diff --git a/OpenNest.Core/CNC/ProgramHighlighting.cs b/OpenNest.Core/CNC/ProgramHighlighting.cs
new file mode 100644
index 0000000..1aeffe6
--- /dev/null
+++ b/OpenNest.Core/CNC/ProgramHighlighting.cs
@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Text.RegularExpressions;
+
+namespace OpenNest.CNC;
+
+public readonly record struct HighlightSpan(int Index, int Length, int RuleIndex);
+
+/// <summary>Computes cosmetic G-code highlight spans without changing the generated text.</summary>
+public static class ProgramHighlighting
+{
+    private static readonly Regex[] Rules =
+    {
+        new(@"^;.*$", RegexOptions.Multiline, TimeSpan.FromMilliseconds(100)),
+        new(@"^G9[01]\b", RegexOptions.Multiline, TimeSpan.FromMilliseconds(100)),
+        new(@"^G00\b", RegexOptions.Multiline, TimeSpan.FromMilliseconds(100)),
+        new(@"^G01\b", RegexOptions.Multiline, TimeSpan.FromMilliseconds(100)),
+        new(@"^G0[23]\b", RegexOptions.Multiline, TimeSpan.FromMilliseconds(100)),
+    };
+
+    /// <summary>
+    /// Materializes all matches in rule/application order (later rules overwrite earlier ones).
+    /// A timeout propagates before any result is published; no partial span list escapes.
+    /// The timeout is per regex match, not a deadline for the complete operation.
+    /// </summary>
+    /// <exception cref="RegexMatchTimeoutException">A rule exceeded its match budget.</exception>
+    public static IReadOnlyList<HighlightSpan> ComputeSpans(string text)
+    {
+        ArgumentNullException.ThrowIfNull(text);
+        var spans = new List<HighlightSpan>();
+        for (var ruleIndex = 0; ruleIndex < Rules.Length; ruleIndex++)
+        {
+            // MatchCollection is lazy: enumerate every rule before returning any spans.
+            foreach (Match match in Rules[ruleIndex].Matches(text))
+                spans.Add(new HighlightSpan(match.Index, match.Length, ruleIndex));
+        }
+        return spans.AsReadOnly();
+    }
+}
diff --git a/OpenNest.Tests/CNC/ProgramHighlightingTests.cs b/OpenNest.Tests/CNC/ProgramHighlightingTests.cs
new file mode 100644
index 0000000..c142b55
--- /dev/null
+++ b/OpenNest.Tests/CNC/ProgramHighlightingTests.cs
@@ -0,0 +1,167 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Text.RegularExpressions;
+using OpenNest.CNC;
+
+namespace OpenNest.Tests.CNC;
+
+public class ProgramHighlightingTests
+{
+    [Theory]
+    [InlineData("G00", 2, 3)]
+    [InlineData("G000", -1, 0)]
+    [InlineData("G00.5", 2, 3)]
+    [InlineData("G00 X1", 2, 3)]
+    [InlineData("G00X1", -1, 0)]
+    [InlineData("G00_", -1, 0)]
+    [InlineData("G00é", -1, 0)]
+    [InlineData("G01", 3, 3)]
+    [InlineData("G010", -1, 0)]
+    [InlineData("G01.5", 3, 3)]
+    [InlineData("G02 X1", 4, 3)]
+    [InlineData("G03 X1", 4, 3)]
+    [InlineData("G020", -1, 0)]
+    [InlineData("G90", 1, 3)]
