line>);
+            var stationary = default(List<Line>);
+            var contacts = default(SlideContactClassifier);
+            for (var i = 0; i < witnesses.Length; i++)
+            {
+                var witness = witnesses[i];
+                results[i] = witness.Distance;
+                if (witness.Distance == double.MaxValue)
+                    continue;
+
+                // GPU finds the nearest event. Prepare the complete boundaries only
+                // once per batch, and share their material-side topology at each offset.
+                if (contacts == null)
+                {
+                    moving = ToLines(_lastMovingData!);
+                    stationary = ToLines(_lastStationaryData!);
+                    contacts = SlideContactClassifier.FromLines(
+                        moving, Vector.Zero, stationary, Vector.Zero).Prepare();
+                }
+
+                var offset = new Vector(offsets[i * 2], offsets[i * 2 + 1]);
+                var push = directions == null ? direction : (PushDirection)directions[i];
+                var unit = SpatialQuery.DirectionToOffset(push, 1);
+                var placed = contacts.At(offset, Vector.Zero);
+                if (placed.Blocks(
+                    new Vector(witness.MovingX, witness.MovingY),
+                    new Vector(witness.StationaryX, witness.StationaryY), unit.X, unit.Y))
+                    continue;
+
+                // A departing or grazing event does not discard the obstacle: replay
+                // all events through the shared resolver to find the next blocking one,
+                // including another contact tied at the same distance.
+                results[i] = SpatialQuery.DirectionalDistance(
+                    moving!, offset.X, offset.Y, stationary!, push, placed);
+            }
+        }
+
+        private static List<Line> ToLines(double[] segments)
+        {
+            var lines = new List<Line>(segments.Length / 4);
+            for (var i = 0; i < segments.Length; i += 4)
+                lines.Add(new Line(segments[i], segments[i + 1], segments[i + 2], segments[i + 3]));
+            return lines;
+        }
 
         private void EnsureStationary(double[] data, int count)
         {
-            // Fast check: if same object or content is identical, skip upload
-            if (
-                _gpuStationaryPrep != null
-                && _lastStationaryData != null
-                && _lastStationaryData.Length == data.Length
-            )
-            {
-                // Reference equality or content equality
-                if (
-                    _lastStationaryData == data
-                    || new ReadOnlySpan<double>(_lastStationaryData).SequenceEqual(
-                        new ReadOnlySpan<double>(data)
-                    )
-                )
-                {
-                    return;
-                }
-            }
+            // Cache the active prefix by value: callers may reuse an array with a
+            // different segment count or mutate its coordinates between batches.
+            var active = data.AsSpan(0, count * 4);
+            if (_gpuStationaryPrep != null && _lastStationaryData != null
+                && active.SequenceEqual(_lastStationaryData))
+                return;
 
             _gpuStationaryRaw?.Dispose();
             _gpuStationaryPrep?.Dispose();
 
-            _gpuStationaryRaw = _accelerator.Allocate1D(data);
+            var snapshot = active.ToArray();
+            _gpuStationaryRaw = _accelerator.Allocate1D(snapshot);
             _gpuStationaryPrep = _accelerator.Allocate1D<double>(count * 10);
 
             _prepareKernel(count, _gpuStationaryRaw.View, _gpuStationaryPrep.View, count);
             _accelerator.Synchronize();
 
-            _lastStationaryData = data; // store reference for next comparison
+            _lastStationaryData = snapshot;
         }
 
         private void EnsureMoving(double[] data, int count)
         {
-            if (
-                _gpuMovingPrep != null
-                && _lastMovingData != null
-                && _lastMovingData.Length == data.Length
-            )
-            {
-                if (
-                    _lastMovingData == data
-                    || new ReadOnlySpan<double>(_lastMovingData).SequenceEqual(
-                        new ReadOnlySpan<double>(data)
-                    )
-                )
-                {
-                    return;
-                }
-            }
+            // Cache the active prefix by value: callers may reuse an array with a
+            // different segment count or mutate its coordinates between batches.
+            var active = data.AsSpan(0, count * 4);
+            if (_gpuMovingPrep != null && _lastMovingData != null
+                && active.SequenceEqual(_lastMovingData))
+                return;
 
             _gpuMovingRaw?.Dispose();
             _gpuMovingPrep?.Dispose();
 
-            _gpuMovingRaw = _accelerator.Allocate1D(data);
+            var snapshot = active.ToArray();
+            _gpuMovingRaw = _accelerator.Allocate1D(snapshot);
             _gpuMovingPrep = _accelerator.Allocate1D<double>(count * 10);
 
             _prepareKernel(count, _gpuMovingRaw.View, _gpuMovingPrep.View, count);
             _accelerator.Synchronize();
 
-            _lastMovingData = data;
+            _lastMovingData = snapshot;
         }
 
         private void EnsureOffsetBuffers(int offsetCount)
@@ -262,7 +317,7 @@ namespace OpenNest.Gpu
             _gpuDirs?.Dispose();
 
             _gpuOffsets = _accelerator.Allocate1D<double>(newCapacity * 2);
-            _gpuResults = _accelerator.Allocate1D<double>(newCapacity);
+            _gpuResults = _accelerator.Allocate1D<ContactWitness>(newCapacity);
             _gpuDirs = _accelerator.Allocate1D<int>(newCapacity);
 
             _offsetCapacity = newCapacity;
@@ -293,8 +348,8 @@ namespace OpenNest.Gpu
             var dy = y2 - y1;
 
             // invD is used for parameter 't'. We use a small epsilon for stability.
-            prepared[index * 10 + 4] = (XMath.Abs(dx) < 1e-9) ? 0 : 1.0 / dx;
-            prepared[index * 10 + 5] = (XMath.Abs(dy) < 1e-9) ? 0 : 1.0 / dy;
+            prepared[index * 10 + 4] = (XMath.Abs(dx) < Tolerance.Epsilon) ? 0 : 1.0 / dx;
+            prepared[index * 10 + 5] = (XMath.Abs(dy) < Tolerance.Epsilon) ? 0 : 1.0 / dy;
 
             prepared[index * 10 + 6] = XMath.Min(x1, x2);
             prepared[index * 10 + 7] = XMath.Max(x1, x2);
@@ -304,12 +359,48 @@ namespace OpenNest.Gpu
 
         // ── Main Slide Kernels ───────────────────────────────────────
 
+        // Public because ILGPU's CPU backend emits kernel argument types in a separate assembly.
+        public struct ContactWitness
+        {
+            public double Distance;
+            public double MovingX;
+            public double MovingY;
+            public double StationaryX;
+            public double StationaryY;
+        }
+
+        private static void Consider(
+            ref ContactWitness nearest,
+            double distance,
+            double vx,
+            double vy,
+            int rayDirection,
+            bool vertexMoves
+        )
+        {
+            var snapped = distance > Tolerance.Epsilon ? distance : 0;
+            if (snapped >= nearest.Distance)
+                return;
+
+            // Use the unsnapped hit for incidence; snapping a tiny gap to zero must
+            // not move the witness off the other boundary.
+            var dirX = rayDirection == 2 ? -1 : rayDirection == 3 ? 1 : 0;
+            var dirY = rayDirection == 0 ? 1 : rayDirection == 1 ? -1 : 0;
+            var hx = vx + distance * dirX;
+            var hy = vy + distance * dirY;
+            nearest.Distance = snapped;
+            nearest.MovingX = vertexMoves ? vx : hx;
+            nearest.MovingY = vertexMoves ? vy : hy;
+            nearest.StationaryX = vertexMoves ? hx : vx;
+            nearest.StationaryY = vertexMoves ? hy : vy;
+        }
+
         private static void SlideKernel(
             Index1D index,
             ArrayView1D<double, Stride1D.Dense> stationaryPrep,
             ArrayView1D<double, Stride1D.Dense> movingPrep,
             ArrayView1D<double, Stride1D.Dense> offsets,
-            ArrayView1D<double, Stride1D.Dense> results,
+            ArrayView1D<ContactWitness, Stride1D.Dense> results,
             int sCount,
             int mCount,
             int direction
@@ -337,7 +428,7 @@ namespace OpenNest.Gpu
             ArrayView1D<double, Stride1D.Dense> stationaryPrep,
             ArrayView1D<double, Stride1D.Dense> movingPrep,
             ArrayView1D<double, Stride1D.Dense> offsets,
-            ArrayView1D<double, Stride1D.Dense> results,
+            ArrayView1D<ContactWitness, Stride1D.Dense> results,
             ArrayView1D<int, Stride1D.Dense> directions,
             int sCount,
             int mCount
@@ -361,7 +452,7 @@ namespace OpenNest.Gpu
             );
         }
 
-        private static double ComputeSlideLean(
+        private static ContactWitness ComputeSlideLean(
             ArrayView1D<double, Stride1D.Dense> sPrep,
             ArrayView1D<double, Stride1D.Dense> mPrep,
             double dx,
@@ -371,20 +462,20 @@ namespace OpenNest.Gpu
             int direction
         )
         {
-            const double eps = 0.00001;
-            var minDist = double.MaxValue;
+            const double eps = Tolerance.Epsilon;
+            var nearest = new ContactWitness { Distance = double.MaxValue };
             var horizontal = direction >= 2;
             var oppDir = direction ^ 1;
 
             // ── Forward Pass: moving vertices vs stationary edges ─────
-            for (int i = 0; i < mCount; i++)
+            for (var i = 0; i < mCount; i++)
             {
                 var m1x = mPrep[i * 10 + 0] + dx;
                 var m1y = mPrep[i * 10 + 1] + dy;
                 var m2x = mPrep[i * 10 + 2] + dx;
                 var m2y = mPrep[i * 10 + 3] + dy;
 
-                for (int j = 0; j < sCount; j++)
+                for (var j = 0; j < sCount; j++)
                 {
                     var sMin = horizontal ? sPrep[j * 10 + 8] : sPrep[j * 10 + 6];
                     var sMax = horizontal ? sPrep[j * 10 + 9] : sPrep[j * 10 + 7];
@@ -394,8 +485,8 @@ namespace OpenNest.Gpu
                     if (mv1 >= sMin - eps && mv1 <= sMax + eps)
                     {
                         var d = RayEdgeLean(m1x, m1y, sPrep, j, direction, eps);
-                        if (d < minDist)
-                            minDist = d;
+                        Consider(ref nearest, d, m1x, m1y,
+                            direction, vertexMoves: true);
                     }
 
                     // Test moving vertex 2 against stationary edge j
@@ -403,21 +494,21 @@ namespace OpenNest.Gpu
                     if (mv2 >= sMin - eps && mv2 <= sMax + eps)
                     {
                         var d = RayEdgeLean(m2x, m2y, sPrep, j, direction, eps);
-                        if (d < minDist)
-                            minDist = d;
+                        Consider(ref nearest, d, m2x, m2y,
+                            direction, vertexMoves: true);
                     }
                 }
             }
 
             // ── Reverse Pass: stationary vertices vs moving edges ─────
-            for (int i = 0; i < sCount; i++)
+            for (var i = 0; i < sCount; i++)
             {
                 var s1x = sPrep[i * 10 + 0];
                 var s1y = sPrep[i * 10 + 1];
                 var s2x = sPrep[i * 10 + 2];
                 var s2y = sPrep[i * 10 + 3];
 
-                for (int j = 0; j < mCount; j++)
+                for (var j = 0; j < mCount; j++)
                 {
                     var mMin = horizontal ? (mPrep[j * 10 + 8] + dy) : (mPrep[j * 10 + 6] + dx);
                     var mMax = horizontal ? (mPrep[j * 10 + 9] + dy) : (mPrep[j * 10 + 7] + dx);
@@ -427,8 +518,8 @@ namespace OpenNest.Gpu
                     if (sv1 >= mMin - eps && sv1 <= mMax + eps)
                     {
                         var d = RayEdgeLeanMoving(s1x, s1y, mPrep, j, dx, dy, oppDir, eps);
-                        if (d < minDist)
-                            minDist = d;
+                        Consider(ref nearest, d, s1x, s1y,
+                            oppDir, vertexMoves: false);
                     }
 
                     // Test stationary vertex 2 against moving edge j
@@ -436,13 +527,13 @@ namespace OpenNest.Gpu
                     if (sv2 >= mMin - eps && sv2 <= mMax + eps)
                     {
                         var d = RayEdgeLeanMoving(s2x, s2y, mPrep, j, dx, dy, oppDir, eps);
-                        if (d < minDist)
-                            minDist = d;
+                        Consider(ref nearest, d, s2x, s2y,
+                            oppDir, vertexMoves: false);
                     }
                 }
             }
 
-            return minDist;
+            return nearest;
         }
 
         private static double RayEdgeLean(
@@ -472,9 +563,7 @@ namespace OpenNest.Gpu
                 var ix = p1x + t * (p2x - p1x);
                 var dist = (direction == 2) ? (vx - ix) : (ix - vx);
 
-                if (dist > eps)
-                    return dist;
-                return (dist >= -eps) ? 0.0 : double.MaxValue;
+                return dist >= -eps ? dist : double.MaxValue;
             }
             else // Vertical (Up=0, Down=1)
             {
@@ -489,9 +578,7 @@ namespace OpenNest.Gpu
                 var iy = p1y + t * (p2y - p1y);
                 var dist = (direction == 1) ? (vy - iy) : (iy - vy);
 
-                if (dist > eps)
-                    return dist;
-                return (dist >= -eps) ? 0.0 : double.MaxValue;
+                return dist >= -eps ? dist : double.MaxValue;
             }
         }
 
@@ -524,9 +611,7 @@ namespace OpenNest.Gpu
                 var ix = p1x + t * (p2x - p1x);
                 var dist = (direction == 2) ? (vx - ix) : (ix - vx);
 
-                if (dist > eps)
-                    return dist;
-                return (dist >= -eps) ? 0.0 : double.MaxValue;
+                return dist >= -eps ? dist : double.MaxValue;
             }
             else // Vertical
             {
@@ -541,9 +626,7 @@ namespace OpenNest.Gpu
                 var iy = p1y + t * (p2y - p1y);
                 var dist = (direction == 1) ? (vy - iy) : (iy - vy);
 
-                if (dist > eps)
-                    return dist;
-                return (dist >= -eps) ? 0.0 : double.MaxValue;
+                return dist >= -eps ? dist : double.MaxValue;
             }
         }
 

OpenNest.Tests/CNC/RapidEnumeratorTests.cs

@@ -54,6 +54,28 @@ namespace OpenNest.Tests.CNC
             Assert.Equal(new Vector(106, 206), segments[1].To);
         }
 
+        [Fact]
+        public void Enumerate_IncrementalProgramWithNonZeroFirstRapid_DoesNotDoubleCountIt()
+        {
+            // Lead-in programs (ContourCuttingStrategy output) are Incremental and
+            // start with a nonzero rapid from the part origin to the first pierce.
+            var pgm = new Program(Mode.Incremental);
+            pgm.Codes.Add(new RapidMove(2, 3));
+            pgm.Codes.Add(new LinearMove(5, 0));
+            pgm.Codes.Add(new RapidMove(1, 1));
+
+            var segments = RapidEnumerator.Enumerate(
+                pgm,
+                basePos: new Vector(100, 200),
+                startPos: new Vector(0, 0)
+            );
+
+            Assert.Equal(2, segments.Count);
+            Assert.Equal(new Vector(102, 203), segments[0].To);
+            Assert.Equal(new Vector(107, 203), segments[1].From);
+            Assert.Equal(new Vector(108, 204), segments[1].To);
+        }
+
         [Fact]
         public void Enumerate_SubProgramCall_RapidEndsAtAbsoluteHolePierce()
         {

OpenNest.Tests/CincinnatiCIFiber/CIFiberPostProcessorTests.cs

@@ -222,6 +222,35 @@ public class CIFiberPostProcessorTests
         Assert.Contains("13.2.4.1", ex.Message);
     }
 
+    [Fact]
+    public void Post_CutOff_PostsLastWithoutCompensationOrLeadIn()
+    {
+        var nest = MakeSquareWithHoleNest();
+        var plate = nest.Plates[0];
+
+        // Vertical cut-off at X=20: a bare rapid + line, no lead-in.
+        var cutOff = new CutOff(new Vector(20, 0), CutOffAxis.Vertical);
+        cutOff.Regenerate(plate, new CutOffSettings());
+        plate.Parts.Insert(0, new Part(cutOff.Drawing, Vector.Zero));
+
+        var lines = Lines(Post(nest));
+
+        // Cut-off follows the real part despite being first in plate order.
+        var partEnd = lines.IndexOf("( PART END )");
+        var cutOffPart = lines.IndexOf("( Part #2 )");
+        Assert.True(partEnd >= 0 && cutOffPart > partEnd);
+
+        var n3 = lines.IndexOf("N3:");
+        Assert.True(n3 > cutOffPart);
+        Assert.Equal("/L \"L0\"", lines[n3 + 1]);
+        Assert.Equal("V.E.R3=3", lines[n3 + 2]);
+        Assert.StartsWith("G0X20Y", lines[n3 + 3]);
+        Assert.Equal("/L \"L4\"", lines[n3 + 4]);
+        Assert.Equal("/L \"L6\"", lines[n3 + 5]);
+        Assert.StartsWith("G1X20Y", lines[n3 + 6]);
+        Assert.Equal("/L \"ZHSOFF\"", lines[n3 + 7]);
+    }
+
     [Fact]
     public void Post_ValidatesTableSize()
     {

