fix(ui): detach disposed plate views and keep repeated layout parts

A disposed PlateView stayed subscribed to its plate, so a plate that
outlived the view kept rebuilding the disposed view's layout and
raising its events; Dispose now detaches the part added, removed and
reordered handlers.

ObservableList.Reorder keeps repeated references, but the reorder
handler mapped each part to one layout, so [a, a, b] collapsed to one
shared layout for both occurrences of a. It now reuses each existing
layout once and creates one only when none is left.

OpenNest.WinForms.Tests/Controls/PlateViewReorderTests.cs

@@ -29,6 +29,47 @@ public class PlateViewReorderTests
         Assert.Empty(view.LayoutParts);
     }, TimeSpan.FromMinutes(1), "The STA test did not complete.");
 
+    [Fact]
+    public void KeepsOneLayoutPerOccurrenceOfARepeatedPart() => StaTestThread.Run(() =>
+    {
+        var plate = new Plate(100, 100);
+        var repeated = new Part(new Drawing("repeated", Square()), new Vector(1, 1));
+        var other = new Part(new Drawing("other", Square()), new Vector(12, 1));
+        plate.Parts.Add(repeated);
+        plate.Parts.Add(repeated);
+        plate.Parts.Add(other);
+        using var view = new PlateView { Plate = plate };
+        var before = view.LayoutParts.ToArray();
+
+        plate.Parts.Reorder(new[] { other, repeated, repeated });
+
+        var after = view.LayoutParts;
+        Assert.Equal(new[] { other, repeated, repeated }, after.Select(layout => layout.BasePart));
+        Assert.Same(before[2], after[0]);
+        Assert.Equal(3, after.Distinct().Count());
+        Assert.Equal(before.ToHashSet(), after.ToHashSet());
+    }, TimeSpan.FromMinutes(1), "The STA test did not complete.");
+
+    [Fact]
+    public void DisposedViewIgnoresItsRetainedPlate() => StaTestThread.Run(() =>
+    {
+        var plate = new Plate(100, 100);
+        foreach (var x in new[] { 1.0, 12.0 })
+            plate.Parts.Add(new Part(new Drawing($"part {x}", Square()), new Vector(x, 1)));
+        var parts = plate.Parts.ToArray();
+        var view = new PlateView { Plate = plate };
+        var raised = 0;
+        view.PartsReordered += (_, _) => raised++;
+        var layouts = view.LayoutParts.ToArray();
+
+        view.Dispose();
+        plate.Parts.Reorder(new[] { parts[1], parts[0] });
+        plate.Parts.Add(new Part(new Drawing("late", Square()), new Vector(23, 1)));
+
+        Assert.Equal(0, raised);
+        Assert.Equal(layouts, view.LayoutParts);
+    }, TimeSpan.FromMinutes(1), "The STA test did not complete.");
+
     private static CNC.Program Square()
     {
         var program = new CNC.Program();

OpenNest/Controls/PlateView.cs

@@ -627,6 +627,13 @@ namespace OpenNest.Controls
         {
             if (disposing)
             {
+                // A plate can outlive its view; stop it calling back into a disposed one.
+                if (plate != null)
+                {
+                    plate.PartAdded -= plate_PartAdded;
+                    plate.PartRemoved -= plate_PartRemoved;
+                    plate.PartsReordered -= plate_PartsReordered;
+                }
                 overlapOverlay?.Dispose();
                 hoverTimer?.Dispose();
                 hoverTimer = null;
@@ -830,11 +837,18 @@ namespace OpenNest.Controls
         {
             // Part numbers are drawn from the layout order, and an applied cutting plan can also
             // replace programs, so follow the plate's order and rebuild every part's graphics.
-            var layouts = new Dictionary<Part, LayoutPart>(ReferenceEqualityComparer.Instance);
+            // A reorder keeps repeated references, so reuse each existing layout once.
+            var layouts = new Dictionary<Part, Queue<LayoutPart>>(ReferenceEqualityComparer.Instance);
             foreach (var layout in parts)
-                layouts.TryAdd(layout.BasePart, layout);
+            {
+                if (!layouts.TryGetValue(layout.BasePart, out var queue))
+                    layouts.Add(layout.BasePart, queue = new Queue<LayoutPart>());
+                queue.Enqueue(layout);
+            }
             var reordered = plate.Parts
-                .Select(part => layouts.TryGetValue(part, out var layout) ? layout : LayoutPart.Create(part, this))
+                .Select(part => layouts.TryGetValue(part, out var queue) && queue.Count > 0
+                    ? queue.Dequeue()
+                    : LayoutPart.Create(part, this))
                 .ToList();
             parts.Clear();
             parts.AddRange(reordered);