+    [InlineData("G91", 1, 3)]
+    [InlineData("G901", -1, 0)]
+    [InlineData("G910", -1, 0)]
+    [InlineData("G90.5", 1, 3)]
+    [InlineData("g00 X1", -1, 0)]
+    [InlineData(" G00 X1", -1, 0)]
+    [InlineData(";)", 0, 2)]
+    [InlineData("; G00 X1", 0, 8)]
+    [InlineData("X1 ; comment", -1, 0)]
+    [InlineData("G00 X1 ; comment", 2, 3)]
+    public void SingleLineRetainsExistingAnchorsAndWordBoundaries(
+        string text, int ruleIndex, int length)
+    {
+        var spans = ComputeSpans(text);
+        if (ruleIndex < 0)
+            Assert.Empty(spans);
+        else
+            Assert.Equal(new[] { new OracleSpan(0, length, ruleIndex) }, spans);
+    }
+
+    [Theory]
+    [InlineData("\n", 0)]
+    [InlineData("\r\n", 1)]
+    public void MultilineCommentsRetainCarriageReturnAndCommandsOnlyColorTheirPrefix(
+        string newline, int carriageReturnLength)
+    {
+        var lines = new[] { ";)", "G91", "G00 X1", "G01 Y2", "G02 X3", "G03 X4", "X1 ; mid-line" };
+        var text = string.Join(newline, lines) + newline;
+        var starts = new List<int>();
+        var index = 0;
+        foreach (var line in lines)
+        {
+            starts.Add(index);
+            index += line.Length + newline.Length;
+        }
+
+        Assert.Equal(new[]
+        {
+            new OracleSpan(starts[0], 2 + carriageReturnLength, 0),
+            new OracleSpan(starts[1], 3, 1),
+            new OracleSpan(starts[2], 3, 2),
+            new OracleSpan(starts[3], 3, 3),
+            new OracleSpan(starts[4], 3, 4),
+            new OracleSpan(starts[5], 3, 4),
+        }, ComputeSpans(text));
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("\n\n")]
+    [InlineData("X1 Y2")]
+    [InlineData("x\rG00")]
+    public void EmptyOrUnmatchedTextHasNoSpans(string text) => Assert.Empty(ComputeSpans(text));
+
+    [Fact]
+    public void RulesStayInApplicationOrderRatherThanTextOrderAndNeverOverlap()
+    {
+        const string text = "G03\nG01\nG00\nG91\n;)\nG02\nG90\nG00";
+        var spans = ComputeSpans(text);
+        Assert.Equal(new[]
+        {
+            new OracleSpan(16, 2, 0),
+            new OracleSpan(12, 3, 1),
+            new OracleSpan(23, 3, 1),
+            new OracleSpan(8, 3, 2),
+            new OracleSpan(27, 3, 2),
+            new OracleSpan(4, 3, 3),
+            new OracleSpan(0, 3, 4),
+            new OracleSpan(19, 3, 4),
+        }, spans);
+        Assert.Equal(spans.Select(s => s.RuleIndex).OrderBy(i => i), spans.Select(s => s.RuleIndex));
+        var coloredIndices = spans.SelectMany(s => Enumerable.Range(s.Index, s.Length)).ToArray();
+        Assert.Equal(coloredIndices.Length, coloredIndices.Distinct().Count());
+    }
+
+    // Frozen oracle: the original five inline rules from ProgramEditorControl.
+    // Keep this independent of production patterns and timeout configuration.