OpenNest.Tests/Data/NestDefaultsTests.cs

@@ -0,0 +1,209 @@
+using OpenNest.Data;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Data;
+
+public class NestDefaultsTests : IDisposable
+{
+    private readonly string _testDir;
+    private readonly string _path;
+
+    public NestDefaultsTests()
+    {
+        _testDir = Path.Combine(Path.GetTempPath(), "OpenNestTests", Guid.NewGuid().ToString());
+        Directory.CreateDirectory(_testDir);
+        _path = Path.Combine(_testDir, "defaults.json");
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_testDir))
+            Directory.Delete(_testDir, true);
+    }
+
+    [Fact]
+    public void Save_ThenLoad_RoundTrips()
+    {
+        var original = new NestDefaults
+        {
+            Units = Units.Millimeters,
+            Size = new Size(1220, 2440),
+            Quadrant = 3,
+            PartSpacing = 2.5,
+            EdgeSpacing = new Spacing(1.5, 3, 1.5, 3),
+        };
+
+        original.Save(_path);
+        var loaded = NestDefaults.Load(_path, out var status);
+
+        Assert.Equal(NestDefaultsStatus.Ok, status);
+        Assert.Equal(Units.Millimeters, loaded.Units);
+        Assert.Equal(original.Size, loaded.Size);
+        Assert.Equal(3, loaded.Quadrant);
+        Assert.Equal(2.5, loaded.PartSpacing);
+        Assert.Equal(original.EdgeSpacing.Left, loaded.EdgeSpacing.Left);
+        Assert.Equal(original.EdgeSpacing.Bottom, loaded.EdgeSpacing.Bottom);
+        Assert.Equal(original.EdgeSpacing.Right, loaded.EdgeSpacing.Right);
+        Assert.Equal(original.EdgeSpacing.Top, loaded.EdgeSpacing.Top);
+    }
+
+    [Fact]
+    public void Save_CreatesMissingDirectory()
+    {
+        var nested = Path.Combine(_testDir, "nested", "defaults.json");
+        new NestDefaults().Save(nested);
+        Assert.True(File.Exists(nested));
+    }
+
+    [Fact]
+    public void Load_MissingFile_ReturnsFallback()
+    {
+        var loaded = NestDefaults.Load(_path, out var status);
+
+        Assert.Equal(NestDefaultsStatus.Missing, status);
+        AssertFallback(loaded);
+    }
+
+    [Fact]
+    public void Load_CorruptJson_ReturnsFallbackButReportsInvalid()
+    {
+        File.WriteAllText(_path, "{ this is not json");
+
+        var loaded = NestDefaults.Load(_path, out var status);
+
+        Assert.Equal(NestDefaultsStatus.Invalid, status);
+        AssertFallback(loaded);
+    }
+
+    [Fact]
+    public void Load_PartialFile_MergesPerField()
+    {
+        File.WriteAllText(_path, """{ "units": "millimeters", "partSpacing": 4 }""");
+
+        var loaded = NestDefaults.Load(_path, out var status);
+
+        Assert.Equal(NestDefaultsStatus.Ok, status);
+        Assert.Equal(Units.Millimeters, loaded.Units);
+        Assert.Equal(4, loaded.PartSpacing);
+        // Untouched fields keep fallback values.
+        Assert.Equal(new Size(100, 100), loaded.Size);
+        Assert.Equal(1, loaded.Quadrant);
+        Assert.Equal(new Spacing(1, 1, 1, 1), loaded.EdgeSpacing);
+    }
+
+    [Fact]
+    public void Load_OutOfRangeValues_FallBackPerField()
+    {
+        File.WriteAllText(
+            _path,
+            """
+            {
+              "units": "furlongs",
+              "size": { "width": -50, "length": 100 },
+              "quadrant": 9,
+              "partSpacing": -1,
+              "edgeSpacing": { "left": 1, "bottom": -2, "right": 1, "top": 1 }
+            }
+            """
+        );
+
+        var loaded = NestDefaults.Load(_path, out var status);
+
+        // The file parses; only the invalid values fall back, so no warning.
+        Assert.Equal(NestDefaultsStatus.Ok, status);
+        Assert.Equal(Units.Inches, loaded.Units);
+        Assert.Equal(new Size(100, 100), loaded.Size);
+        Assert.Equal(1, loaded.Quadrant);
+        Assert.Equal(1, loaded.PartSpacing);
+        Assert.Equal(new Spacing(1, 1, 1, 1), loaded.EdgeSpacing);
+    }
+
+    [Fact]
+    public void Load_NonFiniteValues_FallBack()
+    {
+        File.WriteAllText(_path, """{ "partSpacing": 1e400 }""");
+
+        var loaded = NestDefaults.Load(_path);
+
+        // 1e400 deserializes to Infinity, which is rejected.
+        Assert.Equal(1, loaded.PartSpacing);
+    }
+
+    [Fact]
+    public void Load_UnknownFieldsAndFutureVersion_Ignored()
+    {
+        File.WriteAllText(
+            _path,
+            """
+            {
+              "version": 2,
+              "units": "inches",
+              "partSpacing": 2,
+              "futureFeature": { "enabled": true },
+              "material": { "name": "steel" }
+            }
+            """
+        );
+
+        var loaded = NestDefaults.Load(_path, out var status);
+
+        Assert.Equal(NestDefaultsStatus.Ok, status);
+        Assert.Equal(2, loaded.PartSpacing);
+    }
+
+    [Fact]
+    public void FromNest_CapturesUnitsAndPlateDefaults()
+    {
+        var nest = new Nest
+        {
+            Units = Units.Millimeters,
+        };
+        nest.PlateDefaults.Size = new Size(60, 120);
+        nest.PlateDefaults.Quadrant = 2;
+        nest.PlateDefaults.PartSpacing = 0.5;
+        nest.PlateDefaults.EdgeSpacing = new Spacing(2, 2, 2, 2);
+
+        var captured = NestDefaults.FromNest(nest);
+
+        Assert.Equal(Units.Millimeters, captured.Units);
+        Assert.Equal(new Size(60, 120), captured.Size);
+        Assert.Equal(2, captured.Quadrant);
+        Assert.Equal(0.5, captured.PartSpacing);
+        Assert.Equal(new Spacing(2, 2, 2, 2), captured.EdgeSpacing);
+    }
+
+    [Fact]
+    public void ApplyTo_SetsUnitsAndPlateDefaults_AndDoesNotAliasSourceNest()
+    {
+        var source = new Nest();
+        source.PlateDefaults.Size = new Size(48, 96);
+        source.PlateDefaults.EdgeSpacing = new Spacing(1.25, 1.25, 1.25, 1.25);
+        var defaults = NestDefaults.FromNest(source);
+
+        // Mutating the source nest afterwards must not change the capture.
+        source.PlateDefaults.Size = new Size(1, 1);
+        source.PlateDefaults.EdgeSpacing = new Spacing(9, 9, 9, 9);
+
+        var target = new Nest();
+        defaults.ApplyTo(target);
+
+        Assert.Equal(new Size(48, 96), target.PlateDefaults.Size);
+        Assert.Equal(
+            new Spacing(1.25, 1.25, 1.25, 1.25),
+            target.PlateDefaults.EdgeSpacing
+        );
+
+        // And the applied target owns its own values too.
+        target.PlateDefaults.Size = new Size(2, 2);
+        Assert.Equal(new Size(48, 96), defaults.Size);
+    }
+
+    private static void AssertFallback(NestDefaults loaded)
+    {
+        Assert.Equal(Units.Inches, loaded.Units);
+        Assert.Equal(new Size(100, 100), loaded.Size);
+        Assert.Equal(1, loaded.Quadrant);
+        Assert.Equal(1, loaded.PartSpacing);
+        Assert.Equal(new Spacing(1, 1, 1, 1), loaded.EdgeSpacing);
+    }
+}

OpenNest.Tests/Fill/CompactorTests.cs

@@ -298,22 +298,111 @@ namespace OpenNest.Tests.Fill
             Assert.Equal(32, moving.BoundingBox.Left, 7);
         }
 
+        [Theory]
+        [InlineData(PushDirection.Right)]
+        [InlineData(PushDirection.Up)]
+        [InlineData(PushDirection.Down)]
+        public void Push_WithSpacing_ContactFromPreviousPushDoesNotBlockOtherDirections(
+            PushDirection next
+        )
+        {
+            var workArea = new Box(0, 0, 100, 100);
+            var obstacle = MakeRectPart(20, 40, 10, 10);
+            var moving = MakeRectPart(60, 40, 10, 10);
+            var parts = new List<Part> { moving };
+            var obstacles = new List<Part> { obstacle };
+
+            Assert.True(Compactor.Push(parts, obstacles, workArea, 2, PushDirection.Left) > 0);
+            Assert.Equal(32, moving.BoundingBox.Left, 7);
+
+            var before = moving.Location;
+            var distance = Compactor.Push(parts, obstacles, workArea, 2, next);
+
+            Assert.True(distance > 1, $"Push {next} after contact moved only {distance:R}");
+            Assert.NotEqual(before, moving.Location);
+            AssertClearance(moving, obstacle, 2);
+        }
+
+        [Fact]
+        public void Push_WithSpacing_ContactStillBlocksTheSameDirection()
+        {
+            var workArea = new Box(0, 0, 100, 100);
+            var obstacle = MakeRectPart(20, 40, 10, 10);
+            var moving = MakeRectPart(60, 40, 10, 10);
+            var parts = new List<Part> { moving };
+            var obstacles = new List<Part> { obstacle };
+
+            Compactor.Push(parts, obstacles, workArea, 2, PushDirection.Left);
+            var distance = Compactor.Push(parts, obstacles, workArea, 2, PushDirection.Left);
+
+            Assert.Equal(0, distance);
+            Assert.Equal(32, moving.BoundingBox.Left, 7);
+            AssertClearance(moving, obstacle, 2);
+        }
+
+        [Theory]
+        [InlineData(0)]
+        [InlineData(2)]
+        public void Push_SlidingAlongWall_StopsAtItsLaterHook(double spacing)
+        {
+            var hook = MakeTrianglePart(new Vector(10, 10), new Vector(20, 10),
+                new Vector(20, 50), new Vector(50, 50), new Vector(50, 60), new Vector(10, 60));
+            var moving = MakeRectPart(20 + spacing, 20, 5, 5);
+            var distance = Compactor.Push(new List<Part> { moving }, new List<Part> { hook },
+                new Box(0, 0, 100, 100), spacing, PushDirection.Up);
+            Assert.Equal(25 - spacing, distance, 7);
+            Assert.False(moving.Intersects(hook, out _));
+            if (spacing > 0)
+                AssertClearance(moving, hook, spacing);
+        }
+
+        [Theory]
+        [InlineData(0, false)]
+        [InlineData(2, false)]
+        [InlineData(0, true)]
+        [InlineData(2, true)]
+        public void Push_InsideStationaryHole_CannotPassThroughItsWall(double spacing, bool plateEntry)
+        {
+            var program = MakeRectDrawing(60, 60).Program;
+            program.Codes.Add(new OpenNest.CNC.RapidMove(new Vector(10, 10)));
+            program.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(50, 10)));
+            program.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(50, 50)));
+            program.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(10, 50)));
+            program.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(10, 10)));
+            var frame = new Part(new Drawing("frame", program));
+            var moving = MakeRectPart(10 + spacing, 20, 5, 5);
+            var plate = new Plate(100, 100) { PartSpacing = spacing };
+            plate.Parts.Add(frame);
+            plate.Parts.Add(moving);
+            var distance = plateEntry
+                ? Compactor.Push(new List<Part> { moving }, plate, PushDirection.Right)
+                : Compactor.Push(new List<Part> { moving }, new List<Part> { frame },
+                    new Box(0, 0, 100, 100), spacing, PushDirection.Right);
+            Assert.Equal(35 - 2 * spacing, distance, 7);
+            // Independent rectangular-hole oracle; Part.Intersects ignores cutouts.
+            Assert.Equal(50 - spacing, moving.BoundingBox.Right, 7);
+            Assert.InRange(moving.BoundingBox.Bottom, 10 + spacing, 50 - spacing);
+            Assert.InRange(moving.BoundingBox.Top, 10 + spacing, 50 - spacing);
+            if (spacing > 0)
+                AssertClearance(moving, frame, spacing);
+        }
+
         private static void AssertClearance(Part moving, Part obstacle, double spacing)
         {
             var clearance = double.MaxValue;
             foreach (var a in PartGeometry.GetPartLines(moving))
-            foreach (var b in PartGeometry.GetPartLines(obstacle))
-            {
-                Assert.False(Intersect.Intersects(a, b, out _));
-                clearance = System.Math.Min(
-                    clearance,
-                    a.StartPoint.DistanceTo(b.ClosestPointTo(a.StartPoint))
-                );
-                clearance = System.Math.Min(
-                    clearance,
-                    b.StartPoint.DistanceTo(a.ClosestPointTo(b.StartPoint))
-                );
-            }
+                foreach (var b in PartGeometry.GetPartLines(obstacle))
+                {
+                    Assert.False(Intersect.Intersects(a, b, out _));
+                    clearance = System.Math.Min(
+                        clearance,
+                        a.StartPoint.DistanceTo(b.ClosestPointTo(a.StartPoint))
+                    );
+                    clearance = System.Math.Min(
+                        clearance,
+                        b.StartPoint.DistanceTo(a.ClosestPointTo(b.StartPoint))
+                    );
+                }
             Assert.True(
                 clearance >= spacing - 1e-7,
                 $"Clearance {clearance:R} is less than spacing {spacing:R}"

OpenNest.Tests/Geometry/ClearanceTests.cs

@@ -0,0 +1,286 @@
+using System;
+using System.Collections.Generic;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Geometry;
+
+public class ClearanceTests
+{
+    private const double Tol = 1e-9;
+
+    private static Polygon Square(double x, double y, double w, double h)
+    {
+        var p = new Polygon
+        {
+            Vertices = new List<Vector>
+            {
+                new Vector(x, y),
+                new Vector(x + w, y),
+                new Vector(x + w, y + h),
+                new Vector(x, y + h),
+            }
+        };
+        p.Close();
+        p.UpdateBounds();
+        return p;
+    }
+
+    private static Polygon Triangle(params double[] xy)
+    {
+        var p = new Polygon();
+        for (var i = 0; i + 1 < xy.Length; i += 2)
+            p.Vertices.Add(new Vector(xy[i], xy[i + 1]));
+        p.Close();
+        p.UpdateBounds();
+        return p;
+    }
+
+    // ---- Separation ----
+
+    [Fact]
+    public void Between_SeparatedHorizontally_DistanceAndDirection()
+    {
+        var a = Square(0, 0, 1, 1);
+        var b = Square(3, 0, 1, 1);
+
+        var r = Clearance.Between(a, b);
+
+        Assert.Equal(2.0, r.Distance, 6);
+        // Pushing a away from b means moving left.
+        Assert.Equal(-1.0, r.Direction.X, 6);
+        Assert.Equal(0.0, r.Direction.Y, 6);
+    }
+
+    [Fact]
+    public void Between_SeparatedDiagonally_CornerDistance()
+    {
+        var a = Square(0, 0, 1, 1);
+        var b = Square(2, 2, 1, 1);
+
+        var r = Clearance.Between(a, b);
+
+        Assert.Equal(System.Math.Sqrt(2.0), r.Distance, 6);
+        Assert.Equal(-1 / System.Math.Sqrt(2), r.Direction.X, 6);
+        Assert.Equal(-1 / System.Math.Sqrt(2), r.Direction.Y, 6);
+    }
+
+    [Fact]
+    public void Between_Touching_ZeroDistance()
+    {
+        var a = Square(0, 0, 1, 1);
+        var b = Square(1, 0, 2, 1);
+
+        var r = Clearance.Between(a, b);
+
+        Assert.True(System.Math.Abs(r.Distance) < 1e-6, $"expected ~0, got {r.Distance}");
+        var mag = System.Math.Sqrt(
+            r.Direction.X * r.Direction.X + r.Direction.Y * r.Direction.Y
+        );
+        Assert.Equal(1.0, mag, 6);
+    }
+
+    [Fact]
+    public void Between_VertexToEdge_DistanceIsPerpendicular()
+    {
+        // Triangle above a wide square; the base sits 3 above the square's top edge.
+        var a = Triangle(1, 3, 3, 3, 2, 4);
+        var b = Square(0, -4, 10, 4); // top edge at y = 0
+
+        var r = Clearance.Between(a, b);
+
+        Assert.Equal(3.0, r.Distance, 6); // base y=3 to y=0
+        Assert.Equal(0.0, r.Direction.X, 6);
+        Assert.Equal(1.0, r.Direction.Y, 6);
+    }
+
+    [Fact]
+    public void Between_ParallelStaggeredEdges_MinimumAcrossAllPairs()
+    {
+        // Two L-ish shapes (as simple polys) offset so the true minimum is
+        // between mid-edges, not vertices.
+        var a = Square(0, 0, 4, 1);
+        var b = Square(1, 2, 1, 3);
+
+        var r = Clearance.Between(a, b);
+
+        Assert.Equal(1.0, r.Distance, 6);
+        Assert.Equal(-1.0, r.Direction.Y, 6);
+    }
+
+    // ---- Penetration ----
+
+    [Fact]
+    public void Between_OverlappingSquares_MinimumTranslationAxis()
+    {
+        // Overlap 0.5 in X, 1.0 in Y -> cheapest exit is X.
+        var a = Square(0, 0, 1, 1);
+        var b = Square(0.5, 0, 1.5, 1);
+
+        var r = Clearance.Between(a, b);
+
+        Assert.Equal(-0.5, r.Distance, 6);
+        Assert.Equal(-1.0, r.Direction.X, 6); // push a left, out of b
+        Assert.Equal(0.0, r.Direction.Y, 6);
+    }
+
+    [Fact]
+    public void Between_OverlappingVerticallyCheaper_ExitsInY()
+    {
+        // Overlap 0.8 in X, 0.2 in Y -> cheapest exit is Y.
+        var a = Square(0, 0, 1, 1);
+        var b = Square(0.2, 0.8, 1.2, 1.8);
+
+        var r = Clearance.Between(a, b);
+
+        Assert.Equal(-0.2, r.Distance, 6);
+        Assert.Equal(0.0, r.Direction.X, 6);
+        Assert.Equal(-1.0, r.Direction.Y, 6);
+    }
+
+    [Fact]
+    public void Between_ContainedSquare_ExitsThroughNearestWall()
+    {
+        // Inner square near the left wall: the translation that ENDS the overlap
+        // carries its right edge (x=1.2) past the outer's left edge (x=0).
+        var outer = Square(0, 0, 10, 10);
+        var inner = Square(0.2, 4, 1, 1);
+
+        var r = Clearance.Between(inner, outer);
+
+        Assert.Equal(-1.2, r.Distance, 6);
+        Assert.Equal(-1.0, r.Direction.X, 6);
+    }
+
+    [Fact]
+    public void Between_ConcentricSquares_DepthIsExitTranslation()
+    {
+        var outer = Square(0, 0, 10, 10);
+        var inner = Square(2, 2, 4, 4); // spans [2,6]; leftmost exit carries 6 to 0
+
+        var r = Clearance.Between(inner, outer);
+
+        Assert.Equal(-6.0, r.Distance, 6);
+        Assert.Equal(0.0, r.Direction.X, 6);
+        Assert.Equal(-1.0, r.Direction.Y, 6);
+    }
+
+    [Fact]
+    public void Between_TrianglesPenetrating_ReportsNegativeDepth()
+    {
+        var a = Triangle(0, 0, 4, 0, 2, 3);
+        var b = Triangle(1, 0, 5, 0, 3, 3);
+
+        var r = Clearance.Between(a, b);
+
+        Assert.True(r.Distance < 0, $"expected penetration, got {r.Distance}");
+    }
+
+    // ---- Direction is actionable: moving a by -Distance * dir clears contact ----
+
+    [Fact]
+    public void Between_PenetrationApplyingDirection_EndsContact()
+    {
+        var a = Square(0, 0, 1, 1);
+        var b = Square(0.3, 0, 1.6, 2);
+
+        var r = Clearance.Between(a, b);
+
+        var moved = (Polygon)a.Clone();
+        moved.Offset(r.Direction * (-r.Distance + 0.001));
+        moved.UpdateBounds();
+
+        Assert.False(Collision.HasOverlap(moved, b));
+    }
+
+    [Fact]
+    public void Between_SeparationApplyingDirection_NeverReducesDistance()
+    {
+        var a = Square(0, 0, 1, 1);
+        var b = Square(4, 1, 2, 2);
+
+        var r = Clearance.Between(a, b);
+        Assert.True(r.Distance > 0);
+
+        // A tiny step along the reported direction must not move closer.
+        var moved = (Polygon)a.Clone();
+        moved.Offset(r.Direction * (r.Distance / 2));
+        moved.UpdateBounds();
+
+        var r2 = Clearance.Between(moved, b);
+        Assert.True(
+            r2.Distance >= r.Distance - Tol,
+            $"moving along dir reduced clearance {r.Distance} -> {r2.Distance}"
+        );
+    }
+
+    // ---- Determinism ----
+
+    [Fact]
+    public void Between_RepeatedCalls_IdenticalResult()
+    {
+        var a = Square(0, 0, 1, 1);
+        var b = Square(0.5, 0.25, 2, 1.5);
+
+        var r1 = Clearance.Between(a, b);
+        var r2 = Clearance.Between(a, b);
+
+        Assert.Equal(r1.Distance, r2.Distance);
+        Assert.Equal(r1.Direction.X, r2.Direction.X);
+        Assert.Equal(r1.Direction.Y, r2.Direction.Y);
+    }
+
+    [Fact]
+    public void Between_SymmetricSwap_MirrorsDirection()
+    {
+        var a = Square(0, 0, 1, 1);
+        var b = Square(0.5, 0, 1.5, 1);
+
+        var ab = Clearance.Between(a, b);
+        var ba = Clearance.Between(b, a);
+
+        Assert.Equal(ab.Distance, ba.Distance, 6);
+        Assert.Equal(-ab.Direction.X, ba.Direction.X, 6);
+        Assert.Equal(-ab.Direction.Y, ba.Direction.Y, 6);
+    }
+
+    // ---- Agreement with the Collision oracle ----
+
+    [Fact]
+    public void Between_SignMatchesCollisionVerdict()
+    {
+        var polygons = new List<Polygon>
+        {
+            Square(0, 0, 1, 1),
+            Square(1, 0, 2, 1),
+            Square(0.5, 0, 1.5, 1),
+            Square(0.25, 0.25, 0.75, 0.75),
+            Square(5, 5, 6, 6),
+            Triangle(0, 0, 2, 0, 1, 2),
+            Triangle(0.5, -1, 2.5, -1, 1.5, 1),
+        };
+
+        for (var i = 0; i < polygons.Count; i++)
+        {
+            for (var j = i + 1; j < polygons.Count; j++)
+            {
+                var overlaps = Collision.HasOverlap(polygons[i], polygons[j]);
+                var r = Clearance.Between(polygons[i], polygons[j]);
+
+                if (overlaps)
+                {
+                    Assert.True(
+                        r.Distance <= Tol,
+                        $"pair {i},{j}: Collision overlaps but clearance {r.Distance}"
+                    );
+                }
+                else
+                {
+                    Assert.True(
+                        r.Distance >= -Tol,
+                        $"pair {i},{j}: Collision clear but clearance {r.Distance}"
+                    );
+                }
+            }
+        }
+    }
+}

