Highlight local-reference matches on click

Phase 2.2 of the references/tooltips work — clicking a local reference
(parameter, variable, label, ...) highlights every occurrence in the
current document with a colored background. Cross-document references
clear any existing marks before navigating.

Assisted-by: Claude:claude-opus-4-7:Claude Code

ILSpy/TextView/DecompilerTextView.axaml.cs

@@ -16,11 +16,13 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System.Collections.Generic;
 using System.ComponentModel;
 using System.Linq;
 
 using Avalonia.Controls;
 using Avalonia.Input;
+using Avalonia.Media;
 using Avalonia.Threading;
 
 using AvaloniaEdit.Folding;
@@ -30,7 +32,14 @@ namespace ILSpy.TextView
 {
 	public partial class DecompilerTextView : UserControl
 	{
+		// Local-reference highlight colours, mirroring ILSpy/Themes/generic.xaml. Use the same
+		// light-yellow "GreenYellow" for matches and a softer green for the actual definition.
+		static readonly Color LocalMatchBackground = Colors.GreenYellow;
+		static readonly Color LocalDefinitionBackground = Color.FromArgb(0x80, 0xA0, 0xFF, 0xA0);
+
 		readonly ReferenceElementGenerator referenceElementGenerator;
+		readonly TextMarkerService textMarkerService;
+		readonly List<TextMarker> localReferenceMarks = new();
 		RichTextColorizer? activeColorizer;
 		FoldingManager? activeFoldingManager;
 
@@ -44,6 +53,11 @@ namespace ILSpy.TextView
 			referenceElementGenerator = new ReferenceElementGenerator(static segment => segment.Reference != null);
 			referenceElementGenerator.ReferenceClicked += OnReferenceClicked;
 			Editor.TextArea.TextView.ElementGenerators.Add(referenceElementGenerator);
+
+			// Background renderer that paints local-reference highlights. Lives once for the
+			// lifetime of the view; the marks themselves are cleared and rebuilt per click.
+			textMarkerService = new TextMarkerService(Editor.TextArea.TextView);
+			Editor.TextArea.TextView.BackgroundRenderers.Add(textMarkerService);
 		}
 
 		protected override void OnDataContextChanged(System.EventArgs e)
@@ -72,6 +86,7 @@ namespace ILSpy.TextView
 
 		void ApplyDocument(DecompilerTabPageModel model)
 		{
+			ClearLocalReferenceMarks();
 			Editor.SyntaxHighlighting = HighlightingService.GetByExtension(model.SyntaxExtension);
 			Editor.Document.Text = model.Text;
 
@@ -113,6 +128,16 @@ namespace ILSpy.TextView
 			if (DataContext is not DecompilerTabPageModel model || segment.Reference == null)
 				return;
 
+			// Local references stay inside this document — paint every match and let the user
+			// scrub through them. Cross-document references clear any existing marks since the
+			// view is about to refresh anyway.
+			if (segment.IsLocal)
+			{
+				HighlightLocalReferences(model, segment.Reference);
+				return;
+			}
+			ClearLocalReferenceMarks();
+
 			// In-document jumps win when the definition is in this same view.
 			if (model.DefinitionLookup is { } lookup)
 			{
@@ -129,5 +154,27 @@ namespace ILSpy.TextView
 			// Otherwise let the host route the reference (assembly-tree navigation, new tab, ...).
 			model.RaiseNavigateRequested(segment);
 		}
+
+		void HighlightLocalReferences(DecompilerTabPageModel model, object reference)
+		{
+			ClearLocalReferenceMarks();
+			if (model.References == null)
+				return;
+			foreach (var r in model.References)
+			{
+				if (!ReferenceEquals(reference, r.Reference) && !reference.Equals(r.Reference))
+					continue;
+				var mark = textMarkerService.Create(r.StartOffset, r.Length);
+				mark.BackgroundColor = r.IsDefinition ? LocalDefinitionBackground : LocalMatchBackground;
+				localReferenceMarks.Add(mark);
+			}
+		}
+
+		void ClearLocalReferenceMarks()
+		{
+			foreach (var mark in localReferenceMarks)
+				textMarkerService.Remove(mark);
+			localReferenceMarks.Clear();
+		}
 	}
 }