+    private static IReadOnlyList<OracleSpan> ComputeLegacySpans(string text)
+    {
+        var rules = new[]
+        {
+            new Regex(@"^;.*$", RegexOptions.Multiline),
+            new Regex(@"^G9[01]\b", RegexOptions.Multiline),
+            new Regex(@"^G00\b", RegexOptions.Multiline),
+            new Regex(@"^G01\b", RegexOptions.Multiline),
+            new Regex(@"^G0[23]\b", RegexOptions.Multiline),
+        };
+        var spans = new List<OracleSpan>();
+        for (var ruleIndex = 0; ruleIndex < rules.Length; ruleIndex++)
+            foreach (Match match in rules[ruleIndex].Matches(text))
+                spans.Add(new OracleSpan(match.Index, match.Length, ruleIndex));
+        return spans;
+    }
+
+    [Fact]
+    public void AllFiveCachedRulesHaveUnchangedPatternsAndFinite100MillisecondBudgets()
+    {
+        var rules = Assert.IsType<Regex[]>(typeof(ProgramHighlighting)
+            .GetField("Rules", BindingFlags.Static | BindingFlags.NonPublic)!.GetValue(null));
+        Assert.Equal(new[] { @"^;.*$", @"^G9[01]\b", @"^G00\b", @"^G01\b", @"^G0[23]\b" },
+            rules.Select(r => r.ToString()));
+        Assert.All(rules, rule =>
+        {
+            Assert.Equal(RegexOptions.Multiline, rule.Options);
+            Assert.NotEqual(Regex.InfiniteMatchTimeout, rule.MatchTimeout);
+            Assert.Equal(TimeSpan.FromMilliseconds(100), rule.MatchTimeout);
+        });
+    }
+
+    [Fact]
+    public void GeneratedSizedFixtureRetainsEverySpanAndTheOriginalText()
+    {
+        var text = string.Concat(Enumerable.Repeat(
+            "G91\r\n; Perimeter (CCW)\r\nG00 X1 Y2\r\nG01 X2 Y3\r\nG02 X1 Y0 I1 J2\r\nG03 X0 Y1 I2 J3\r\n", 5000));
+        var original = text;
+        var spans = ComputeSpans(text);
+        Assert.Equal(30000, spans.Count);
+        Assert.Equal(original, text);
+    }
+
+    [Fact]
+    public void PublishedSpansAreReadOnlyAndIndependentOfSubsequentCalls()
+    {
+        var spans = ProgramHighlighting.ComputeSpans("G00");
+        Assert.Throws<NotSupportedException>(() => ((IList<HighlightSpan>)spans)
+            .Add(new HighlightSpan(0, 3, 4)));
+        Assert.Empty(ProgramHighlighting.ComputeSpans(string.Empty));
+        Assert.Equal(new[] { new HighlightSpan(0, 3, 2) }, spans);
+    }
+
+    private static IReadOnlyList<OracleSpan> ComputeSpans(string text)
+    {
+        var expected = ComputeLegacySpans(text);
+        var actual = ProgramHighlighting.ComputeSpans(text)
+            .Select(s => new OracleSpan(s.Index, s.Length, s.RuleIndex)).ToArray();
+        Assert.Equal(expected, actual);
+        return actual;
+    }
+
+    private readonly record struct OracleSpan(int Index, int Length, int RuleIndex);
+}
diff --git a/OpenNest.WinForms.Tests/Controls/ProgramEditorControlTests.cs b/OpenNest.WinForms.Tests/Controls/ProgramEditorControlTests.cs
new file mode 100644
index 0000000..903a510
--- /dev/null
+++ b/OpenNest.WinForms.Tests/Controls/ProgramEditorControlTests.cs
@@ -0,0 +1,281 @@
+using System.Drawing;
+using System.Reflection;
+using System.Runtime.ExceptionServices;
+using System.Text.RegularExpressions;
+using System.Windows.Forms;
+using OpenNest.CNC;
+using OpenNest.Controls;
+using OpenNest.Converters;
+using OpenNest.Geometry;
+
+namespace OpenNest.WinForms.Tests.Controls;
+
+public class ProgramEditorControlTests
+{
+    private static readonly Color BaseColor = Color.FromArgb(180, 200, 180);
+
+    [Fact]
+    public void TimeoutDuringInitialLoadKeepsCompleteTextAndBaseColorWithoutAParentHandle() => RunSta(() =>
+    {
+        using var control = new TestEditor { TimeoutFromCall = 1 };
+        var status = GetControl<Label>(control, "lblGcode");
+        Assert.False(control.IsHandleCreated);
+        Assert.False(status.IsHandleCreated);
+
+        control.LoadEntities(CreateEntities());
+
+        Assert.True(control.IsLoaded);
+        Assert.False(control.IsDirty);
+        Assert.NotNull(control.Program);
+        Assert.True(control.SpansCollectedBeforeTimeout > 0);
+        AssertCompleteText(control);
+        AssertUniformBaseColor(GetControl<RichTextBox>(control, "gcodeEditor"));
+        Assert.Equal("G-Code (highlighting timed out)", status.Text);
+        Assert.Equal(3, GetControl<EntityView>(control, "preview").Entities.Count);
+    });
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void ReverseAndReorderFinishModelPreviewAndEventsWithOrWithoutTimeout(bool timeout) => RunSta(() =>
+    {
+        using var control = new TestEditor { TimeoutFromCall = timeout ? 2 : int.MaxValue };
+        using var form = new Form { Width = 900, Height = 600 };
+        control.Dock = DockStyle.Fill;
+        form.Controls.Add(control);
+        form.Show();
+        var changes = 0;
+        control.ProgramChanged += (_, _) => changes++;
+
+        control.LoadEntities(CreateEntities());
+        Assert.Equal(0, changes); // Loading did not raise ProgramChanged before this repair either.