OpenNest.Tests/Geometry/SlideContactTests.cs

@@ -0,0 +1,241 @@
+using OpenNest.Engine.BestFit;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Geometry;
+
+public class SlideContactTests
+{
+    public static IEnumerable<object[]> LinePaths()
+    {
+        foreach (var path in new[] { "axis", "offset", "edges", "vector", "entities", "cpu-lines", "cpu-entities" })
+            foreach (var reverse in new[] { false, true })
+                yield return new object[] { path, reverse };
+    }
+
+    [Theory]
+    [MemberData(nameof(LinePaths))]
+    public void TouchingRectangles_LeaveOrSlideButCannotEnter(string path, bool reverse)
+    {
+        var stationary = Rect(0, 0, 2, 2);
+        var moving = Rect(2, 0, 2, 2);
+        if (reverse)
+        {
+            Reverse(stationary);
+            Reverse(moving);
+        }
+        Assert.Equal(0, Distance(path, moving, stationary, PushDirection.Left));
+        Assert.Equal(double.MaxValue, Distance(path, moving, stationary, PushDirection.Right));
+        Assert.Equal(double.MaxValue, Distance(path, moving, stationary, PushDirection.Up));
+        Assert.Equal(double.MaxValue, Distance(path, moving, stationary, PushDirection.Down));
+    }
+
+    [Theory]
+    [MemberData(nameof(LinePaths))]
+    public void SlidingContact_StillStopsAtLaterHookOnSameObstacle(string path, bool reverse)
+    {
+        var stationary = Loop((0, 0), (2, 0), (2, 4), (5, 4), (5, 6), (0, 6));
+        var moving = Rect(2, 0, 1, 1);
+        if (reverse)
+        {
+            Reverse(stationary);
+            Reverse(moving);
+        }
+        Assert.Equal(3, Distance(path, moving, stationary, PushDirection.Up), 9);
+    }
+
+    [Theory]
+    [MemberData(nameof(LinePaths))]
+    public void HoleContact_LeavingWallStillStopsAtOppositeWall(string path, bool reverse)
+    {
+        var stationary = Rect(0, 0, 10, 10);
+        stationary.AddRange(Rect(2, 2, 6, 6)); // depth, not winding, defines the hole
+        var moving = Rect(2, 3, 1, 1);
+        if (reverse)
+        {
+            Reverse(stationary);
+            Reverse(moving);
+        }
+        Assert.Equal(0, Distance(path, moving, stationary, PushDirection.Left));
+        Assert.Equal(5, Distance(path, moving, stationary, PushDirection.Right), 9);
+        Assert.Equal(4, Distance(path, moving, stationary, PushDirection.Up), 9);
+    }
+
+    [Theory]
+    [InlineData(false, 0)]
+    [InlineData(true, 0)]
+    [InlineData(false, 0.37)]
+    [InlineData(true, 0.37)]
+    public void RotatedHook_StopsAtFirstBlockingContact(bool cpu, double angle)
+    {
+        var stationary = Loop((0, 0), (2, 0), (2, 4), (5, 4), (5, 6), (0, 6)).Cast<Entity>().ToList();
+        var moving = Rect(2, 0, 1, 1).Cast<Entity>().ToList();
+        foreach (var entity in stationary.Concat(moving))
+        {
+            entity.Rotate(angle);
+            entity.Offset(17, -23);
+        }
+        Assert.Equal(3, EntityDistance(cpu, moving, stationary, new Vector(0, 1).Rotate(angle)), 8);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void Circles_TangentEscapeAndEnteringContact(bool cpu)
+    {
+        var moving = new List<Entity> { new Circle(2, 0, 1) };
+        var stationary = new List<Entity> { new Circle(0, 0, 1) };
+        Assert.Equal(0, EntityDistance(cpu, moving, stationary, new Vector(-1, 0)));
+        Assert.Equal(double.MaxValue, EntityDistance(cpu, moving, stationary, new Vector(1, 0)));
+        Assert.Equal(double.MaxValue, EntityDistance(cpu, moving, stationary, new Vector(0, 1)));
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void CircleInsideHole_TangentBlocksButDepartureFindsFarSide(bool cpu)
+    {
+        var moving = new List<Entity> { new Circle(3, 0, 1) };
+        var stationary = new List<Entity> { new Circle(0, 0, 6), new Circle(0, 0, 4) };
+        Assert.Equal(0, EntityDistance(cpu, moving, stationary, new Vector(1, 0)));
+        Assert.Equal(0, EntityDistance(cpu, moving, stationary, new Vector(0, 1)));
+        Assert.Equal(6, EntityDistance(cpu, moving, stationary, new Vector(-1, 0)), 9);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void PositiveGrazingContact_DoesNotHideLaterCircle(bool cpu)
+    {
+        var moving = new List<Entity> { new Circle(0, 0, 1) };
+        var stationary = new List<Entity> { new Circle(4, 2, 1), new Circle(10, 0, 1) };
+        Assert.Equal(8, EntityDistance(cpu, moving, stationary, new Vector(1, 0)), 9);
+    }
+
+    [Fact]
+    public void ReusedEdgeArrays_KeepTopologyAfterPreviousQuerySortedThem()
+    {
+        var moving = Rect(2, 0, 2, 2).Select(l => (l.StartPoint, l.EndPoint)).ToArray();
+        var stationary = Rect(0, 0, 2, 2).Select(l => (l.StartPoint, l.EndPoint)).ToArray();
+        Assert.Equal(0, SpatialQuery.DirectionalDistance(moving, Vector.Zero, stationary, Vector.Zero, PushDirection.Left));
+        Assert.Equal(double.MaxValue, SpatialQuery.DirectionalDistance(moving, Vector.Zero, stationary, Vector.Zero, PushDirection.Right));
+        Assert.Equal(double.MaxValue, SpatialQuery.DirectionalDistance(moving, Vector.Zero, stationary, Vector.Zero, PushDirection.Up));
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void FullCircleArcSeam_AllowsSeparation(bool cpu)
+    {
+        var moving = new List<Entity> { new Arc(0, 0, 1, 0, 2 * System.Math.PI) };
+        var stationary = new List<Entity> { new Circle(2, 0, 1) };
+        Assert.Equal(double.MaxValue, EntityDistance(cpu, moving, stationary, new Vector(-1, 0)));
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void ConcaveCorner_DoesNotBlockSlidingAlongItsStraightSide(bool cpu)
+    {
+        var moving = Rect(3, -1, 2, 1).Cast<Entity>().ToList();
+        var stationary = new List<Entity>
+        {
+            new Arc(0, 0, 5, System.Math.PI / 2, 0, true),
+            new Line(5, 0, 10, 0),
+            new Arc(0, 0, 10, 0, System.Math.PI / 2),
+            new Line(0, 10, 0, 5),
+        };
+        Assert.Equal(double.MaxValue, EntityDistance(cpu, moving, stationary, new Vector(1, 0)));
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void ThinRing_ArcSeamDoesNotChangeHoleClassification(bool cpu)
+    {
+        var a = System.Math.PI / 72;
+        var moving = new List<Entity> { new Circle(8.995, 0, 1) };
+        var stationary = new List<Entity>
+        {
+            new Circle(0, 0, 10),
+            new Arc(0, 0, 9.995, a, a + System.Math.PI),
+            new Arc(0, 0, 9.995, a + System.Math.PI, a + 2 * System.Math.PI),
+        };
+        Assert.Equal(17.99, EntityDistance(cpu, moving, stationary, new Vector(-1, 0)), 9);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void CircleAgainstInclinedWall_UsesInteriorCurveContact(bool cpu)
+    {
+        var moving = new List<Entity> { new Circle(0, 0, 1) };
+        var stationary = Loop((-10, 0), (10, 10), (10, 12), (-10, 2)).Cast<Entity>().ToList();
+        Assert.Equal(5 - System.Math.Sqrt(1.25), EntityDistance(cpu, moving, stationary, new Vector(0, 1)), 9);
+    }
+
+    [Fact]
+    public void GpuAdapter_ArbitraryDirectionUsesExactCpuFallback()
+    {
+        using var axesOnly = new RejectSlideComputer();
+        var distance = new GpuDistanceComputer(axesOnly).ComputeDistances(Rect(10, 10, 2, 2), Rect(0, 0, 2, 2),
+            new[] { new SlideOffset(0, 0, 0.6, 0.8) })[0];
+        Assert.Equal(8 / 0.6, distance, 9);
+    }
+
+    private sealed class RejectSlideComputer : ISlideComputer
+    {
+        public double[] ComputeBatch(double[] s, int sc, double[] m, int mc, double[] o, int oc, PushDirection d) =>
+            throw new InvalidOperationException("Non-cardinal direction reached axis-only GPU");
+        public double[] ComputeBatchMultiDir(double[] s, int sc, double[] m, int mc, double[] o, int oc, int[] d) =>
+            throw new InvalidOperationException("Non-cardinal direction reached axis-only GPU");
+        public void Dispose() { }
+    }
+
+    [Fact]
+    public void OpenBoundaries_RemainConservative()
+    {
+        var moving = new List<Line> { new Line(2, 0, 2, 2) };
+        var stationary = new List<Line> { new Line(2, 0, 2, 2) };
+        Assert.Equal(0, SpatialQuery.DirectionalDistance(moving, stationary, PushDirection.Right));
+    }
+
+    private static double EntityDistance(bool cpu, List<Entity> moving, List<Entity> stationary, Vector direction) =>
+        cpu ? new CpuDistanceComputer().ComputeDistances(stationary, moving,
+            new[] { new SlideOffset(0, 0, direction.X, direction.Y) })[0]
+            : SpatialQuery.DirectionalDistance(moving, stationary, direction);
+
+    private static double Distance(string path, List<Line> moving, List<Line> stationary, PushDirection direction)
+    {
+        var unit = SpatialQuery.DirectionToOffset(direction, 1);
+        // A nonzero template origin catches mixed local/world contact coordinates.
+        var origin = new Vector(13, -7);
+        var local = moving.Select(l => new Line(l.StartPoint - origin, l.EndPoint - origin)).ToList();
+        return path switch
+        {
+            "axis" => SpatialQuery.DirectionalDistance(moving, stationary, direction),
+            "offset" => SpatialQuery.DirectionalDistance(local, origin.X, origin.Y, stationary, direction),
+            "edges" => SpatialQuery.DirectionalDistance(local.Select(l => (l.StartPoint, l.EndPoint)).ToArray(), origin,
+                stationary.Select(l => (l.StartPoint, l.EndPoint)).ToArray(), Vector.Zero, direction),
+            "vector" => SpatialQuery.DirectionalDistance(moving, stationary, unit),
+            "entities" => SpatialQuery.DirectionalDistance(moving.Cast<Entity>().ToList(), stationary.Cast<Entity>().ToList(), unit),
+            "cpu-lines" => new CpuDistanceComputer().ComputeDistances(stationary, local,
+                new[] { new SlideOffset(origin.X, origin.Y, unit.X, unit.Y) })[0],
+            "cpu-entities" => new CpuDistanceComputer().ComputeDistances(stationary.Cast<Entity>().ToList(), local.Cast<Entity>().ToList(),
+                new[] { new SlideOffset(origin.X, origin.Y, unit.X, unit.Y) })[0],
+            _ => throw new ArgumentOutOfRangeException(nameof(path)),
+        };
+    }
+
+    private static void Reverse(List<Line> lines)
+    {
+        lines.Reverse();
+        foreach (var line in lines)
+            line.Reverse();
+    }
+
+    private static List<Line> Rect(double x, double y, double w, double h) =>
+        Loop((x, y), (x + w, y), (x + w, y + h), (x, y + h));
+
+    private static List<Line> Loop(params (double X, double Y)[] points) =>
+        points.Select((p, i) => new Line(p.X, p.Y, points[(i + 1) % points.Length].X, points[(i + 1) % points.Length].Y)).ToList();
+}

