feat(diff): tint diff rows and changed words via background renderers

src/ClaudeDo.Ui/Views/Controls/DiffTextView.axaml.cs

@@ -6,6 +6,7 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Media;
 using AvaloniaEdit;
+using AvaloniaEdit.Document;
 using AvaloniaEdit.Editing;
 using AvaloniaEdit.Rendering;
 using AvaloniaEdit.TextMate;
@@ -68,6 +69,7 @@ public partial class DiffTextView : UserControl
         InitializeComponent();
         _leftTm = LeftEditor.InstallTextMate(Registry);
         _rightTm = RightEditor.InstallTextMate(Registry);
+        InstallRenderers();
         Rebuild();
     }
 
@@ -113,6 +115,7 @@ public partial class DiffTextView : UserControl
         RebuildMargins();
         ApplyWrap();
         ApplyGrammar(File?.Path);
+        InvalidateRenderers();
     }
 
     /// Row index i is document line i + 1, so slot 0 stays null and lookups can pass the
@@ -175,6 +178,27 @@ public partial class DiffTextView : UserControl
             ? brush
             : new SolidColorBrush(fallback);
 
+    private void InstallRenderers()
+    {
+        var add    = Brush("RunningTintBrush",  Color.Parse("#1F7C9166"));
+        var del    = Brush("ErrorTintBrush",    Color.Parse("#1FC87060"));
+        var filler = Brush("DiffFillerBrush",   Color.Parse("#0AFFFFFF"));
+        var gap    = Brush("DiffGapBrush",      Color.Parse("#14FFFFFF"));
+        var wordAdd = Brush("DiffWordAddBrush", Color.Parse("#556FA86B"));
+        var wordDel = Brush("DiffWordDelBrush", Color.Parse("#55C87060"));
+
+        LeftEditor.TextArea.TextView.BackgroundRenderers.Add(new DiffLineRenderer(LeftRow, add, del, filler, gap));
+        LeftEditor.TextArea.TextView.BackgroundRenderers.Add(new WordDiffRenderer(LeftRow, wordAdd, wordDel));
+        RightEditor.TextArea.TextView.BackgroundRenderers.Add(new DiffLineRenderer(RightRow, add, del, filler, gap));
+        RightEditor.TextArea.TextView.BackgroundRenderers.Add(new WordDiffRenderer(RightRow, wordAdd, wordDel));
+    }
+
+    private void InvalidateRenderers()
+    {
+        LeftEditor.TextArea.TextView.InvalidateVisual();
+        RightEditor.TextArea.TextView.InvalidateVisual();
+    }
+
     /// <summary>What one document line represents, for the margin and the background renderers.</summary>
     internal sealed record RowInfo(
         AlignedSide Kind, int? OldNo, int? NewNo, IReadOnlyList<TextSpan> Spans);
@@ -245,4 +269,88 @@ public partial class DiffTextView : UserControl
             context.DrawText(text, new Point(x, y));
         }
     }
+
+    /// <summary>Tints whole rows by their diff role.</summary>
+    private sealed class DiffLineRenderer : IBackgroundRenderer
+    {
+        private readonly Func<int, RowInfo?> _rows;
+        private readonly IBrush _add, _del, _filler, _gap;
+
+        public DiffLineRenderer(Func<int, RowInfo?> rows, IBrush add, IBrush del, IBrush filler, IBrush gap)
+        {
+            _rows = rows; _add = add; _del = del; _filler = filler; _gap = gap;
+        }
+
+        public KnownLayer Layer => KnownLayer.Background;
+
+        public void Draw(TextView textView, DrawingContext drawingContext)
+        {
+            if (!textView.VisualLinesValid) return;
+            foreach (var visualLine in textView.VisualLines)
+            {
+                var row = _rows(visualLine.FirstDocumentLine.LineNumber);
+                var brush = row?.Kind switch
+                {
+                    AlignedSide.Add    => _add,
+                    AlignedSide.Del    => _del,
+                    AlignedSide.Filler => _filler,
+                    AlignedSide.Gap    => _gap,
+                    _                  => null,
+                };
+                if (brush is null) continue;
+
+                var top = visualLine.VisualTop - textView.ScrollOffset.Y;
+                drawingContext.FillRectangle(brush, new Rect(0, top, textView.Bounds.Width, visualLine.Height));
+            }
+        }
+    }
+
+    /// <summary>Tints the changed character ranges inside a row, on top of the row tint.</summary>
+    private sealed class WordDiffRenderer : IBackgroundRenderer
+    {
+        private readonly Func<int, RowInfo?> _rows;
+        private readonly IBrush _add, _del;
+
+        public WordDiffRenderer(Func<int, RowInfo?> rows, IBrush add, IBrush del)
+        {
+            _rows = rows; _add = add; _del = del;
+        }
+
+        // Above the line tint but still behind the text.
+        public KnownLayer Layer => KnownLayer.Selection;
+
+        public void Draw(TextView textView, DrawingContext drawingContext)
+        {
+            if (!textView.VisualLinesValid) return;
+            foreach (var visualLine in textView.VisualLines)
+            {
+                var documentLine = visualLine.FirstDocumentLine;
+                var row = _rows(documentLine.LineNumber);
+                if (row is null || row.Spans.Count == 0) continue;
+
+                var brush = row.Kind == AlignedSide.Add ? _add : _del;
+                foreach (var span in row.Spans)
+                {
+                    var offset = documentLine.Offset + span.Start;
+                    // Spans are computed against the row text; clamp in case the document
+                    // and the row lookup ever disagree rather than drawing past the line.
+                    if (offset < documentLine.Offset || offset + span.Length > documentLine.EndOffset) continue;
+
+                    var builder = new BackgroundGeometryBuilder { AlignToWholePixels = true, CornerRadius = 2 };
+                    builder.AddSegment(textView, new Seg(offset, span.Length));
+                    if (builder.CreateGeometry() is { } geometry)
+                        drawingContext.DrawGeometry(brush, null, geometry);
+                }
+            }
+        }
+    }
+
+    /// <summary>A minimal <see cref="ISegment"/> for geometry queries.</summary>
+    private readonly struct Seg : ISegment
+    {
+        public Seg(int offset, int length) { Offset = offset; Length = length; }
+        public int Offset { get; }
+        public int Length { get; }
+        public int EndOffset => Offset + Length;
+    }
 }