ILSpy/TextView/TextMarkerService.cs

@@ -0,0 +1,141 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+using Avalonia;
+using Avalonia.Media;
+using Avalonia.Threading;
+
+using AvaloniaEdit.Document;
+using AvaloniaEdit.Rendering;
+
+namespace ILSpy.TextView
+{
+	/// <summary>
+	/// Manages background-coloured text marks over the editor — used for local-reference
+	/// highlighting today, and a useful primitive for future features (errors, search hits, ...).
+	/// Slimmed down from the WPF host's <c>TextMarkerService</c>: only the colored background
+	/// path is implemented; underline marker types (squiggly / dotted / solid) and per-marker
+	/// foreground / typeface tweaks are deliberately omitted until something actually needs them.
+	/// </summary>
+	sealed class TextMarkerService : IBackgroundRenderer
+	{
+		readonly AvaloniaEdit.Rendering.TextView textView;
+		TextSegmentCollection<TextMarker>? markers;
+
+		public TextMarkerService(AvaloniaEdit.Rendering.TextView textView)
+		{
+			this.textView = textView ?? throw new ArgumentNullException(nameof(textView));
+			textView.DocumentChanged += OnDocumentChanged;
+			OnDocumentChanged(null, EventArgs.Empty);
+		}
+
+		void OnDocumentChanged(object? sender, EventArgs e)
+		{
+			markers = textView.Document != null ? new TextSegmentCollection<TextMarker>(textView.Document) : null;
+		}
+
+		public TextMarker Create(int startOffset, int length)
+		{
+			if (markers == null)
+				throw new InvalidOperationException("Cannot create a marker when not attached to a document");
+			var textLength = textView.Document.TextLength;
+			if (startOffset < 0 || startOffset > textLength)
+				throw new ArgumentOutOfRangeException(nameof(startOffset));
+			if (length < 0 || startOffset + length > textLength)
+				throw new ArgumentOutOfRangeException(nameof(length));
+
+			var marker = new TextMarker(this, startOffset, length);
+			markers.Add(marker);
+			return marker;
+		}
+
+		public void Remove(TextMarker marker)
+		{
+			if (markers != null && markers.Remove(marker))
+				Redraw(marker);
+		}
+
+		public void RemoveAll(Predicate<TextMarker> predicate)
+		{
+			if (markers == null)
+				return;
+			foreach (var m in markers.ToArray())
+				if (predicate(m))
+					Remove(m);
+		}
+
+		internal void Redraw(ISegment segment) => textView.Redraw(segment);
+
+		// IBackgroundRenderer
+
+		public KnownLayer Layer => KnownLayer.Selection; // draw behind selection
+
+		public void Draw(AvaloniaEdit.Rendering.TextView textView, DrawingContext drawingContext)
+		{
+			if (markers == null || !textView.VisualLinesValid)
+				return;
+			var visualLines = textView.VisualLines;
+			if (visualLines.Count == 0)
+				return;
+
+			int viewStart = visualLines.First().FirstDocumentLine.Offset;
+			int viewEnd = visualLines.Last().LastDocumentLine.EndOffset;
+			foreach (var marker in markers.FindOverlappingSegments(viewStart, viewEnd - viewStart))
+			{
+				if (marker.BackgroundColor is not { } bg)
+					continue;
+				var geo = new BackgroundGeometryBuilder { AlignToWholePixels = true, CornerRadius = 3 };
+				geo.AddSegment(textView, marker);
+				var geometry = geo.CreateGeometry();
+				if (geometry != null)
+					drawingContext.DrawGeometry(new SolidColorBrush(bg), null, geometry);
+			}
+		}
+	}
+
+	/// <summary>
+	/// A single highlighted span. Setting <see cref="BackgroundColor"/> triggers a redraw of
+	/// just that segment.
+	/// </summary>
+	sealed class TextMarker : TextSegment
+	{
+		readonly TextMarkerService service;
+		Color? backgroundColor;
+
+		public TextMarker(TextMarkerService service, int startOffset, int length)
+		{
+			this.service = service;
+			StartOffset = startOffset;
+			Length = length;
+		}
+
+		public Color? BackgroundColor {
+			get => backgroundColor;
+			set {
+				if (backgroundColor == value)
+					return;
+				backgroundColor = value;
+				service.Redraw(this);
+			}
+		}
+	}
+}