OpenNest.WinForms.Tests/GpuSlideContactTests.cs

@@ -0,0 +1,308 @@
+using System.Reflection;
+using ILGPU.Runtime;
+using OpenNest.Geometry;
+using OpenNest.Gpu;
+using OpenNest.Math;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace OpenNest.WinForms.Tests;
+
+// The real ILGPU kernels run on its deterministic CPU accelerator, not a mock or
+// CpuDistanceComputer. This also runs without GPU hardware on Windows CI.
+public sealed class GpuSlideContactFixture : IDisposable
+{
+    public GpuSlideComputer Computer { get; } = new GpuSlideComputer(preferCPU: true);
+
+    public void Dispose() => Computer.Dispose();
+}
+
+public class GpuSlideContactTests : IClassFixture<GpuSlideContactFixture>
+{
+    private readonly GpuSlideComputer computer;
+    private readonly ITestOutputHelper output;
+
+    public GpuSlideContactTests(GpuSlideContactFixture fixture, ITestOutputHelper output)
+    {
+        computer = fixture.Computer;
+        this.output = output;
+    }
+
+    public static IEnumerable<object[]> Paths()
+    {
+        foreach (var multiDir in new[] { false, true })
+            foreach (var reverse in new[] { false, true })
+                for (var turns = 0; turns < 4; turns++)
+                    yield return new object[] { multiDir, reverse, turns };
+    }
+
+    [Fact]
+    public void Kernels_ExecuteOnCpuAccelerator()
+    {
+        var accelerator = Assert.IsAssignableFrom<Accelerator>(typeof(GpuSlideComputer)
+            .GetField("_accelerator", BindingFlags.Instance | BindingFlags.NonPublic)!
+            .GetValue(computer));
+        output.WriteLine($"ILGPU backend: {accelerator.AcceleratorType}; device: {accelerator.Name}");
+        Assert.Equal(AcceleratorType.CPU, accelerator.AcceleratorType);
+    }
+
+    [Theory]
+    [MemberData(nameof(Paths))]
+    public void TouchingRectangles_EnterBlocksButDepartureAndTangentsDoNot(
+        bool multiDir, bool reverse, int turns)
+    {
+        var stationary = Rect(0, 0, 2, 2);
+        var moving = Rect(2, 0, 2, 2);
+        AssertSlide(multiDir, reverse, turns, stationary, moving, PushDirection.Left, 0);
+        AssertSlide(multiDir, reverse, turns, stationary, moving, PushDirection.Right, double.MaxValue);
+        AssertSlide(multiDir, reverse, turns, stationary, moving, PushDirection.Up, double.MaxValue);
+        AssertSlide(multiDir, reverse, turns, stationary, moving, PushDirection.Down, double.MaxValue);
+    }
+
+    [Theory]
+    [MemberData(nameof(Paths))]
+    public void SlidingContact_StopsAtLaterHookOnSameObstacle(
+        bool multiDir, bool reverse, int turns)
+    {
+        var stationary = Loop((0, 0), (2, 0), (2, 4), (5, 4), (5, 6), (0, 6));
+        AssertSlide(multiDir, reverse, turns, stationary, Rect(2, 0, 1, 1), PushDirection.Up, 3);
+    }
+
+    [Theory]
+    [MemberData(nameof(Paths))]
+    public void PositiveGrazingContact_StopsAtLaterFeatureOnSameObstacle(
+        bool multiDir, bool reverse, int turns)
+    {
+        var stationary = Loop((4, 1), (10, 1), (10, -2), (12, -2), (12, 3), (4, 3));
+        AssertSlide(multiDir, reverse, turns, stationary, Rect(0, 0, 1, 1), PushDirection.Right, 9);
+    }
+
+    [Theory]
+    [MemberData(nameof(Paths))]
+    public void PositiveGrazingContact_WithoutLaterBlockerIsUnbounded(
+        bool multiDir, bool reverse, int turns)
+    {
+        AssertSlide(multiDir, reverse, turns, Rect(4, 1, 2, 2), Rect(0, 0, 1, 1),
+            PushDirection.Right, double.MaxValue);
+    }
+
+    [Theory]
+    [MemberData(nameof(Paths))]
+    public void HoleContact_LeavingWallStillStopsAtOppositeWall(
+        bool multiDir, bool reverse, int turns)
+    {
+        var stationary = Rect(0, 0, 10, 10);
+        stationary.AddRange(Rect(2, 2, 6, 6)); // Hole depth must not depend on winding.
+        AssertSlide(multiDir, reverse, turns, stationary, Rect(2, 3, 1, 1), PushDirection.Right, 5);
+        AssertSlide(multiDir, reverse, turns, stationary, Rect(2, 3, 1, 1), PushDirection.Up, 4);
+    }
+
+    [Theory]
+    [MemberData(nameof(Paths))]
+    public void ReverseRayWitness_StationaryVertexHitsMiddleOfMovingEdge(
+        bool multiDir, bool reverse, int turns)
+    {
+        // No moving vertex can hit the shorter stationary rectangle.
+        AssertSlide(multiDir, reverse, turns, Rect(5, 2, 1, 1), Rect(0, 0, 1, 6),
+            PushDirection.Right, 4);
+    }
+
+    [Theory]
+    [MemberData(nameof(Paths))]
+    public void SnappedZeroDistance_KeepsUnsnappedWitnessOnBothBoundaries(
+        bool multiDir, bool reverse, int turns)
+    {
+        var gap = Tolerance.Epsilon / 2;
+        AssertSlide(multiDir, reverse, turns, Rect(1 + gap, 0, 1, 1), Rect(0, 0, 1, 1),
+            PushDirection.Right, 0);
+        AssertSlide(multiDir, reverse, turns, Rect(1 + gap, 2, 1, 1), Rect(0, 0, 1, 6),
+            PushDirection.Right, 0);
+    }
+
+    [Theory]
+    [MemberData(nameof(Paths))]
+    public void ToleranceNearMiss_DoesNotBecomeABlockingWitness(
+        bool multiDir, bool reverse, int turns)
+    {
+        AssertSlide(multiDir, reverse, turns, Rect(4, 0, 1, 1),
+            Rect(0, 1 + Tolerance.Epsilon / 2, 1, 1), PushDirection.Right, double.MaxValue);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void NearParallelEdges_UseTheSharedAxisRayTolerance(bool multiDir)
+    {
+        var dy = Tolerance.Epsilon / 2;
+        AssertSlide(multiDir, false, 0,
+            new List<Line> { new Line(2, 0, 3, dy) },
+            new List<Line> { new Line(0, 0, 1, dy) }, PushDirection.Right, double.MaxValue);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void TiedGrazingAndBlockingContacts_DoNotDropBlockingContact(bool multiDir)
+    {
+        var stationary = Rect(4, 1, 1, 1);
+        stationary.AddRange(Rect(4, -2, 1, 2.5));
+        var moving = Loop((1, 1), (0, 1), (0, 0), (1, 0));
+        AssertSlide(multiDir, false, 0, stationary, moving, PushDirection.Right, 3);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void OpenBoundaries_RemainConservative(bool multiDir)
+    {
+        AssertSlide(multiDir, false, 0,
+            new List<Line> { new Line(2, 0, 2, 2) },
+            new List<Line> { new Line(2, 0, 2, 2) }, PushDirection.Right, 0);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void EmptyGeometryAndOffsets_ReturnNoHitWithoutStaleResults(bool multiDir)
+    {
+        var rectangle = SpatialQuery.FlattenLines(Rect(0, 0, 1, 1));
+        var offsets = new[] { 0.0, 0.0, 2.0, 0.0 };
+        var directions = new[] { (int)PushDirection.Left, (int)PushDirection.Left };
+        Assert.Equal(new[] { 0.0, 1.0 },
+            Compute(multiDir, rectangle, 4, rectangle, 4, offsets, 2, directions, PushDirection.Left));
+        Assert.Empty(Compute(multiDir, rectangle, 4, rectangle, 4, offsets, 0, directions));
+        Assert.All(Compute(multiDir, Array.Empty<double>(), 0, rectangle, 4, offsets, 2, directions),
+            distance => Assert.Equal(double.MaxValue, distance));
+        Assert.All(Compute(multiDir, rectangle, 4, Array.Empty<double>(), 0, offsets, 2, directions),
+            distance => Assert.Equal(double.MaxValue, distance));
+        Assert.Equal(new[] { 0.0, 1.0 },
+            Compute(multiDir, rectangle, 4, rectangle, 4, offsets, 2, directions, PushDirection.Left));
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void ReusedOffsetBuffers_GrowShrinkAndRespectActivePrefixes(bool multiDir)
+    {
+        var stationary = Rect(0, 0, 2, 2);
+        var moving = Rect(0, 0, 1, 1);
+        var stationaryData = SpatialQuery.FlattenLines(stationary);
+        var movingData = SpatialQuery.FlattenLines(moving);
+        // Odd lengths exercise rounded-up thread groups and retained excess capacity.
+        foreach (var count in new[] { 1, 37, 3, 65, 2, 97, 0, 5, 129, 1 })
+        {
+            var offsets = new double[(count + 7) * 2];
+            var directions = new int[count + 7];
+            var expected = new double[count];
+            for (var i = 0; i < count; i++)
+            {
+                var dx = 2 + i % 4;
+                var dy = i % 3;
+                var direction = multiDir ? (PushDirection)(i % 4) : PushDirection.Left;
+                offsets[i * 2] = dx;
+                offsets[i * 2 + 1] = dy;
+                directions[i] = (int)direction;
+                expected[i] = SpatialQuery.DirectionalDistance(moving, dx, dy, stationary, direction);
+            }
+            var actual = Compute(multiDir, stationaryData, 4, movingData, 4, offsets, count,
+                directions, PushDirection.Left);
+            Assert.Equal(expected, actual);
+        }
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void ReusedSegmentArrays_ChangingCountsAndCoordinatesRefreshesBothCaches(bool multiDir)
+    {
+        var stationary = SpatialQuery.FlattenLines(Rect(4, 0, 1, 1).Concat(Rect(2, 0, 1, 1)).ToList());
+        var moving = SpatialQuery.FlattenLines(Rect(0, 0, 1, 1).Concat(Rect(2, 0, 1, 1)).ToList());
+        var offsets = new[] { 0.0, 0.0 };
+        var directions = new[] { (int)PushDirection.Right };
+        foreach (var counts in new[] { (4, 4, 3.0), (8, 4, 1.0), (4, 8, 1.0), (4, 4, 3.0) })
+            Assert.Equal(counts.Item3,
+                Compute(multiDir, stationary, counts.Item1, moving, counts.Item2, offsets, 1, directions)[0]);
+
+        for (var i = 0; i < stationary.Length; i += 2)
+            stationary[i] += 1;
+        Assert.Equal(4, Compute(multiDir, stationary, 4, moving, 4, offsets, 1, directions)[0]);
+        for (var i = 0; i < moving.Length; i += 2)
+            moving[i] -= 1;
+        Assert.Equal(5, Compute(multiDir, stationary, 4, moving, 4, offsets, 1, directions)[0]);
+        computer.InvalidateStationary();
+        computer.InvalidateMoving();
+        Assert.Equal(5, Compute(multiDir, stationary, 4, moving, 4, offsets, 1, directions)[0]);
+    }
+
+    [Fact]
+    public void MultiDir_UsesEachOffsetAndDirectionIndependently()
+    {
+        var stationary = SpatialQuery.FlattenLines(Rect(0, 0, 2, 2));
+        var moving = SpatialQuery.FlattenLines(Rect(0, 0, 1, 1));
+        var offsets = new[] { 2.0, 0.0, 2.0, 0.0, 2.0, 0.0, 2.0, 0.0, -3.0, 0.0, 0.0, -4.0 };
+        var directions = new[] { PushDirection.Left, PushDirection.Right, PushDirection.Up,
+            PushDirection.Down, PushDirection.Right, PushDirection.Up }.Select(d => (int)d).ToArray();
+        Assert.Equal(new[] { 0.0, double.MaxValue, double.MaxValue, double.MaxValue, 2.0, 3.0 },
+            computer.ComputeBatchMultiDir(stationary, 4, moving, 4, offsets, 6, directions));
+    }
+
+    private void AssertSlide(bool multiDir, bool reverse, int turns, List<Line> stationary,
+        List<Line> moving, PushDirection direction, double expected)
+    {
+        // Exact quarter turns cover all axis signs without trigonometric rounding.
+        // Offset the world and template independently to expose mixed-frame witnesses.
+        var origin = new Vector(13, -7);
+        stationary = Transform(stationary, turns, new Vector(17, -23), reverse);
+        moving = Transform(moving, turns, new Vector(17, -23) - origin, reverse);
+        for (var turn = 0; turn < turns; turn++)
+            direction = direction switch
+            {
+                PushDirection.Right => PushDirection.Up,
+                PushDirection.Up => PushDirection.Left,
+                PushDirection.Left => PushDirection.Down,
+                _ => PushDirection.Right,
+            };
+
+        Assert.Equal(expected,
+            SpatialQuery.DirectionalDistance(moving, origin.X, origin.Y, stationary, direction), 9);
+        var actual = Compute(multiDir, SpatialQuery.FlattenLines(stationary), stationary.Count,
+            SpatialQuery.FlattenLines(moving), moving.Count, new[] { origin.X, origin.Y }, 1,
+            new[] { (int)direction }, direction);
+        Assert.Single(actual);
+        Assert.Equal(expected, actual[0], 9);
+    }
+
+    private double[] Compute(bool multiDir, double[] stationary, int stationaryCount,
+        double[] moving, int movingCount, double[] offsets, int count, int[] directions,
+        PushDirection direction = PushDirection.Right) =>
+        multiDir
+            ? computer.ComputeBatchMultiDir(stationary, stationaryCount, moving, movingCount,
+                offsets, count, directions)
+            : computer.ComputeBatch(stationary, stationaryCount, moving, movingCount,
+                offsets, count, direction);
+
+    private static List<Line> Transform(List<Line> lines, int turns, Vector origin, bool reverse)
+    {
+        Vector Map(Vector point)
+        {
+            for (var i = 0; i < turns; i++)
+                point = new Vector(-point.Y, point.X);
+            return point + origin;
+        }
+        var result = lines.Select(line => new Line(Map(line.StartPoint), Map(line.EndPoint))).ToList();
+        if (reverse)
+        {
+            result.Reverse();
+            foreach (var line in result)
+                line.Reverse();
+        }
+        return result;
+    }
+
+    private static List<Line> Rect(double x, double y, double width, double height) =>
+        Loop((x, y), (x + width, y), (x + width, y + height), (x, y + height));
+
+    private static List<Line> Loop(params (double X, double Y)[] points) =>
+        points.Select((point, i) => new Line(point.X, point.Y,
+            points[(i + 1) % points.Length].X, points[(i + 1) % points.Length].Y)).ToList();
+}

OpenNest/Controls/PlateView.cs

@@ -10,13 +10,13 @@ using System.Threading.Tasks;
 using System.Windows.Forms;
 using OpenNest.Actions;
 using OpenNest.Collections;
+using OpenNest.Engine;
 using OpenNest.Engine.Fill;
+using OpenNest.Engine.Jobs.Placement;
 using OpenNest.Forms;
 using OpenNest.Geometry;
 using OpenNest.Math;
 using Timer = System.Timers.Timer;
-using OpenNest.Engine;
-using OpenNest.Engine.Jobs.Placement;
 
 namespace OpenNest.Controls
 {
@@ -562,9 +562,6 @@ namespace OpenNest.Controls
             Plate.Parts.Add(part);
         }
 
-        public void SetStationaryParts(List<Part> parts) =>
-            previewManager.SetStationaryParts(parts);
-
         public void SetActiveParts(List<Part> parts) => previewManager.SetActiveParts(parts);
 
         public void ClearPreviewParts() => previewManager.ClearPreviewParts();
@@ -578,23 +575,12 @@ namespace OpenNest.Controls
             var cts = new CancellationTokenSource();
             var progressForm = new NestProgressForm(cts, showPlateRow: false);
 
-            var previewPlate = new Plate(Plate.Size)
-            {
-                Quadrant = Plate.Quadrant,
-                PartSpacing = Plate.PartSpacing,
-            };
-            previewPlate.EdgeSpacing = Plate.EdgeSpacing;
-            progressForm.PreviewPlate = previewPlate;
-
             var progress = new Progress<NestProgress>(p =>
             {
                 progressForm.UpdateProgress(p);
 
                 if (p.IsOverallBest)
-                {
-                    progressForm.UpdatePreview(p.BestParts);
                     SetActiveParts(p.BestParts);
-                }
 
                 ActiveWorkArea = p.ActiveWorkArea;
             });
@@ -749,6 +735,8 @@ namespace OpenNest.Controls
 
         public void PushSelected(PushDirection direction) => selection.PushSelected(direction);
 
+        public bool ExpandSelected() => selection.ExpandSelected();
+
         public void RotateSelectedParts(double angle) => selection.RotateSelectedParts(angle);
 
         protected override void UpdateMatrix()

OpenNest/Controls/PreviewManager.cs

@@ -6,7 +6,6 @@ namespace OpenNest.Controls
     internal class PreviewManager
     {
         private readonly PlateView view;
-        private readonly List<LayoutPart> stationaryParts = new List<LayoutPart>();
         private readonly List<LayoutPart> activeParts = new List<LayoutPart>();
 
         public PreviewManager(PlateView view)
@@ -14,32 +13,11 @@ namespace OpenNest.Controls
             this.view = view;
         }
 
-        public IReadOnlyList<LayoutPart> PreviewParts =>
-            activeParts.Count > 0 ? activeParts : stationaryParts;
+        public IReadOnlyList<LayoutPart> PreviewParts => activeParts;
 
-        public Brush PreviewBrush =>
-            activeParts.Count > 0
-                ? view.ColorScheme.ActivePreviewPartBrush
-                : view.ColorScheme.PreviewPartBrush;
+        public Brush PreviewBrush => view.ColorScheme.ActivePreviewPartBrush;
 
-        public Pen PreviewPen =>
-            activeParts.Count > 0
-                ? view.ColorScheme.ActivePreviewPartPen
-                : view.ColorScheme.PreviewPartPen;
-
-        public void SetStationaryParts(List<Part> parts)
-        {
-            stationaryParts.Clear();
-            activeParts.Clear();
-
-            if (parts != null)
-            {
-                foreach (var part in parts)
-                    stationaryParts.Add(LayoutPart.Create(part, view));
-            }
-
-            view.Invalidate();
-        }
+        public Pen PreviewPen => view.ColorScheme.ActivePreviewPartPen;
 
         public void SetActiveParts(List<Part> parts)
         {
@@ -56,7 +34,6 @@ namespace OpenNest.Controls
 
         public void ClearPreviewParts()
         {
-            stationaryParts.Clear();
             activeParts.Clear();
             view.Invalidate();
         }
@@ -69,19 +46,16 @@ namespace OpenNest.Controls
                     view.Plate.Parts.Add(part);
             }
 
-            stationaryParts.Clear();
             activeParts.Clear();
         }
 
         public void Update()
         {
-            stationaryParts.ForEach(p => p.Update(view));
             activeParts.ForEach(p => p.Update(view));
         }
 
         public void Clear()
         {
-            stationaryParts.Clear();
             activeParts.Clear();
         }
     }

OpenNest/Controls/SelectionManager.cs

@@ -167,6 +167,36 @@ namespace OpenNest.Controls
             view.Invalidate();
         }
 
+        /// <summary>
+        /// Grows the part-to-part spacing of the selection as far as the plate
+        /// allows (Expander). Returns false when nothing could be done.
+        /// </summary>
+        public bool ExpandSelected()
+        {
+            var movingParts = selectedParts.Select(p => p.BasePart).ToList();
+
+            if (movingParts.Count < 2)
+                return false;
+
+            var result = Expander.Expand(movingParts, view.Plate);
+
+            if (result.Cancelled)
+                return false;
+
+            selectedParts.ForEach(p => p.IsDirty = true);
+
+            if (view.Plate.CutOffs.Count > 0)
+                view.Plate.RegenerateCutOffs(view.CutOffSettings);
+
+            view.Invalidate();
+
+            view.Status =
+                $"Expanded spacing to {result.AchievedSpacing:0.###}"
+                + (result.Violations.Count > 0 ? $" - {result.Violations.Count} blocked pairs" : "");
+
+            return true;
+        }
+
         public LayoutPart GetPartAtControlPoint(Point pt)
         {
             var pt2 = view.PointControlToGraph(pt);

OpenNest/Forms/BomImportForm.cs

@@ -5,6 +5,7 @@ using System.Drawing;
 using System.IO;
 using System.Linq;
 using System.Windows.Forms;
+using OpenNest.Data;
 using OpenNest.Geometry;
 using OpenNest.IO;
 using OpenNest.IO.Bom;
@@ -16,7 +17,7 @@ namespace OpenNest.Forms
         private List<BomPartRow> _parts;
         private Dictionary<string, GroupSettings> _groupSettings;
         private bool _suppressRegroup;
-        private Nest.PlateSettings _templateDefaults;
+        private NestDefaults _defaults;
 
         public Form MdiParentForm { get; set; }
 
@@ -25,37 +26,14 @@ namespace OpenNest.Forms
             InitializeComponent();
             _parts = new List<BomPartRow>();
             _groupSettings = new Dictionary<string, GroupSettings>();
-            _templateDefaults = LoadTemplateDefaults();
-            ApplyTemplateDefaults();
+            _defaults = NestDefaults.Load(NestDefaults.DefaultPath);
+            ApplyDefaults();
         }
 
-        private Nest.PlateSettings LoadTemplateDefaults()
+        private void ApplyDefaults()
         {
-            var templatePath = Properties.Settings.Default.NestTemplatePath;
-            if (File.Exists(templatePath))
-            {
-                try
-                {
-                    var nest = new NestReader(templatePath).Read();
-                    return nest.PlateDefaults;
-                }
-                catch { }
-            }
-
-            // Fallback defaults matching CreateDefaultNest
-            return new Nest.PlateSettings
-            {
-                Size = new Geometry.Size(100, 100),
-                Quadrant = 1,
-                PartSpacing = 1,
-                EdgeSpacing = new Spacing(1, 1, 1, 1),
-            };
-        }
-
-        private void ApplyTemplateDefaults()
-        {
-            txtPlateWidth.Text = _templateDefaults.Size.Width.ToString("0.####");
-            txtPlateLength.Text = _templateDefaults.Size.Length.ToString("0.####");
+            txtPlateWidth.Text = _defaults.Size.Width.ToString("0.####");
+            txtPlateLength.Text = _defaults.Size.Length.ToString("0.####");
         }
 
         #region File Browsing
@@ -147,9 +125,9 @@ namespace OpenNest.Forms
             var matchedPaths = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
 
             foreach (var group in analysis.Groups)
-            foreach (var part in group.Parts)
-                if (part.DxfPath != null)
-                    matchedPaths[part.Item.FileName ?? ""] = part.DxfPath;
+                foreach (var part in group.Parts)
+                    if (part.DxfPath != null)
+                        matchedPaths[part.Item.FileName ?? ""] = part.DxfPath;
 
             _parts = new List<BomPartRow>();
 