+        Assert.False(control.IsDirty);
+        AssertCompleteText(control);
+        var list = GetControl<ListBox>(control, "contourList");
+        list.SelectedIndex = 0;
+        var firstContour = Assert.IsType<ContourInfo>(list.Items[0]);
+        var oldDirection = firstContour.DirectionLabel;
+        var oldProgram = control.Program;
+        var preview = GetControl<EntityView>(control, "preview");
+        var oldPreview = preview.Entities.ToArray();
+
+        GetControl<Button>(control, "reverseButton").PerformClick();
+
+        Assert.NotSame(oldProgram, control.Program);
+        Assert.NotEqual(oldDirection, firstContour.DirectionLabel);
+        Assert.True(control.IsDirty);
+        Assert.Equal(1, changes);
+        AssertPreviewRebuilt(preview, oldPreview);
+        Assert.Equal(Assert.IsType<Circle>(firstContour.Shape.Entities[0]).Rotation,
+            Assert.IsType<Circle>(preview.Entities[0]).Rotation);
+        AssertCompleteText(control);
+        AssertHighlightingOutcome(control, timeout);
+
+        oldProgram = control.Program;
+        oldPreview = preview.Entities.ToArray();
+        GetField<ToolStripMenuItem>(control, "menuMoveDown").PerformClick();
+
+        Assert.NotSame(oldProgram, control.Program);
+        Assert.Same(firstContour, list.Items[1]);
+        Assert.Equal(1, list.SelectedIndex);
+        Assert.True(control.IsDirty);
+        Assert.Equal(2, changes);
+        AssertPreviewRebuilt(preview, oldPreview);
+        AssertCompleteText(control);
+        AssertHighlightingOutcome(control, timeout);
+        Assert.Equal(3, control.MatchCalls);
+        if (timeout)
+            Assert.True(control.SpansCollectedBeforeTimeout > 0);
+    });
+
+    [Fact]
+    public void SuccessfulRetryAndClearRemoveTheNonmodalTimeoutIndication() => RunSta(() =>
+    {
+        using var control = new TestEditor { TimeoutFromCall = 1 };
+        control.LoadEntities(CreateEntities());
+        var status = GetControl<Label>(control, "lblGcode");
+        Assert.Contains("timed out", status.Text);
+
+        control.TimeoutFromCall = int.MaxValue;
+        control.LoadEntities(CreateEntities());
+        Assert.Equal("G-Code", status.Text);
+        AssertCompleteText(control);
+        var editor = GetControl<RichTextBox>(control, "gcodeEditor");
+        editor.Select(0, 3);
+        Assert.Equal(Color.FromArgb(200, 140, 220), editor.SelectionColor);
+
+        control.TimeoutFromCall = 1;
+        control.LoadEntities(CreateEntities());
+        Assert.Contains("timed out", status.Text);
+        control.Clear();
+        Assert.Equal("G-Code", status.Text);
+        Assert.Empty(editor.Text);
+        Assert.False(control.IsLoaded);
+        Assert.False(control.IsDirty);
+        Assert.Null(control.Program);
+    });
+
+    [Fact]
+    public void OrdinaryHighlightingPreservesAllColorsAndUnmatchedCoordinateText() => RunSta(() =>
+    {
+        using var control = new TestEditor();
+        var editor = GetControl<RichTextBox>(control, "gcodeEditor");