@@ -290,10 +268,10 @@ namespace OpenNest.Forms
 
             var defaultWidth = double.TryParse(txtPlateWidth.Text, out var w)
                 ? w
-                : _templateDefaults.Size.Width;
+                : _defaults.Size.Width;
             var defaultLength = double.TryParse(txtPlateLength.Text, out var l)
                 ? l
-                : _templateDefaults.Size.Length;
+                : _defaults.Size.Length;
 
             var groups = _parts
                 .Where(p =>
@@ -336,11 +314,11 @@ namespace OpenNest.Forms
                     group.Sum(p => p.Qty ?? 0),
                     existing ? gs.PlateWidth : defaultWidth,
                     existing ? gs.PlateLength : defaultLength,
-                    existing ? gs.PartSpacing : _templateDefaults.PartSpacing,
-                    existing ? gs.EdgeLeft : _templateDefaults.EdgeSpacing.Left,
-                    existing ? gs.EdgeBottom : _templateDefaults.EdgeSpacing.Bottom,
-                    existing ? gs.EdgeRight : _templateDefaults.EdgeSpacing.Right,
-                    existing ? gs.EdgeTop : _templateDefaults.EdgeSpacing.Top
+                    existing ? gs.PartSpacing : _defaults.PartSpacing,
+                    existing ? gs.EdgeLeft : _defaults.EdgeSpacing.Left,
+                    existing ? gs.EdgeBottom : _defaults.EdgeSpacing.Bottom,
+                    existing ? gs.EdgeRight : _defaults.EdgeSpacing.Right,
+                    existing ? gs.EdgeTop : _defaults.EdgeSpacing.Top
                 );
             }
 
@@ -374,23 +352,23 @@ namespace OpenNest.Forms
                 {
                     PlateWidth = row["Plate Width"] is double pw
                         ? pw
-                        : _templateDefaults.Size.Width,
+                        : _defaults.Size.Width,
                     PlateLength = row["Plate Length"] is double pl
                         ? pl
-                        : _templateDefaults.Size.Length,
+                        : _defaults.Size.Length,
                     PartSpacing = row["Part Spacing"] is double ps
                         ? ps
-                        : _templateDefaults.PartSpacing,
+                        : _defaults.PartSpacing,
                     EdgeLeft = row["Edge Left"] is double el
                         ? el
-                        : _templateDefaults.EdgeSpacing.Left,
+                        : _defaults.EdgeSpacing.Left,
                     EdgeBottom = row["Edge Bottom"] is double eb
                         ? eb
-                        : _templateDefaults.EdgeSpacing.Bottom,
+                        : _defaults.EdgeSpacing.Bottom,
                     EdgeRight = row["Edge Right"] is double er
                         ? er
-                        : _templateDefaults.EdgeSpacing.Right,
-                    EdgeTop = row["Edge Top"] is double et ? et : _templateDefaults.EdgeSpacing.Top,
+                        : _defaults.EdgeSpacing.Right,
+                    EdgeTop = row["Edge Top"] is double et ? et : _defaults.EdgeSpacing.Top,
                 };
             }
         }
@@ -434,10 +412,10 @@ namespace OpenNest.Forms
 
             var defaultWidth = double.TryParse(txtPlateWidth.Text, out var dw)
                 ? dw
-                : _templateDefaults.Size.Width;
+                : _defaults.Size.Width;
             var defaultLength = double.TryParse(txtPlateLength.Text, out var dl)
                 ? dl
-                : _templateDefaults.Size.Length;
+                : _defaults.Size.Length;
 
             var groups = _parts
                 .Where(p =>
@@ -477,11 +455,11 @@ namespace OpenNest.Forms
                 var hasSettings = _groupSettings.TryGetValue(key, out var gs);
                 var plateWidth = hasSettings ? gs.PlateWidth : defaultWidth;
                 var plateLength = hasSettings ? gs.PlateLength : defaultLength;
-                var partSpacing = hasSettings ? gs.PartSpacing : _templateDefaults.PartSpacing;
-                var edgeLeft = hasSettings ? gs.EdgeLeft : _templateDefaults.EdgeSpacing.Left;
-                var edgeBottom = hasSettings ? gs.EdgeBottom : _templateDefaults.EdgeSpacing.Bottom;
-                var edgeRight = hasSettings ? gs.EdgeRight : _templateDefaults.EdgeSpacing.Right;
-                var edgeTop = hasSettings ? gs.EdgeTop : _templateDefaults.EdgeSpacing.Top;
+                var partSpacing = hasSettings ? gs.PartSpacing : _defaults.PartSpacing;
+                var edgeLeft = hasSettings ? gs.EdgeLeft : _defaults.EdgeSpacing.Left;
+                var edgeBottom = hasSettings ? gs.EdgeBottom : _defaults.EdgeSpacing.Bottom;
+                var edgeRight = hasSettings ? gs.EdgeRight : _defaults.EdgeSpacing.Right;
+                var edgeTop = hasSettings ? gs.EdgeTop : _defaults.EdgeSpacing.Top;
 
                 var nestName = $"{jobName} - {thickness:0.###} {material}";
                 var nest = new Nest(nestName);
@@ -490,7 +468,7 @@ namespace OpenNest.Forms
                 nest.PlateDefaults.Size = new Geometry.Size(plateWidth, plateLength);
                 nest.Thickness = thickness;
                 nest.Material = new Material(material);
-                nest.PlateDefaults.Quadrant = _templateDefaults.Quadrant;
+                nest.PlateDefaults.Quadrant = _defaults.Quadrant;
                 nest.PlateDefaults.PartSpacing = partSpacing;
                 nest.PlateDefaults.EdgeSpacing = new Spacing(
                     edgeLeft,

OpenNest/Forms/EditNestForm.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.ComponentModel;
 using System.Diagnostics;
 using System.Drawing;
@@ -301,16 +301,11 @@ namespace OpenNest.Forms
         public void SaveAs()
         {
             var dlg = new SaveFileDialog();
-            dlg.Filter = $"{NestFormat.FileFilter}|Template File|*.nstdot";
+            dlg.Filter = NestFormat.FileFilter;
             dlg.FileName = Nest.Name;
 
             if (dlg.ShowDialog() == System.Windows.Forms.DialogResult.OK)
-            {
-                if (dlg.FilterIndex == 2)
-                    SaveTemplate(dlg.FileName);
-                else
-                    SaveAs(dlg.FileName);
-            }
+                SaveAs(dlg.FileName);
         }
 
         public void SaveAs(string path)
@@ -319,17 +314,6 @@ namespace OpenNest.Forms
             Text = Document.Name;
         }
 
-        public void SaveTemplate(string path)
-        {
-            var nst = new Nest();
-            nst.Name = Path.GetFileNameWithoutExtension(path);
-            nst.PlateDefaults = Nest.PlateDefaults;
-            nst.Units = Nest.Units;
-
-            var writer = new NestWriter(nst);
-            writer.Write(path);
-        }
-
         public void Import()
         {
             var dlg = new OpenFileDialog();
@@ -943,9 +927,9 @@ namespace OpenNest.Forms
 
             // Refresh all parts to use the updated programs
             foreach (var plate in Nest.Plates)
-            foreach (var part in plate.Parts)
-                if (!part.BaseDrawing.IsCutOff)
-                    part.Update();
+                foreach (var part in plate.Parts)
+                    if (!part.BaseDrawing.IsCutOff)
+                        part.Update();
 
             UpdateDrawingList();
             PlateView.Invalidate();

OpenNest/Forms/MainForm.Designer.cs

@@ -65,6 +65,7 @@
             mnuToolsMeasureArea = new System.Windows.Forms.ToolStripMenuItem();
             mnuToolsBestFitViewer = new System.Windows.Forms.ToolStripMenuItem();
             mnuToolsPatternTile = new System.Windows.Forms.ToolStripMenuItem();
+            mnuToolsExpandSpacing = new System.Windows.Forms.ToolStripMenuItem();
             mnuToolsAlign = new System.Windows.Forms.ToolStripMenuItem();
             mnuToolsAlignLeft = new System.Windows.Forms.ToolStripMenuItem();
             mnuToolsAlignRight = new System.Windows.Forms.ToolStripMenuItem();
@@ -81,6 +82,8 @@
             mnuSetRotationIncrement = new System.Windows.Forms.ToolStripMenuItem();
             toolStripMenuItem15 = new System.Windows.Forms.ToolStripSeparator();
             mnuToolsMachineConfig = new System.Windows.Forms.ToolStripMenuItem();
+            mnuToolsNestDefaults = new System.Windows.Forms.ToolStripMenuItem();
+            mnuToolsSaveCurrentAsDefaults = new System.Windows.Forms.ToolStripMenuItem();
             mnuToolsOptions = new System.Windows.Forms.ToolStripMenuItem();
             mnuNest = new System.Windows.Forms.ToolStripMenuItem();
             mnuNestEdit = new System.Windows.Forms.ToolStripMenuItem();
@@ -419,7 +422,7 @@
             // 
             // mnuTools
             // 
-            mnuTools.DropDownItems.AddRange(new System.Windows.Forms.ToolStripItem[] { mnuToolsMeasureArea, mnuToolsBestFitViewer, mnuToolsPatternTile, mnuToolsAlign, toolStripMenuItem14, mnuSetOffsetIncrement, mnuSetRotationIncrement, toolStripMenuItem15, mnuToolsMachineConfig, mnuToolsOptions });
+            mnuTools.DropDownItems.AddRange(new System.Windows.Forms.ToolStripItem[] { mnuToolsMeasureArea, mnuToolsBestFitViewer, mnuToolsPatternTile, mnuToolsExpandSpacing, mnuToolsAlign, toolStripMenuItem14, mnuSetOffsetIncrement, mnuSetRotationIncrement, toolStripMenuItem15, mnuToolsMachineConfig, mnuToolsNestDefaults, mnuToolsSaveCurrentAsDefaults, mnuToolsOptions });
             mnuTools.Name = "mnuTools";
             mnuTools.Size = new System.Drawing.Size(47, 20);
             mnuTools.Text = "&Tools";
@@ -445,6 +448,13 @@
             this.mnuToolsPatternTile.Text = "Pattern Tile";
             this.mnuToolsPatternTile.Click += PatternTile_Click;
             //
+            // mnuToolsExpandSpacing
+            //
+            this.mnuToolsExpandSpacing.Name = "mnuToolsExpandSpacing";
+            this.mnuToolsExpandSpacing.Size = new System.Drawing.Size(214, 22);
+            this.mnuToolsExpandSpacing.Text = "Expand Spacing";
+            this.mnuToolsExpandSpacing.Click += ExpandSpacing_Click;
+            //
             // mnuToolsAlign
             //
             mnuToolsAlign.DropDownItems.AddRange(new System.Windows.Forms.ToolStripItem[] { mnuToolsAlignLeft, mnuToolsAlignRight, mnuToolsAlignTop, mnuToolsAlignBottom, toolStripMenuItem11, mnuToolsAlignHorizontal, mnuToolsAlignVertically, toolStripMenuItem8, mnuToolsEvenlySpaceHorizontal, mnuToolsEvenlySpaceVertical });
@@ -551,6 +561,20 @@
             mnuToolsMachineConfig.Text = "Machine Configuration...";
             mnuToolsMachineConfig.Click += MachineConfig_Click;
             //
+            // mnuToolsNestDefaults
+            //
+            mnuToolsNestDefaults.Name = "mnuToolsNestDefaults";
+            mnuToolsNestDefaults.Size = new System.Drawing.Size(214, 22);
+            mnuToolsNestDefaults.Text = "Nest Defaults...";
+            mnuToolsNestDefaults.Click += NestDefaults_Click;
+            //
+            // mnuToolsSaveCurrentAsDefaults
+            //
+            mnuToolsSaveCurrentAsDefaults.Name = "mnuToolsSaveCurrentAsDefaults";
+            mnuToolsSaveCurrentAsDefaults.Size = new System.Drawing.Size(214, 22);
+            mnuToolsSaveCurrentAsDefaults.Text = "Save Current Plate as Defaults";
+            mnuToolsSaveCurrentAsDefaults.Click += SaveCurrentAsDefaults_Click;
+            //
             // mnuToolsOptions
             //
             mnuToolsOptions.Name = "mnuToolsOptions";
@@ -1217,6 +1241,8 @@
         private System.Windows.Forms.ToolStripSeparator toolStripMenuItem5;
         private System.Windows.Forms.ToolStripMenuItem mnuTools;
         private System.Windows.Forms.ToolStripMenuItem mnuToolsMachineConfig;
+        private System.Windows.Forms.ToolStripMenuItem mnuToolsNestDefaults;
+        private System.Windows.Forms.ToolStripMenuItem mnuToolsSaveCurrentAsDefaults;
         private System.Windows.Forms.ToolStripMenuItem mnuToolsOptions;
         private System.Windows.Forms.ToolStripMenuItem mnuNest;
         private System.Windows.Forms.ToolStripMenuItem mnuNestEdit;
@@ -1306,6 +1332,7 @@
         private System.Windows.Forms.ToolStripMenuItem mnuToolsMeasureArea;
         private System.Windows.Forms.ToolStripMenuItem mnuToolsBestFitViewer;
         private System.Windows.Forms.ToolStripMenuItem mnuToolsPatternTile;
+        private System.Windows.Forms.ToolStripMenuItem mnuToolsExpandSpacing;
         private System.Windows.Forms.ToolStripButton btnSaveAs;
         private System.Windows.Forms.ToolStripMenuItem centerPartsToolStripMenuItem;
         private System.Windows.Forms.ToolStripStatusLabel gpuStatusLabel;

OpenNest/Forms/MainForm.cs

@@ -10,16 +10,16 @@ using System.Windows.Forms;
 using OpenNest.Actions;
 using OpenNest.Collections;
 using OpenNest.Data;
+using OpenNest.Engine;
 using OpenNest.Engine.BestFit;
 using OpenNest.Engine.Fill;
+using OpenNest.Engine.Jobs;
+using OpenNest.Engine.Jobs.Adapters;
+using OpenNest.Engine.Jobs.Placement;
 using OpenNest.Geometry;
 using OpenNest.Gpu;
 using OpenNest.IO;
 using OpenNest.Properties;
-using OpenNest.Engine;
-using OpenNest.Engine.Jobs;
-using OpenNest.Engine.Jobs.Adapters;
-using OpenNest.Engine.Jobs.Placement;
 
 namespace OpenNest.Forms
 {
@@ -90,14 +90,38 @@ namespace OpenNest.Forms
         private Nest CreateDefaultNest()
         {
             var nest = new Nest();
-            nest.Units = Properties.Settings.Default.DefaultUnit;
-            nest.PlateDefaults.EdgeSpacing = new Spacing(1, 1, 1, 1);
-            nest.PlateDefaults.PartSpacing = 1;
-            nest.PlateDefaults.Size = new OpenNest.Geometry.Size(100, 100);
-            nest.PlateDefaults.Quadrant = 1;
+            LoadNestDefaults().ApplyTo(nest);
             return nest;
         }
 
+        /// <summary>
+        /// Loads the persisted nest defaults. When no usable file exists,
+        /// units come from the legacy DefaultUnit setting; a corrupt file
+        /// warns once per session.
+        /// </summary>
+        private NestDefaults LoadNestDefaults()
+        {
+            var defaults = NestDefaults.Load(NestDefaults.DefaultPath, out var status);
+            if (status == NestDefaultsStatus.Ok)
+                return defaults;
+
+            defaults.Units = Properties.Settings.Default.DefaultUnit;
+
+            if (status == NestDefaultsStatus.Invalid && !defaultsWarned)
+            {
+                defaultsWarned = true;
+                MessageBox.Show(
+                    $"The nest defaults file could not be read:\n{NestDefaults.DefaultPath}\n\nBuilt-in defaults will be used. Re-save your defaults from Tools > Nest Defaults.",
+                    "Nest Defaults",
+                    MessageBoxButtons.OK,
+                    MessageBoxIcon.Warning
+                );
+            }
+            return defaults;
+        }
+
+        private bool defaultsWarned;
+
         private string GetNestName(DateTime date, int id)
         {
             var year = (date.Year % 100).ToString("D2");
@@ -184,6 +208,8 @@ namespace OpenNest.Forms
             mnuWindow.Visible = hasValue;
             mnuToolsAlign.Visible = hasValue;
             mnuToolsMeasureArea.Visible = hasValue;
+            mnuToolsExpandSpacing.Visible = hasValue;
+            mnuToolsSaveCurrentAsDefaults.Visible = hasValue;
 
             toolStripMenuItem14.Visible = hasValue;
             mnuSetOffsetIncrement.Visible = hasValue;
@@ -453,10 +479,52 @@ namespace OpenNest.Forms
         {
             base.OnLoad(e);
 
+            MigrateNestTemplate();
+
             if (Settings.Default.CreateNewNestOnOpen)
                 New_Click(this, new EventArgs());
         }
 
+        /// <summary>
+        /// One-time upgrade: converts a legacy .nstdot nest template
+        /// (NestTemplatePath setting) into defaults.json, then clears the
+        /// setting so the template mechanism is never consulted again.
+        /// </summary>
+        private void MigrateNestTemplate()
+        {
+            var templatePath = Settings.Default.NestTemplatePath;
+            if (string.IsNullOrWhiteSpace(templatePath))
+                return;
+
+            // A failed conversion keeps the setting populated so the user
+            // can still find their template file.
+            var converted = true;
+            if (File.Exists(templatePath) && !File.Exists(NestDefaults.DefaultPath))
+            {
+                try
+                {
+                    var nest = new NestReader(templatePath).Read();
+                    NestDefaults.FromNest(nest).Save(NestDefaults.DefaultPath);
+                }
+                catch (Exception ex)
+                {
+                    converted = false;
+                    MessageBox.Show(
+                        $"The nest template could not be converted to the new defaults file:\n{templatePath}\n\n{ex.Message}\n\nIt will no longer be loaded automatically. Set defaults under Tools > Nest Defaults.",
+                        "Nest Template",
+                        MessageBoxButtons.OK,
+                        MessageBoxIcon.Warning
+                    );
+                }
+            }
+
+            if (!converted)
+                return;
+
+            Settings.Default.NestTemplatePath = "";
+            Settings.Default.Save();
+        }
+
         protected override void OnClosing(System.ComponentModel.CancelEventArgs e)
         {
             base.OnClosing(e);
@@ -472,30 +540,7 @@ namespace OpenNest.Forms
             var windowState =
                 ActiveMdiChild != null ? ActiveMdiChild.WindowState : FormWindowState.Maximized;
 
-            Nest nest;
-
-            if (File.Exists(Properties.Settings.Default.NestTemplatePath))
-            {
-                try
-                {
-                    var reader = new NestReader(Properties.Settings.Default.NestTemplatePath);
-                    nest = reader.Read();
-                }
-                catch (Exception ex)
-                {
-                    MessageBox.Show(
-                        $"Failed to load nest template:\n{ex.Message}\n\nA default nest will be created instead.",
-                        "Template Error",
-                        MessageBoxButtons.OK,
-                        MessageBoxIcon.Warning
-                    );
-                    nest = CreateDefaultNest();
-                }
-            }
-            else
-            {
-                nest = CreateDefaultNest();
-            }
+            var nest = CreateDefaultNest();
 
             nest.DateCreated = DateTime.Now;
             nest.DateLastModified = DateTime.Now;
@@ -741,6 +786,20 @@ namespace OpenNest.Forms
             }
         }
 
+        private void ExpandSpacing_Click(object sender, EventArgs e)
+        {
+            if (activeForm == null)
+                return;
+
+            if (!activeForm.PlateView.ExpandSelected())
+                MessageBox.Show(
+                    "Select at least two parts on the plate to expand.",
+                    "Expand Spacing",
+                    MessageBoxButtons.OK,
+                    MessageBoxIcon.Information
+                );
+        }
+
         private void PatternTile_Click(object sender, EventArgs e)
         {
             if (activeForm == null)
@@ -819,6 +878,25 @@ namespace OpenNest.Forms
             form.ShowDialog();
         }
 
+        private void NestDefaults_Click(object sender, EventArgs e)
+        {
+            using (var form = new NestDefaultsForm(NestDefaults.Load(NestDefaults.DefaultPath)))
+            {
+                if (form.ShowDialog(this) == DialogResult.OK)
+                    form.GetDefaults().Save(NestDefaults.DefaultPath);
+            }
+        }
+
+        private void SaveCurrentAsDefaults_Click(object sender, EventArgs e)
+        {
+            if (activeForm == null)
+                return;
+
+            NestDefaults
+                .FromPlate(activeForm.Nest.Units, activeForm.PlateView.Plate)
+                .Save(NestDefaults.DefaultPath);
+        }
+
         private void MachineConfig_Click(object sender, EventArgs e)
         {
             var appDataPath = Path.Combine(
@@ -1040,7 +1118,6 @@ namespace OpenNest.Forms
 
             nestingCts = new CancellationTokenSource();
             var progressForm = new NestProgressForm(nestingCts, showPlateRow: true);
-            progressForm.PreviewPlate = CreatePreviewPlate(activeForm.PlateView.Plate);
 
             var jobEngineName = EngineSelection.IsFillStrategy(EngineSelection.EngineName)
                 ? null
@@ -1052,9 +1129,7 @@ namespace OpenNest.Forms
                 progressForm.UpdateProgress(p);
 
                 if (p.IsOverallBest)
-                    progressForm.UpdatePreview(p.BestParts);
-
-                activeForm.PlateView.SetActiveParts(p.BestParts);
+                    activeForm.PlateView.SetActiveParts(p.BestParts);
                 activeForm.PlateView.ActiveWorkArea = p.ActiveWorkArea;
             });
 
@@ -1152,7 +1227,7 @@ namespace OpenNest.Forms
                 {
                     if (pr.IsNew)
                     {
-                        var plate = GetOrCreatePlate(progressForm);
+                        var plate = GetOrCreatePlate();
                         plate.Size = pr.Plate.Size;
                         plate.Parts.AddRange(pr.Parts);
                     }
@@ -1172,7 +1247,7 @@ namespace OpenNest.Forms
                 if (remaining.Count == 0 || token.IsCancellationRequested)
                     break;
 
-                var plate = GetOrCreatePlate(progressForm);
+                var plate = GetOrCreatePlate();
 
                 var placed = await NestSinglePlateAsync(
                     plate,
@@ -1242,7 +1317,7 @@ namespace OpenNest.Forms
                 if (parts.Count == 0)
                     continue;
 
-                var plate = GetOrCreatePlate(progressForm);
+                var plate = GetOrCreatePlate();
                 plate.Size = sheet.Stock.Size;
                 plate.Parts.AddRange(parts);
             }
@@ -1260,11 +1335,10 @@ namespace OpenNest.Forms
                 );
         }
 
-        private Plate GetOrCreatePlate(NestProgressForm progressForm)
+        private Plate GetOrCreatePlate()
         {
             var plate = activeForm.PlateManager.GetOrCreateEmpty();
             activeForm.PlateManager.LoadLast();
-            progressForm.PreviewPlate = CreatePreviewPlate(plate);
             return plate;
         }
 
@@ -1340,17 +1414,6 @@ namespace OpenNest.Forms
             return true;
         }
 
-        private static Plate CreatePreviewPlate(Plate source)
-        {
-            var plate = new Plate(source.Size)
-            {
-                Quadrant = source.Quadrant,
-                PartSpacing = source.PartSpacing,
-            };
-            plate.EdgeSpacing = source.EdgeSpacing;
-            return plate;
-        }
-
         private void SequenceAllPlates_Click(object sender, EventArgs e)
         {
             if (activeForm == null)

OpenNest/Forms/NestDefaultsForm.Designer.cs

@@ -0,0 +1,364 @@
+namespace OpenNest.Forms
+{
+    partial class NestDefaultsForm
+    {
+        /// <summary>
+        /// Required designer variable.
+        /// </summary>
+        private System.ComponentModel.IContainer components = null;
+
+        /// <summary>
+        /// Clean up any resources being used.
+        /// </summary>
+        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing && (components != null))
+            {
+                components.Dispose();
+            }
+            base.Dispose(disposing);
+        }
+
+        #region Windows Form Designer generated code
+
+        /// <summary>
+        /// Required method for Designer support - do not modify
+        /// the contents of this method with the code editor.
+        /// </summary>
+        private void InitializeComponent()
+        {
+            this.tableLayoutPanel1 = new System.Windows.Forms.TableLayoutPanel();
+            this.unitsLabel = new System.Windows.Forms.Label();
+            this.unitsCombo = new System.Windows.Forms.ComboBox();
+            this.widthLabel = new System.Windows.Forms.Label();
+            this.widthBox = new OpenNest.Controls.NumericUpDown();
+            this.lengthLabel = new System.Windows.Forms.Label();
+            this.lengthBox = new OpenNest.Controls.NumericUpDown();
+            this.partSpacingLabel = new System.Windows.Forms.Label();
+            this.partSpacingBox = new OpenNest.Controls.NumericUpDown();
+            this.edgeLeftLabel = new System.Windows.Forms.Label();
+            this.edgeLeftBox = new OpenNest.Controls.NumericUpDown();
+            this.edgeBottomLabel = new System.Windows.Forms.Label();
+            this.edgeBottomBox = new OpenNest.Controls.NumericUpDown();
+            this.edgeRightLabel = new System.Windows.Forms.Label();
+            this.edgeRightBox = new OpenNest.Controls.NumericUpDown();
+            this.edgeTopLabel = new System.Windows.Forms.Label();
+            this.edgeTopBox = new OpenNest.Controls.NumericUpDown();
+            this.quadrantLabel = new System.Windows.Forms.Label();
+            this.quadrantSelect1 = new OpenNest.Controls.QuadrantSelect();
+            this.saveButton = new System.Windows.Forms.Button();
+            this.cancelButton = new System.Windows.Forms.Button();
+            ((System.ComponentModel.ISupportInitialize)(this.widthBox)).BeginInit();
+            ((System.ComponentModel.ISupportInitialize)(this.lengthBox)).BeginInit();
+            ((System.ComponentModel.ISupportInitialize)(this.partSpacingBox)).BeginInit();
+            ((System.ComponentModel.ISupportInitialize)(this.edgeLeftBox)).BeginInit();
+            ((System.ComponentModel.ISupportInitialize)(this.edgeBottomBox)).BeginInit();
+            ((System.ComponentModel.ISupportInitialize)(this.edgeRightBox)).BeginInit();
+            ((System.ComponentModel.ISupportInitialize)(this.edgeTopBox)).BeginInit();
+            this.tableLayoutPanel1.SuspendLayout();
+            this.SuspendLayout();
+            //
+            // tableLayoutPanel1
+            //
+            this.tableLayoutPanel1.ColumnCount = 2;
+            this.tableLayoutPanel1.ColumnStyles.Add(new System.Windows.Forms.ColumnStyle());
+            this.tableLayoutPanel1.ColumnStyles.Add(new System.Windows.Forms.ColumnStyle(System.Windows.Forms.SizeType.Percent, 100F));
+            this.tableLayoutPanel1.Controls.Add(this.unitsLabel, 0, 0);
+            this.tableLayoutPanel1.Controls.Add(this.unitsCombo, 1, 0);
+            this.tableLayoutPanel1.Controls.Add(this.widthLabel, 0, 1);
+            this.tableLayoutPanel1.Controls.Add(this.widthBox, 1, 1);
+            this.tableLayoutPanel1.Controls.Add(this.lengthLabel, 0, 2);
+            this.tableLayoutPanel1.Controls.Add(this.lengthBox, 1, 2);
+            this.tableLayoutPanel1.Controls.Add(this.partSpacingLabel, 0, 3);
+            this.tableLayoutPanel1.Controls.Add(this.partSpacingBox, 1, 3);
+            this.tableLayoutPanel1.Controls.Add(this.edgeLeftLabel, 0, 4);
+            this.tableLayoutPanel1.Controls.Add(this.edgeLeftBox, 1, 4);
+            this.tableLayoutPanel1.Controls.Add(this.edgeBottomLabel, 0, 5);
+            this.tableLayoutPanel1.Controls.Add(this.edgeBottomBox, 1, 5);
+            this.tableLayoutPanel1.Controls.Add(this.edgeRightLabel, 0, 6);
+            this.tableLayoutPanel1.Controls.Add(this.edgeRightBox, 1, 6);
+            this.tableLayoutPanel1.Controls.Add(this.edgeTopLabel, 0, 7);
+            this.tableLayoutPanel1.Controls.Add(this.edgeTopBox, 1, 7);
+            this.tableLayoutPanel1.Controls.Add(this.quadrantLabel, 0, 8);
+            this.tableLayoutPanel1.Controls.Add(this.quadrantSelect1, 1, 8);
+            this.tableLayoutPanel1.Location = new System.Drawing.Point(12, 12);
+            this.tableLayoutPanel1.Name = "tableLayoutPanel1";
+            this.tableLayoutPanel1.RowCount = 9;
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 11.11111F));
+            this.tableLayoutPanel1.Size = new System.Drawing.Size(410, 260);
+            this.tableLayoutPanel1.TabIndex = 0;
+            //
+            // unitsLabel
+            //
+            this.unitsLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.unitsLabel.AutoSize = true;
+            this.unitsLabel.Location = new System.Drawing.Point(3, 9);
+            this.unitsLabel.Name = "unitsLabel";
+            this.unitsLabel.Size = new System.Drawing.Size(100, 16);
+            this.unitsLabel.TabIndex = 0;
+            this.unitsLabel.Text = "Units:";
+            //
+            // unitsCombo
+            //
+            this.unitsCombo.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.unitsCombo.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList;
+            this.unitsCombo.Items.AddRange(new object[] { "Inches", "Millimeters" });
+            this.unitsCombo.Location = new System.Drawing.Point(109, 5);
+            this.unitsCombo.Name = "unitsCombo";
+            this.unitsCombo.Size = new System.Drawing.Size(298, 24);
+            this.unitsCombo.TabIndex = 1;
+            //
+            // widthLabel
+            //
+            this.widthLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.widthLabel.AutoSize = true;
+            this.widthLabel.Location = new System.Drawing.Point(3, 37);
+            this.widthLabel.Name = "widthLabel";
+            this.widthLabel.Size = new System.Drawing.Size(100, 16);
+            this.widthLabel.TabIndex = 2;
+            this.widthLabel.Text = "Plate width:";
+            //
+            // widthBox
+            //
+            this.widthBox.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.widthBox.DecimalPlaces = 4;
+            this.widthBox.Location = new System.Drawing.Point(109, 33);
+            this.widthBox.Maximum = new decimal(new int[] { 1000000, 0, 0, 0 });
+            this.widthBox.Minimum = new decimal(new int[] { 1, 0, 0, 196608 });
+            this.widthBox.Name = "widthBox";
+            this.widthBox.Size = new System.Drawing.Size(298, 22);
+            this.widthBox.Suffix = "";
+            this.widthBox.TabIndex = 3;
+            //
+            // lengthLabel
+            //
+            this.lengthLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.lengthLabel.AutoSize = true;
+            this.lengthLabel.Location = new System.Drawing.Point(3, 65);
+            this.lengthLabel.Name = "lengthLabel";
+            this.lengthLabel.Size = new System.Drawing.Size(100, 16);
+            this.lengthLabel.TabIndex = 4;
+            this.lengthLabel.Text = "Plate length:";
+            //
+            // lengthBox
+            //
+            this.lengthBox.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.lengthBox.DecimalPlaces = 4;
+            this.lengthBox.Location = new System.Drawing.Point(109, 61);
+            this.lengthBox.Maximum = new decimal(new int[] { 1000000, 0, 0, 0 });
+            this.lengthBox.Minimum = new decimal(new int[] { 1, 0, 0, 196608 });
+            this.lengthBox.Name = "lengthBox";
+            this.lengthBox.Size = new System.Drawing.Size(298, 22);
+            this.lengthBox.Suffix = "";
+            this.lengthBox.TabIndex = 5;
+            //
+            // partSpacingLabel
+            //
+            this.partSpacingLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.partSpacingLabel.AutoSize = true;
+            this.partSpacingLabel.Location = new System.Drawing.Point(3, 93);
+            this.partSpacingLabel.Name = "partSpacingLabel";
+            this.partSpacingLabel.Size = new System.Drawing.Size(100, 16);
+            this.partSpacingLabel.TabIndex = 6;
+            this.partSpacingLabel.Text = "Part spacing:";
+            //
+            // partSpacingBox
+            //
+            this.partSpacingBox.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.partSpacingBox.DecimalPlaces = 4;
+            this.partSpacingBox.Location = new System.Drawing.Point(109, 89);
+            this.partSpacingBox.Maximum = new decimal(new int[] { 1000000, 0, 0, 0 });
+            this.partSpacingBox.Name = "partSpacingBox";
+            this.partSpacingBox.Size = new System.Drawing.Size(298, 22);
+            this.partSpacingBox.Suffix = "";
+            this.partSpacingBox.TabIndex = 7;
+            //
+            // edgeLeftLabel
+            //
+            this.edgeLeftLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.edgeLeftLabel.AutoSize = true;
+            this.edgeLeftLabel.Location = new System.Drawing.Point(3, 121);
+            this.edgeLeftLabel.Name = "edgeLeftLabel";
+            this.edgeLeftLabel.Size = new System.Drawing.Size(100, 16);
+            this.edgeLeftLabel.TabIndex = 8;
+            this.edgeLeftLabel.Text = "Edge left:";
+            //
+            // edgeLeftBox
+            //
+            this.edgeLeftBox.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.edgeLeftBox.DecimalPlaces = 4;
+            this.edgeLeftBox.Location = new System.Drawing.Point(109, 117);
+            this.edgeLeftBox.Maximum = new decimal(new int[] { 1000000, 0, 0, 0 });
+            this.edgeLeftBox.Name = "edgeLeftBox";
+            this.edgeLeftBox.Size = new System.Drawing.Size(298, 22);
+            this.edgeLeftBox.Suffix = "";
+            this.edgeLeftBox.TabIndex = 9;
+            //
+            // edgeBottomLabel
+            //
+            this.edgeBottomLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.edgeBottomLabel.AutoSize = true;
+            this.edgeBottomLabel.Location = new System.Drawing.Point(3, 149);
+            this.edgeBottomLabel.Name = "edgeBottomLabel";
+            this.edgeBottomLabel.Size = new System.Drawing.Size(100, 16);
+            this.edgeBottomLabel.TabIndex = 10;
+            this.edgeBottomLabel.Text = "Edge bottom:";
+            //
+            // edgeBottomBox
+            //
+            this.edgeBottomBox.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.edgeBottomBox.DecimalPlaces = 4;
+            this.edgeBottomBox.Location = new System.Drawing.Point(109, 145);
+            this.edgeBottomBox.Maximum = new decimal(new int[] { 1000000, 0, 0, 0 });
+            this.edgeBottomBox.Name = "edgeBottomBox";
+            this.edgeBottomBox.Size = new System.Drawing.Size(298, 22);
+            this.edgeBottomBox.Suffix = "";
+            this.edgeBottomBox.TabIndex = 11;
+            //
+            // edgeRightLabel
+            //
+            this.edgeRightLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.edgeRightLabel.AutoSize = true;
+            this.edgeRightLabel.Location = new System.Drawing.Point(3, 177);
+            this.edgeRightLabel.Name = "edgeRightLabel";
+            this.edgeRightLabel.Size = new System.Drawing.Size(100, 16);
+            this.edgeRightLabel.TabIndex = 12;
+            this.edgeRightLabel.Text = "Edge right:";
+            //
+            // edgeRightBox
+            //
+            this.edgeRightBox.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.edgeRightBox.DecimalPlaces = 4;
+            this.edgeRightBox.Location = new System.Drawing.Point(109, 173);
+            this.edgeRightBox.Maximum = new decimal(new int[] { 1000000, 0, 0, 0 });
+            this.edgeRightBox.Name = "edgeRightBox";
+            this.edgeRightBox.Size = new System.Drawing.Size(298, 22);
+            this.edgeRightBox.Suffix = "";
+            this.edgeRightBox.TabIndex = 13;
+            //
+            // edgeTopLabel
+            //
+            this.edgeTopLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.edgeTopLabel.AutoSize = true;
+            this.edgeTopLabel.Location = new System.Drawing.Point(3, 205);
+            this.edgeTopLabel.Name = "edgeTopLabel";
+            this.edgeTopLabel.Size = new System.Drawing.Size(100, 16);
+            this.edgeTopLabel.TabIndex = 14;
+            this.edgeTopLabel.Text = "Edge top:";
+            //
+            // edgeTopBox
+            //
+            this.edgeTopBox.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.edgeTopBox.DecimalPlaces = 4;
+            this.edgeTopBox.Location = new System.Drawing.Point(109, 201);
+            this.edgeTopBox.Maximum = new decimal(new int[] { 1000000, 0, 0, 0 });
+            this.edgeTopBox.Name = "edgeTopBox";
+            this.edgeTopBox.Size = new System.Drawing.Size(298, 22);
+            this.edgeTopBox.Suffix = "";
+            this.edgeTopBox.TabIndex = 15;
+            //
+            // quadrantLabel
+            //
+            this.quadrantLabel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
+            this.quadrantLabel.AutoSize = true;
+            this.quadrantLabel.Location = new System.Drawing.Point(3, 233);
+            this.quadrantLabel.Name = "quadrantLabel";
+            this.quadrantLabel.Size = new System.Drawing.Size(100, 16);
+            this.quadrantLabel.TabIndex = 16;
+            this.quadrantLabel.Text = "Quadrant:";
+            //
+            // quadrantSelect1
+            //
+            this.quadrantSelect1.Anchor = System.Windows.Forms.AnchorStyles.None;
+            this.quadrantSelect1.BackColor = System.Drawing.Color.White;
+            this.quadrantSelect1.Location = new System.Drawing.Point(198, 225);
+            this.quadrantSelect1.Name = "quadrantSelect1";
+            this.quadrantSelect1.Quadrant = 1;
+            this.quadrantSelect1.Size = new System.Drawing.Size(120, 28);
+            this.quadrantSelect1.TabIndex = 17;
+            //
+            // saveButton
+            //
+            this.saveButton.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Bottom | System.Windows.Forms.AnchorStyles.Right)));
+            this.saveButton.DialogResult = System.Windows.Forms.DialogResult.OK;
+            this.saveButton.Location = new System.Drawing.Point(243, 285);
+            this.saveButton.Name = "saveButton";
+            this.saveButton.Size = new System.Drawing.Size(90, 28);
+            this.saveButton.TabIndex = 1;
+            this.saveButton.Text = "Save";
+            this.saveButton.UseVisualStyleBackColor = true;
+            //
+            // cancelButton
+            //
+            this.cancelButton.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Bottom | System.Windows.Forms.AnchorStyles.Right)));
+            this.cancelButton.DialogResult = System.Windows.Forms.DialogResult.Cancel;
+            this.cancelButton.Location = new System.Drawing.Point(339, 285);
+            this.cancelButton.Name = "cancelButton";
+            this.cancelButton.Size = new System.Drawing.Size(90, 28);
+            this.cancelButton.TabIndex = 2;
+            this.cancelButton.Text = "Cancel";
+            this.cancelButton.UseVisualStyleBackColor = true;
+            //
+            // NestDefaultsForm
+            //
+            this.AcceptButton = this.saveButton;
+            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.None;
+            this.CancelButton = this.cancelButton;
+            this.ClientSize = new System.Drawing.Size(434, 322);
+            this.Controls.Add(this.tableLayoutPanel1);
+            this.Controls.Add(this.saveButton);
+            this.Controls.Add(this.cancelButton);
+            this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedDialog;
+            this.MaximizeBox = false;
+            this.MinimizeBox = false;
+            this.Name = "NestDefaultsForm";
+            this.ShowIcon = false;
+            this.ShowInTaskbar = false;
+            this.StartPosition = System.Windows.Forms.FormStartPosition.CenterParent;
+            this.Text = "Nest Defaults";
+            ((System.ComponentModel.ISupportInitialize)(this.widthBox)).EndInit();
+            ((System.ComponentModel.ISupportInitialize)(this.lengthBox)).EndInit();
+            ((System.ComponentModel.ISupportInitialize)(this.partSpacingBox)).EndInit();
+            ((System.ComponentModel.ISupportInitialize)(this.edgeLeftBox)).EndInit();
+            ((System.ComponentModel.ISupportInitialize)(this.edgeBottomBox)).EndInit();
+            ((System.ComponentModel.ISupportInitialize)(this.edgeRightBox)).EndInit();
+            ((System.ComponentModel.ISupportInitialize)(this.edgeTopBox)).EndInit();
+            this.tableLayoutPanel1.ResumeLayout(false);
+            this.tableLayoutPanel1.PerformLayout();
+            this.ResumeLayout(false);
+        }
+
+        #endregion
+
+        private System.Windows.Forms.TableLayoutPanel tableLayoutPanel1;
+        private System.Windows.Forms.Label unitsLabel;
+        private System.Windows.Forms.ComboBox unitsCombo;
+        private System.Windows.Forms.Label widthLabel;
+        private OpenNest.Controls.NumericUpDown widthBox;
+        private System.Windows.Forms.Label lengthLabel;
+        private OpenNest.Controls.NumericUpDown lengthBox;
+        private System.Windows.Forms.Label partSpacingLabel;
+        private OpenNest.Controls.NumericUpDown partSpacingBox;
+        private System.Windows.Forms.Label edgeLeftLabel;
+        private OpenNest.Controls.NumericUpDown edgeLeftBox;
+        private System.Windows.Forms.Label edgeBottomLabel;
+        private OpenNest.Controls.NumericUpDown edgeBottomBox;
+        private System.Windows.Forms.Label edgeRightLabel;
+        private OpenNest.Controls.NumericUpDown edgeRightBox;
+        private System.Windows.Forms.Label edgeTopLabel;
+        private OpenNest.Controls.NumericUpDown edgeTopBox;
+        private System.Windows.Forms.Label quadrantLabel;
+        private OpenNest.Controls.QuadrantSelect quadrantSelect1;
+        private System.Windows.Forms.Button saveButton;
+        private System.Windows.Forms.Button cancelButton;
+    }
+}