+        editor.Text = ";)\nG91\nG00 X1\nG01 X2\nG02 X3\nG03 X4\nG000";
+        var original = editor.Text;
+
+        ApplyHighlighting(control);
+
+        Assert.Equal(original, editor.Text);
+        Assert.Equal(0, editor.SelectionStart);
+        Assert.Equal(0, editor.SelectionLength);
+        var colors = new[]
+        {
+            Color.FromArgb(120, 120, 140), Color.FromArgb(200, 140, 220),
+            Color.FromArgb(230, 180, 80), Color.FromArgb(130, 200, 140),
+            Color.FromArgb(120, 160, 255),
+        };
+        var spans = ProgramHighlighting.ComputeSpans(original);
+        for (var index = 0; index < editor.TextLength; index++)
+        {
+            if (original[index] is '\r' or '\n')
+                continue;
+            var expected = BaseColor;
+            foreach (var span in spans)
+                if (index >= span.Index && index < span.Index + span.Length)
+                    expected = colors[span.RuleIndex];
+            editor.Select(index, 1);
+            Assert.Equal(expected, editor.SelectionColor);
+        }
+    });
+
+    [Fact]
+    public void UnexpectedApplicationFailureStillRestoresSelectionAndResumesLayout() => RunSta(() =>
+    {
+        using var control = new TestEditor { ReturnInvalidRule = true };
+        var editor = GetControl<RichTextBox>(control, "gcodeEditor");
+        Assert.Throws<IndexOutOfRangeException>(() => control.LoadEntities(CreateEntities()));
+        Assert.Equal(0, editor.SelectionStart);
+        Assert.Equal(0, editor.SelectionLength);
+        var layouts = 0;
+        editor.Layout += (_, _) => layouts++;
+        editor.PerformLayout();
+        Assert.True(layouts > 0); // PerformLayout would be deferred if SuspendLayout were unbalanced.
+    });
+
+    [Fact]
+    public void UnexpectedMatchingFailureIsNotSwallowed() => RunSta(() =>
+    {
+        using var control = new TestEditor { FailMatching = true };
+        var exception = Assert.Throws<InvalidOperationException>(() => control.LoadEntities(CreateEntities()));
+        Assert.Equal("Unexpected matching failure", exception.Message);
+    });
+
+    private sealed class TestEditor : ProgramEditorControl
+    {
+        public int TimeoutFromCall { get; set; } = int.MaxValue;
+        public int MatchCalls { get; private set; }
+        public int SpansCollectedBeforeTimeout { get; private set; }
+        public bool ReturnInvalidRule { get; set; }
+        public bool FailMatching { get; set; }
+
+        internal override IReadOnlyList<HighlightSpan> ComputeHighlightSpans(string text)
+        {
+            MatchCalls++;
+            if (FailMatching)
+                throw new InvalidOperationException("Unexpected matching failure");
+            if (MatchCalls >= TimeoutFromCall)
+            {
+                // Simulate lazy enumeration succeeding for the comment rule, then timing out
+                // in a later rule. The local prefix must never reach the color renderer.