OpenNest/Forms/NestDefaultsForm.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Windows.Forms;
+using OpenNest.Data;
+using OpenNest.Geometry;
+
+namespace OpenNest.Forms
+{
+    /// <summary>
+    /// Edits the persisted nest defaults (Tools &gt; Nest Defaults), written
+    /// to the JSON file used when creating a new nest.
+    /// </summary>
+    public partial class NestDefaultsForm : Form
+    {
+        public NestDefaultsForm(NestDefaults defaults)
+        {
+            InitializeComponent();
+
+            unitsCombo.SelectedIndex = defaults.Units == Units.Millimeters ? 1 : 0;
+            widthBox.Value = Clamp(widthBox, defaults.Size.Width);
+            lengthBox.Value = Clamp(lengthBox, defaults.Size.Length);
+            partSpacingBox.Value = Clamp(partSpacingBox, defaults.PartSpacing);
+            edgeLeftBox.Value = Clamp(edgeLeftBox, defaults.EdgeSpacing.Left);
+            edgeBottomBox.Value = Clamp(edgeBottomBox, defaults.EdgeSpacing.Bottom);
+            edgeRightBox.Value = Clamp(edgeRightBox, defaults.EdgeSpacing.Right);
+            edgeTopBox.Value = Clamp(edgeTopBox, defaults.EdgeSpacing.Top);
+            quadrantSelect1.Quadrant = defaults.Quadrant;
+        }
+
+        public Units Units => unitsCombo.SelectedIndex == 1 ? Units.Millimeters : Units.Inches;
+
+        public NestDefaults GetDefaults()
+        {
+            return new NestDefaults
+            {
+                Units = Units,
+                Size = new Size((double)widthBox.Value, (double)lengthBox.Value),
+                Quadrant = quadrantSelect1.Quadrant,
+                PartSpacing = (double)partSpacingBox.Value,
+                EdgeSpacing = new Spacing(
+                    (double)edgeLeftBox.Value,
+                    (double)edgeBottomBox.Value,
+                    (double)edgeRightBox.Value,
+                    (double)edgeTopBox.Value
+                ),
+            };
+        }
+
+        private static decimal Clamp(NumericUpDown box, double value)
+        {
+            if (double.IsNaN(value) || double.IsInfinity(value))
+                return box.Minimum;
+
+            if (value < (double)box.Minimum)
+                return box.Minimum;
+
+            if (value > (double)box.Maximum)
+                return box.Maximum;
+
+            return (decimal)value;
+        }
+    }
+}

OpenNest/Forms/NestProgressForm.Designer.cs

@@ -17,11 +17,6 @@ namespace OpenNest.Forms
 
         private void InitializeComponent()
         {
-            ColorScheme colorScheme1 = new ColorScheme();
-            CutOffSettings cutOffSettings1 = new CutOffSettings();
-            Plate plate1 = new Plate();
-            Collections.ObservableList<CutOff> observableList_11 = new Collections.ObservableList<CutOff>();
-            Collections.ObservableList<Part> observableList_12 = new Collections.ObservableList<Part>();
             phaseStepper = new OpenNest.Controls.PhaseStepperControl();
             resultsPanel = new System.Windows.Forms.Panel();
             resultsTable = new System.Windows.Forms.TableLayoutPanel();
@@ -46,8 +41,6 @@ namespace OpenNest.Forms
             buttonPanel = new System.Windows.Forms.FlowLayoutPanel();
             stopButton = new System.Windows.Forms.Button();
             acceptButton = new System.Windows.Forms.Button();
-            splitContainer = new System.Windows.Forms.SplitContainer();
-            previewPlateView = new OpenNest.Controls.PlateView();
             statsPanel = new System.Windows.Forms.Panel();
             resultsPanel.SuspendLayout();
             resultsTable.SuspendLayout();
@@ -55,10 +48,6 @@ namespace OpenNest.Forms
             statusPanel.SuspendLayout();
             statusTable.SuspendLayout();
             buttonPanel.SuspendLayout();
-            ((System.ComponentModel.ISupportInitialize)splitContainer).BeginInit();
-            splitContainer.Panel1.SuspendLayout();
-            splitContainer.Panel2.SuspendLayout();
-            splitContainer.SuspendLayout();
             statsPanel.SuspendLayout();
             SuspendLayout();
             // 
@@ -365,74 +354,6 @@ namespace OpenNest.Forms
             acceptButton.UseVisualStyleBackColor = true;
             acceptButton.Click += AcceptButton_Click;
             // 
-            // splitContainer
-            // 
-            splitContainer.Dock = System.Windows.Forms.DockStyle.Fill;
-            splitContainer.FixedPanel = System.Windows.Forms.FixedPanel.Panel2;
-            splitContainer.Location = new System.Drawing.Point(0, 0);
-            splitContainer.Name = "splitContainer";
-            // 
-            // splitContainer.Panel1
-            // 
-            splitContainer.Panel1.Controls.Add(previewPlateView);
-            // 
-            // splitContainer.Panel2
-            // 
-            splitContainer.Panel2.Controls.Add(statsPanel);
-            splitContainer.Size = new System.Drawing.Size(626, 341);
-            splitContainer.SplitterDistance = 356;
-            splitContainer.TabIndex = 0;
-            // 
-            // previewPlateView
-            // 
-            previewPlateView.ActiveWorkArea = null;
-            previewPlateView.AllowPan = true;
-            previewPlateView.AllowSelect = true;
-            previewPlateView.AllowZoom = true;
-            previewPlateView.BackColor = System.Drawing.Color.DarkGray;
-            colorScheme1.BackgroundColor = System.Drawing.Color.DarkGray;
-            colorScheme1.BoundingBoxColor = System.Drawing.Color.FromArgb(128, 128, 255);
-            colorScheme1.EdgeSpacingColor = System.Drawing.Color.FromArgb(180, 180, 180);
-            colorScheme1.LayoutFillColor = System.Drawing.Color.WhiteSmoke;
-            colorScheme1.LayoutOutlineColor = System.Drawing.Color.Gray;
-            colorScheme1.OriginColor = System.Drawing.Color.Gray;
-            colorScheme1.PreviewPartColor = System.Drawing.Color.FromArgb(255, 140, 0);
-            colorScheme1.RapidColor = System.Drawing.Color.DodgerBlue;
-            previewPlateView.ColorScheme = colorScheme1;
-            cutOffSettings1.CutDirection = CutDirection.AwayFromOrigin;
-            cutOffSettings1.MinSegmentLength = 0.05D;
-            cutOffSettings1.Overtravel = 0D;
-            cutOffSettings1.PartClearance = 0.02D;
-            previewPlateView.CutOffSettings = cutOffSettings1;
-            previewPlateView.DebugRemnantPriorities = null;
-            previewPlateView.DebugRemnants = null;
-            previewPlateView.Dock = System.Windows.Forms.DockStyle.Fill;
-            previewPlateView.DrawBounds = true;
-            previewPlateView.DrawCutDirection = false;
-            previewPlateView.DrawOffset = false;
-            previewPlateView.DrawOrigin = true;
-            previewPlateView.DrawPiercePoints = false;
-            previewPlateView.DrawRapid = false;
-            previewPlateView.FillParts = true;
-            previewPlateView.Location = new System.Drawing.Point(0, 0);
-            previewPlateView.Name = "previewPlateView";
-            previewPlateView.OffsetIncrementDistance = 10D;
-            previewPlateView.OffsetTolerance = 0.001D;
-            plate1.CutOffs = observableList_11;
-            plate1.CuttingParameters = null;
-            plate1.GrainAngle = 0D;
-            plate1.Parts = observableList_12;
-            plate1.PartSpacing = 0D;
-            plate1.Quadrant = 1;
-            plate1.Quantity = 0;
-            previewPlateView.Plate = plate1;
-            previewPlateView.RotateIncrementAngle = 10D;
-
-            previewPlateView.ShowBendLines = false;
-            previewPlateView.Size = new System.Drawing.Size(356, 341);
-            previewPlateView.Status = "Select";
-            previewPlateView.TabIndex = 0;
-            // 
             // statsPanel
             // 
             statsPanel.AutoScroll = true;
@@ -450,12 +371,12 @@ namespace OpenNest.Forms
             // 
             AutoScaleDimensions = new System.Drawing.SizeF(7F, 15F);
             AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
-            ClientSize = new System.Drawing.Size(626, 341);
-            Controls.Add(splitContainer);
+            ClientSize = new System.Drawing.Size(266, 341);
+            Controls.Add(statsPanel);
             FormBorderStyle = System.Windows.Forms.FormBorderStyle.SizableToolWindow;
             MaximizeBox = false;
             MinimizeBox = false;
-            MinimumSize = new System.Drawing.Size(550, 380);
+            MinimumSize = new System.Drawing.Size(320, 380);
             Name = "NestProgressForm";
             ShowInTaskbar = false;
             StartPosition = System.Windows.Forms.FormStartPosition.CenterParent;
@@ -471,10 +392,6 @@ namespace OpenNest.Forms
             statusTable.ResumeLayout(false);
             statusTable.PerformLayout();
             buttonPanel.ResumeLayout(false);
-            splitContainer.Panel1.ResumeLayout(false);
-            splitContainer.Panel2.ResumeLayout(false);
-            ((System.ComponentModel.ISupportInitialize)splitContainer).EndInit();
-            splitContainer.ResumeLayout(false);
             statsPanel.ResumeLayout(false);
             statsPanel.PerformLayout();
             ResumeLayout(false);
@@ -506,8 +423,6 @@ namespace OpenNest.Forms
         private System.Windows.Forms.FlowLayoutPanel buttonPanel;
         private System.Windows.Forms.Button acceptButton;
         private System.Windows.Forms.Button stopButton;
-        private System.Windows.Forms.SplitContainer splitContainer;
         private System.Windows.Forms.Panel statsPanel;
-        private Controls.PlateView previewPlateView;
     }
 }

OpenNest/Forms/NestProgressForm.cs

@@ -38,23 +38,11 @@ namespace OpenNest.Forms
             set => acceptButton.Visible = value;
         }
 
-        public Plate PreviewPlate
-        {
-            get => previewPlateView.Plate;
-            set
-            {
-                previewPlateView.Plate = value;
-                previewPlateView.ZoomToFit();
-            }
-        }
-
         public NestProgressForm(CancellationTokenSource cts, bool showPlateRow = true)
         {
             this.cts = cts;
             InitializeComponent();
 
-            previewPlateView.AllowSelect = false;
-
             if (!showPlateRow)
             {
                 plateLabel.Visible = false;
@@ -104,20 +92,6 @@ namespace OpenNest.Forms
                 : progress.Phase.DisplayName();
         }
 
-        public void UpdatePreview(List<Part> bestParts)
-        {
-            if (IsDisposed || !IsHandleCreated)
-                return;
-
-            var plate = previewPlateView.Plate;
-            plate.Parts.Clear();
-
-            foreach (var part in bestParts)
-                plate.Parts.Add((Part)part.Clone());
-
-            previewPlateView.ZoomToFit();
-        }
-
         public void ShowCompleted()
         {
             if (IsDisposed || !IsHandleCreated)
@@ -178,8 +152,6 @@ namespace OpenNest.Forms
             if (!cts.IsCancellationRequested)
                 cts.Cancel();
 
-            previewPlateView.Dispose();
-
             base.OnFormClosing(e);
         }
 

OpenNest/Forms/OptionsForm.Designer.cs

@@ -34,9 +34,6 @@
             this.toolTip1 = new System.Windows.Forms.ToolTip(this.components);
             this.numericUpDown1 = new OpenNest.Controls.NumericUpDown();
             this.tableLayoutPanel1 = new System.Windows.Forms.TableLayoutPanel();
-            this.textBox1 = new System.Windows.Forms.TextBox();
-            this.label3 = new System.Windows.Forms.Label();
-            this.button1 = new System.Windows.Forms.Button();
             this.saveButton = new System.Windows.Forms.Button();
             this.cancelButton = new System.Windows.Forms.Button();
             this.bottomPanel1 = new OpenNest.Controls.BottomPanel();
@@ -94,55 +91,20 @@
             this.tableLayoutPanel1.ColumnStyles.Add(new System.Windows.Forms.ColumnStyle(System.Windows.Forms.SizeType.Percent, 100F));
             this.tableLayoutPanel1.ColumnStyles.Add(new System.Windows.Forms.ColumnStyle(System.Windows.Forms.SizeType.Absolute, 297F));
             this.tableLayoutPanel1.ColumnStyles.Add(new System.Windows.Forms.ColumnStyle(System.Windows.Forms.SizeType.Absolute, 100F));
-            this.tableLayoutPanel1.Controls.Add(this.label1, 0, 1);
-            this.tableLayoutPanel1.Controls.Add(this.textBox1, 1, 0);
-            this.tableLayoutPanel1.Controls.Add(this.label3, 0, 0);
-            this.tableLayoutPanel1.Controls.Add(this.colorSchemeLabel, 0, 2);
-            this.tableLayoutPanel1.Controls.Add(this.colorSchemeCombo, 1, 2);
-            this.tableLayoutPanel1.Controls.Add(this.checkBox1, 0, 3);
-            this.tableLayoutPanel1.Controls.Add(this.numericUpDown1, 1, 1);
-            this.tableLayoutPanel1.Controls.Add(this.button1, 3, 0);
+            this.tableLayoutPanel1.Controls.Add(this.label1, 0, 0);
+            this.tableLayoutPanel1.Controls.Add(this.colorSchemeLabel, 0, 1);
+            this.tableLayoutPanel1.Controls.Add(this.colorSchemeCombo, 1, 1);
+            this.tableLayoutPanel1.Controls.Add(this.checkBox1, 0, 2);
+            this.tableLayoutPanel1.Controls.Add(this.numericUpDown1, 1, 0);
             this.tableLayoutPanel1.Location = new System.Drawing.Point(12, 12);
             this.tableLayoutPanel1.Name = "tableLayoutPanel1";
-            this.tableLayoutPanel1.RowCount = 4;
-            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 25F));
-            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 25F));
-            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 25F));
-            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 25F));
+            this.tableLayoutPanel1.RowCount = 3;
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 33.33333F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 33.33333F));
+            this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 33.33333F));
             this.tableLayoutPanel1.Size = new System.Drawing.Size(684, 160);
             this.tableLayoutPanel1.TabIndex = 0;
             // 
-            // textBox1
-            // 
-            this.textBox1.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
-            this.tableLayoutPanel1.SetColumnSpan(this.textBox1, 2);
-            this.textBox1.Location = new System.Drawing.Point(154, 9);
-            this.textBox1.Name = "textBox1";
-            this.textBox1.Size = new System.Drawing.Size(427, 22);
-            this.textBox1.TabIndex = 1;
-            // 
-            // label3
-            // 
-            this.label3.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
-            this.label3.AutoSize = true;
-            this.label3.Location = new System.Drawing.Point(3, 12);
-            this.label3.Name = "label3";
-            this.label3.Size = new System.Drawing.Size(145, 16);
-            this.label3.TabIndex = 0;
-            this.label3.Text = "Nest Template Path:";
-            //
-            // button1
-            // 
-            this.button1.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Left | System.Windows.Forms.AnchorStyles.Right)));
-            this.button1.Location = new System.Drawing.Point(588, 6);
-            this.button1.Margin = new System.Windows.Forms.Padding(4);
-            this.button1.Name = "button1";
-            this.button1.Size = new System.Drawing.Size(92, 28);
-            this.button1.TabIndex = 2;
-            this.button1.Text = "Browse...";
-            this.button1.UseVisualStyleBackColor = true;
-            this.button1.Click += new System.EventHandler(this.BrowseNestTemplatePath_Click);
-            // 
             // saveButton
             // 
             this.saveButton.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Bottom | System.Windows.Forms.AnchorStyles.Right)));
@@ -258,9 +220,6 @@
         private System.Windows.Forms.ToolTip toolTip1;
         private Controls.BottomPanel bottomPanel1;
         private System.Windows.Forms.TableLayoutPanel tableLayoutPanel1;
-        private System.Windows.Forms.TextBox textBox1;
-        private System.Windows.Forms.Label label3;
-        private System.Windows.Forms.Button button1;
         private System.Windows.Forms.DataGridView strategyGrid;
         private System.Windows.Forms.GroupBox strategyGroupBox;
         private System.Windows.Forms.Label colorSchemeLabel;

OpenNest/Forms/OptionsForm.cs

@@ -72,7 +72,6 @@ namespace OpenNest.Forms
 
         private void LoadSettings()
         {
-            textBox1.Text = Settings.Default.NestTemplatePath;
             checkBox1.Checked = Settings.Default.CreateNewNestOnOpen;
             numericUpDown1.Value = (decimal)Settings.Default.AutoSizePlateFactor;
 
@@ -92,7 +91,6 @@ namespace OpenNest.Forms
 
         private void SaveSettings()
         {
-            Settings.Default.NestTemplatePath = textBox1.Text;
             Settings.Default.CreateNewNestOnOpen = checkBox1.Checked;
             Settings.Default.AutoSizePlateFactor = (double)numericUpDown1.Value;
             Settings.Default.ActiveColorScheme =
@@ -144,14 +142,5 @@ namespace OpenNest.Forms
         {
             SaveSettings();
         }
-
-        private void BrowseNestTemplatePath_Click(object sender, EventArgs e)
-        {
-            var dlg = new OpenFileDialog();
-            dlg.Filter = "Template File|*.nstdot";
-
-            if (dlg.ShowDialog() == DialogResult.OK)
-                textBox1.Text = dlg.FileName;
-        }
     }
 }

OpenNest/Properties/Settings.Designer.cs

@@ -158,6 +158,8 @@ namespace OpenNest.Properties {
         [global::System.Configuration.UserScopedSettingAttribute()]
         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
         [global::System.Configuration.DefaultSettingValueAttribute("")]
+        // Legacy .nstdot nest-template path. No longer read by the app
+        // except by the one-time migration to defaults.json in MainForm.
         public string NestTemplatePath {
             get {
                 return ((string)(this["NestTemplatePath"]));

Posts/OpenNest.Posts.CincinnatiCIFiber/CIFiberProgramWriter.cs

@@ -135,8 +135,14 @@ namespace OpenNest.Posts.CincinnatiCIFiber
 
         private int WriteSheet(Plate plate, TextWriter w, int contourNumber)
         {
+            // Cut-offs run last: severing the sheet first would free the
+            // skeleton before the parts are cut (matches the CL post).
+            var ordered = plate
+                .Parts.Where(p => !p.BaseDrawing.IsCutOff)
+                .Concat(plate.Parts.Where(p => p.BaseDrawing.IsCutOff));
+
             var partNumber = 0;
-            foreach (var part in plate.Parts)
+            foreach (var part in ordered)
             {
                 partNumber++;
                 contourNumber = WritePart(part, partNumber, w, contourNumber);
@@ -158,10 +164,14 @@ namespace OpenNest.Posts.CincinnatiCIFiber
             if (_config.SkipScribe)
                 contours = contours.Where(c => !IsScribeContour(c)).ToList();
 
+            var isCutOff = part.BaseDrawing.IsCutOff;
             foreach (var contour in contours)
             {
                 contourNumber++;
-                WriteContour(contour, contourNumber, w);
+                if (isCutOff)
+                    WriteCutOffContour(contour, contourNumber, w);
+                else
+                    WriteContour(contour, contourNumber, w);
             }
 
             CIFiberFormatter.Line(w, "( PART END )");
@@ -227,6 +237,38 @@ namespace OpenNest.Posts.CincinnatiCIFiber
             CIFiberFormatter.Line(w, SkippableLine(_config.LayerCutEnd));
         }
 
+        /// <summary>
+        /// Cut-offs are open straight lines with no lead-in: the line is the
+        /// beam centreline (CutOffSettings clearance already allows for kerf)
+        /// and an open line has no inside/outside, so no G41/G42 is selected
+        /// and the §13.2.4.1 linear-lead-in rule does not apply. The exterior
+        /// lead layer still runs so the pierce sequence matches a perimeter.
+        /// </summary>
+        private void WriteCutOffContour(CIFiberContour contour, int contourNumber, TextWriter w)
+        {
+            CIFiberFormatter.Line(w, $"N{contourNumber}:");
+            CIFiberFormatter.Line(w, SkippableLine(_config.LayerCancel));
+            CIFiberFormatter.Line(w, $"V.E.R3={contourNumber}");
+            CIFiberFormatter.Line(w, $"G0X{Fmt(contour.Pierce.X)}Y{Fmt(contour.Pierce.Y)}");
+            CIFiberFormatter.Line(w, SkippableLine(_config.LayerExteriorLeadin));
+            CIFiberFormatter.Line(w, SkippableLine(_config.LayerCut));
+
+            var prev = contour.Pierce;
+            if (contour.LeadIn != null)
+            {
+                CIFiberFormatter.Line(w, FormatMotion(contour.LeadIn, prev));
+                prev = contour.LeadIn.EndPoint;
+            }
+
+            foreach (var motion in contour.LeadInExtra.Concat(contour.Cuts))
+            {
+                CIFiberFormatter.Line(w, FormatMotion(motion, prev));
+                prev = motion.EndPoint;
+            }
+
+            CIFiberFormatter.Line(w, SkippableLine(_config.LayerCutEnd));
+        }
+
         private string ResolvePartComment(Part part)
         {
             if (!string.IsNullOrWhiteSpace(_config.PartComment))

README.md

@@ -11,7 +11,7 @@ A Windows desktop application for CNC nesting — imports DXF drawings, arranges
 
 - **Import / export** — DXF & DWG parts (ACadSharp), Excel BOMs, bend-line detection, built-in parametric shapes; export DXF or post-processed G-code.
 - **Nesting** — pluggable whole-job engines (Default, Strip, Vertical/Horizontal Remnant, StockLadder, plus DLL plugins), NFP-based interlocking pair evaluation, gravity compaction, rotation sweeps, multi-plate/multi-material jobs.
-- **Plate operations** — sheet cut-offs, oversized-part splitting (straight, weld-gap tabs, spike-groove), interactive editing.
+- **Plate operations** — sheet cut-offs, oversized-part splitting (straight, weld-gap tabs, spike-groove), interactive editing, and spacing-aware pushes that can slide along or away from touching parts.
 - **CNC output** — configurable lead-ins/outs and tabs, contour editing, user-defined G-code variables (`$name` → `#200+` machine variables), plugin post-processors (Cincinnati CL-707/800/900/940/CLX included).
 
 ## Requirements

docs/cincinnati-ci-fiber-post-output.md

@@ -0,0 +1,76 @@
+# Cincinnati CI Fiber Post Output Reference
+
+Project-written summary of `OpenNest.Posts.CincinnatiCIFiber`, not a vendor
+manual or a machine-operation guide. For the separate CL-series post, see
+[Cincinnati output](cincinnati-post-output.md).
+
+Controller reference used during development: Beckhoff *TF5200 | TwinCAT 3 CNC
+Programming manual*, version 1.33, May 19, 2026
+(`TF5200_programming_manual_en.pdf`). Obtain the applicable documentation from
+Beckhoff and Cincinnati. Vendor PDFs and full-text extracts stay outside source
+control; redistribution permission has not been established.
+
+## Output contract
+
+The current [writer](../Posts/OpenNest.Posts.CincinnatiCIFiber/CIFiberProgramWriter.cs)
+uses the Cincinnati machine-sample convention, not a generic TF5200 laser API:
+
+- Header: nest/configuration/material comments; `V.E.MATERIAL`, `V.E.THICKNESS`,
+  `V.E.X_SIZE`, `V.E.Y_SIZE`, and `V.E.UNIT`. Sheet weight is omitted by default.
+- Startup: `G90`, `L PROGRAMSTART.NC`, then `P3=V.E.R3`, `$GOTO NP3:`, and `N0:`.
+- Parts follow plate order. `V.E.R4` identifies the part within its sheet;
+  numbered contour labels and `V.E.R3` continue across sheets for restart lookup.
+- Each contour: `/L "L0"`, restart number, rapid to pierce, then interior
+  `/L "L2"` with `G41` or exterior `/L "L4"` with `G42`; linear lead-in,
+  `/L "L6"`, cutting moves, and `/L "ZHSOFF"`.
+- Tail: `/L "L0"`, `L PROGRAMEND.NC`, `M50`, `M30`, and `%`.
+- Motion endpoints are sheet-absolute XY. Arc `I`/`J` are offsets from the arc
+  start, matching the G162 convention; the post does not explicitly emit G162.
+- Sheet cut-offs post after every part on their sheet. Each segment is an open
+  line with no lead-in: rapid to its start, `/L "L4"`, `/L "L6"`, the `G1`,
+  and `/L "ZHSOFF"`. No `G41`/`G42` is selected, because the line is the beam
+  centreline and has no inside or outside. Whether the `L4` macro runs
+  correctly without a following lead-in move has not been confirmed on the
+  machine.
+- Hole subprogram geometry is inlined. Suppressed moves and, by default,
+  wholly scribe contours are omitted.
+- Files use UTF-8 without a BOM and CRLF lines. Default accuracy is three decimal
+  places: coordinates trim trailing zeros; dimensional header values retain them.
+
+## Configuration and boundaries
+
+See [CIFiberPostConfig](../Posts/OpenNest.Posts.CincinnatiCIFiber/CIFiberPostConfig.cs)
+for macro names, material mappings, unit codes, precision, and table limits.
+The post is named for the machine family; table size belongs in configuration.
+
+- Assign linear lead-ins before posting, including circular holes. The writer
+  rejects missing or arc-first lead-ins. Its conservative rule cites the G238
+  compensation-selection restriction in TF5200 §13.2.4.1; do not generalize it
+  to every controller compensation mode.
+- `/L` calls are skippable; `L PROGRAMSTART.NC` and `L PROGRAMEND.NC` are not.
+  Macro bodies, process settings, restart handling, and compensation cancellation
+  belong to the machine configuration. The post does not emit an explicit G40.
+- `InchUnitCode` defaults to `1`; `MetricUnitCode` defaults to `0` but remains
+  unconfirmed. Table limits compare directly with nest dimensions, without unit
+  conversion. Do not assume changing the unit code establishes metric support.
+- Multiple nonempty plates are written sequentially, but dimensional header
+  values come only from the first plate and the pallet-change tail occurs once.
+  This is not proof of a qualified multi-sheet machine cycle.
+- Validation can throw after output has started. Discard any output from a failed
+  post; it may be incomplete. Successful generation and tests do not establish
+  that a program is safe to run on a particular machine.
+
+## Verification
+
+[Output-contract tests](../OpenNest.Tests/CincinnatiCIFiber/CIFiberPostProcessorTests.cs)
+cover a square with a hole, coordinate transforms/formatting, lead-in rejection,
+cut-off ordering and output,
+table bounds, and suppression/scribe handling. Run:
+
+```sh
+dotnet test OpenNest.Tests/OpenNest.Tests.csproj --filter FullyQualifiedName~CincinnatiCIFiber
+```
+
+Keep this summary aligned with the implementation and tests. Cite the relevant
+manual section for controller rules and distinguish those rules from
+Cincinnati-specific macros and behavior observed in a machine sample.

docs/cincinnati-post-output.md

@@ -2,8 +2,14 @@
 
 Reference for the G-code structure emitted by `OpenNest.Posts.Cincinnati`.
 Every code listed here maps to a section in the Cincinnati Laser Programming
-Manual (`docs/CINCINNATI LASER PROGRAMMING MANUAL.pdf`, EM-423 R-02/11).
-Section numbers in parentheses (e.g. `§1.52`) refer to the manual.
+Manual (EM-423 R-02/11). Section numbers in parentheses (e.g. `§1.52`)
+refer to that edition. Obtain the applicable manual from Cincinnati; vendor
+manuals are not bundled because redistribution permission has not been established.
+
+This is a project-written implementation reference, not a replacement for the
+machine's programming, operating, or safety documentation. Keep vendor PDFs and
+full-text extracts outside source control; do not re-add them without permission.
+For the separate CI Fiber post, see [CI Fiber output](cincinnati-ci-fiber-post-output.md).
 
 If you add a new emission in the post, either cite the manual section it maps
 to, or flag it here as a known custom extension. "Custom code" in this project

docs/geometry/directional-slides.md

@@ -0,0 +1,37 @@
+# Directional slides and repeated pushes
+
+## Behavior contract
+
+For initially non-overlapping closed material boundaries, a directional slide stops at the first contact that blocks forward motion. Leaving an existing contact or sliding along a straight shared edge is legal. Skipping a contact must not skip the rest of that obstacle: a later hook or the opposite wall of a hole still stops the slide. Distances retain the existing `Tolerance.Epsilon` snapping and `double.MaxValue` no-hit convention; supplied vector directions are unit vectors.
+
+`SpatialQuery.DirectionalDistance` overloads and CPU best-fit batches use the same event sources and contact resolver. Events carry both contact points in their initial world frames, rather than only a snapped distance. The classifier uses closed-loop material sectors, native-curve containment for hole depth, and the curvature of the supporting boundary at a tangential contact. A full-circle arc has no physical corner at its seam. Raw ray helpers remain first-touch primitives, not material-aware slide queries.
+
+Open/incomplete chains and ambiguous contacts conservatively block. This is not an overlap-repair operation or a general replacement for layout validation. Caller-provided contact topology must represent the same boundaries and offsets as the query. Prepared geometry must not be mutated; prepare a classifier before sharing it between parallel queries. Edge-array queries still sort their arrays, and recover loop order from private copies before classifying contacts.
+
+## Callers
+
+- PlateView uses `SelectionManager.PushSelected` → `Compactor.Push`. The zero-spacing nudge-and-discard workaround is removed. Cutout contours on stationary obstacles are retained, and the plate entry's existing-overlap filter accounts for holes rather than relying solely on `Part.Intersects` (which compares outer perimeters).
+- Linear fill inherits the shared native-entity query unchanged. Extents fill passes complete boundary loops instead of direction-filtered fragments.
+- CPU best-fit batches prepare contact topology once and use all vertices plus curve/line interior and curve/curve tangency events. The old leading-half vertex filter cannot establish the next blocker after a skipped touch.
+- GPU kernels retain nearest-hit reduction and return unsnapped contact witnesses. The shared CPU classifier accepts a blocking witness or replays the full query after a nonblocking witness, preserving tied/later blockers. Both batch APIs honor active buffer lengths and refresh mutated/reused segment arrays. The GPU distance adapter sends only exact cardinal directions to the axis-only slide interface; arbitrary directions and native curves use the shared CPU path.
+
+## Regression coverage
+
+`SlideContactTests` exercises cardinal line, translated line, reused edge-array, arbitrary-vector, native-entity, and both CPU batch paths. Cases include winding reversal, nonzero origins, rotated hooks, holes, separating circles, positive-distance grazing followed by a blocker, full-circle arc seams, concave/straight junctions, thin rings, and circle/line interior contact.
+
+`CompactorTests` covers the reported sequence (push left with spacing, then right/up/down), genuine zero-distance blocking, zero/nonzero-spacing later hooks, and inside-hole pushes through both direct and plate entry points. Physical spacing is measured from raw outlines rather than the inflated contours used by the solver.
+
+Verification commands:
+
+```sh
+dotnet test OpenNest.Tests/OpenNest.Tests.csproj --filter 'FullyQualifiedName~SlideContactTests|FullyQualifiedName~CurveContactDistanceTests|FullyQualifiedName~CompactorTests'
+dotnet test OpenNest.Tests/OpenNest.Tests.csproj
+dotnet test OpenNest.Engine.Tests/OpenNest.Engine.Tests.csproj
+dotnet test OpenNest.IO.Tests/OpenNest.IO.Tests.csproj
+```
+
+The isolated repair tree (excluding other sessions' fill-performance and spacing-expander work) passed 130 targeted cases and the full Debug suites: main 1758 passed / 21 skipped, engine 300 passed, IO 41 passed. The main Release suite passed 1728 / 21 skipped. Skips are not counted as passes. The 142 `GpuSlideContactTests` also pass in a net8 harness linking the production GPU class and test source, using ILGPU 1.5.1's CPU accelerator (not a mocked distance solver). The Windows desktop/test project cross-build passes in Release. Neither physical GPU execution nor Windows UI interaction was runtime-verified on Linux.
+
+## Remaining hardening
+
+This repair does not change saved best-fit cache versioning, add a clearance acceptance gate to every fill entry, repair existing overlapping layouts, or claim the earlier real-DXF/grid-validator discrepancies in [pair-spacing checks](pair-spacing.md) are resolved. That document's measured candidate counts describe its earlier tree; removing CPU projection filtering and adding interior curve/line contacts does not substitute for rerunning its corpus. General `Part.Intersects` hole semantics remain unchanged outside Compactor. Profile the new classification path before attempting optimizations; retain the first-blocking-contact regressions.

docs/geometry/pair-spacing.md

@@ -4,7 +4,7 @@
 
 CPU best-fit slides and shared directional-distance queries now check both external and internal curve tangency. A convex offset corner inside a concave slot contacts at the difference of the radii, not their sum. Both forward ray/circle roots must be checked: the nearer root can be outside an arc's angular span while the farther root is the first actual contact. Tangent-point directions differ for internal contact, including when the moving curve is the larger one.
 
-`SpatialQuery.CurveTangencyDistance` shares this calculation between the two callers. It assumes a unit direction, nonnegative radii and world-frame centers. An optional arc supplies only angular limits; null represents a full circle. This helper supplements the existing vertex/line phases rather than replacing them. Equal-radius coincident curves have no isolated internal tangent and remain the vertex phases' responsibility; zero-radius curves are points. No spacing tolerances or acceptance policies were changed.
+The raw `SpatialQuery.CurveTangencyDistance` helper and shared slide events implement this calculation. The subsequent [directional-slide repair](directional-slides.md) routes both callers through material-aware contact events; the measurements below describe the earlier native-tangency repair. It assumes a unit direction, nonnegative radii and world-frame centers. An optional arc supplies only angular limits; null represents a full circle. This helper supplements the existing vertex/line phases rather than replacing them. Equal-radius coincident curves have no isolated internal tangent and remain the vertex phases' responsibility; zero-radius curves are points. No spacing tolerances or acceptance policies were changed.
 
 ### Reproduced U-shaped part