+                var partial = new List<HighlightSpan>();
+                var comments = new Regex(@"^;.*$", RegexOptions.Multiline, TimeSpan.FromMilliseconds(100));
+                foreach (Match match in comments.Matches(text))
+                    partial.Add(new HighlightSpan(match.Index, match.Length, 0));
+                SpansCollectedBeforeTimeout = partial.Count;
+                throw new RegexMatchTimeoutException();
+            }
+            if (ReturnInvalidRule)
+                return new[] { new HighlightSpan(0, 3, 99) };
+            return base.ComputeHighlightSpans(text);
+        }
+    }
+
+    private static List<Entity> CreateEntities() => new()
+    {
+        new Circle(3, 3, 1), new Circle(7, 7, 1), new Circle(10, 10, 10),
+    };
+
+    private static void AssertCompleteText(ProgramEditorControl control)
+    {
+        var contours = GetField<List<ContourInfo>>(control, "contours");
+        var raw = Assert.IsType<string>(typeof(ProgramEditorControl)
+            .GetMethod("FormatProgram", BindingFlags.Static | BindingFlags.NonPublic)!
+            .Invoke(null, new object[] { control.Program, contours }));
+        // RichTextBox performs its existing newline normalization; compare with the same
+        // generated text assigned to an unhighlighted editor, not with another highlight pass.
+        using var unhighlighted = new RichTextBox { Text = raw };
+        var editor = GetControl<RichTextBox>(control, "gcodeEditor");
+        Assert.NotEmpty(raw);
+        Assert.Equal(unhighlighted.Text, editor.Text);
+        Assert.True(editor.ReadOnly);
+    }
+
+    private static void AssertHighlightingOutcome(ProgramEditorControl control, bool timeout)
+    {
+        var editor = GetControl<RichTextBox>(control, "gcodeEditor");
+        Assert.Equal(0, editor.SelectionStart);
+        Assert.Equal(0, editor.SelectionLength);
+        Assert.Equal(timeout ? "G-Code (highlighting timed out)" : "G-Code",
+            GetControl<Label>(control, "lblGcode").Text);
+        if (timeout)
+            AssertUniformBaseColor(editor);
+    }
+
+    private static void AssertUniformBaseColor(RichTextBox editor)
+    {
+        Assert.Equal(0, editor.SelectionStart);
+        Assert.Equal(0, editor.SelectionLength);
+        editor.SelectAll();
+        Assert.Equal(BaseColor, editor.SelectionColor);
+        for (var index = 0; index < editor.TextLength; index++)
+        {
+            if (editor.Text[index] is '\r' or '\n')
+                continue;
+            editor.Select(index, 1);
+            Assert.Equal(BaseColor, editor.SelectionColor);
+        }
+        editor.Select(0, 0);
+    }
+
+    private static void AssertPreviewRebuilt(EntityView preview, Entity[] before)
+    {
+        Assert.Equal(before.Length, preview.Entities.Count);
+        Assert.All(preview.Entities, entity => Assert.DoesNotContain(entity, before));
+    }
+
+    private static T GetControl<T>(Control control, string name) where T : Control =>
+        Assert.IsType<T>(control.Controls.Find(name, true).Single());
+
+    private static T GetField<T>(ProgramEditorControl control, string name) =>
+        Assert.IsType<T>(typeof(ProgramEditorControl)
+            .GetField(name, BindingFlags.Instance | BindingFlags.NonPublic)!.GetValue(control));
+
+    private static void ApplyHighlighting(ProgramEditorControl control) => typeof(ProgramEditorControl)
+        .GetMethod("ApplyHighlighting", BindingFlags.Instance | BindingFlags.NonPublic)!.Invoke(control, null);
+
+    private static void RunSta(Action action)
+    {
+        Exception? error = null;
+        var thread = new Thread(() =>
+        {
+            try { action(); }
+            catch (Exception ex) { error = ex; }
+        })
+        { IsBackground = true };
+        thread.SetApartmentState(ApartmentState.STA);
+        thread.Start();
+        Assert.True(thread.Join(TimeSpan.FromSeconds(30)), "The STA test did not complete.");
+        if (error != null)
+            ExceptionDispatchInfo.Capture(error).Throw();
+    }
+}
diff --git a/OpenNest/Controls/ProgramEditorControl.cs b/OpenNest/Controls/ProgramEditorControl.cs
index 2077b9c..9ba8af9 100644
--- a/OpenNest/Controls/ProgramEditorControl.cs
+++ b/OpenNest/Controls/ProgramEditorControl.cs
@@ -65,6 +65,7 @@ namespace OpenNest.Controls
             preview.Entities.Clear();
             preview.Invalidate();
             gcodeEditor.Clear();
+            lblGcode.Text = "G-Code";
             Program = null;
             isDirty = false;
             isLoaded = false;
@@ -154,44 +155,67 @@ namespace OpenNest.Controls
                 .ToString("0.####", System.Globalization.CultureInfo.InvariantCulture);
         }
 
+        internal virtual IReadOnlyList<HighlightSpan> ComputeHighlightSpans(string text) =>
+            ProgramHighlighting.ComputeSpans(text);
+
         private void ApplyHighlighting()
         {
             var text = gcodeEditor.Text;
             if (string.IsNullOrEmpty(text))
+            {
+                lblGcode.Text = "G-Code";
                 return;
-
-            gcodeEditor.SuspendLayout();
-
-            var rapidColor = Color.FromArgb(230, 180, 80);
-            var linearColor = Color.FromArgb(130, 200, 140);
-            var arcColor = Color.FromArgb(120, 160, 255);
-            var commentColor = Color.FromArgb(120, 120, 140);
-            var modeColor = Color.FromArgb(200, 140, 220);
-            var coordColor = Color.FromArgb(180, 200, 180);
-
-            gcodeEditor.SelectAll();
-            gcodeEditor.SelectionColor = coordColor;
-
-            var rules = new (Regex pattern, Color color)[]
-            {
-                (new Regex(@"^;.*$", RegexOptions.Multiline), commentColor),
-                (new Regex(@"^G9[01]\b", RegexOptions.Multiline), modeColor),
-                (new Regex(@"^G00\b", RegexOptions.Multiline), rapidColor),
-                (new Regex(@"^G01\b", RegexOptions.Multiline), linearColor),
-                (new Regex(@"^G0[23]\b", RegexOptions.Multiline), arcColor),
-            };
-
-            foreach (var (pattern, color) in rules)
-            {
-                foreach (Match match in pattern.Matches(text))
-                {
-                    gcodeEditor.Select(match.Index, match.Length);
-                    gcodeEditor.SelectionColor = color;
-                }
             }
 
-            gcodeEditor.Select(0, 0);
-            gcodeEditor.ResumeLayout();
+            IReadOnlyList<HighlightSpan> spans;
+            var timedOut = false;
+            try
+            {
+                // Complete lazy regex enumeration before applying any rule colors.
+                spans = ComputeHighlightSpans(text);
+            }
+            catch (RegexMatchTimeoutException)
+            {
+                spans = Array.Empty<HighlightSpan>();
+                timedOut = true;
+            }
+
+            var colors = new[]
+            {
+                Color.FromArgb(120, 120, 140), // Comment
+                Color.FromArgb(200, 140, 220), // Mode
+                Color.FromArgb(230, 180, 80),  // Rapid
+                Color.FromArgb(130, 200, 140), // Linear
+                Color.FromArgb(120, 160, 255), // Arc
+            };
+            var coordColor = Color.FromArgb(180, 200, 180);
+
+            gcodeEditor.SuspendLayout();
+            try
+            {
+                gcodeEditor.SelectAll();
+                gcodeEditor.SelectionColor = coordColor;
+
+                foreach (var span in spans)
+                {
+                    gcodeEditor.Select(span.Index, span.Length);
+                    gcodeEditor.SelectionColor = colors[span.RuleIndex];
+                }
+
+                // Reuse the existing toolbar label; no modal UI or handle-dependent invoke.
+                lblGcode.Text = timedOut ? "G-Code (highlighting timed out)" : "G-Code";
+            }
+            finally
+            {
+                try
+                {
+                    gcodeEditor.Select(0, 0);
+                }
+                finally
+                {
+                    gcodeEditor.ResumeLayout();
+                }
+            }
         }
 
         private void RefreshPreview()