feat(diff): draw old and new line numbers in a custom margin

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

@@ -1,9 +1,13 @@
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.IO;
 using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Media;
 using AvaloniaEdit;
+using AvaloniaEdit.Editing;
+using AvaloniaEdit.Rendering;
 using AvaloniaEdit.TextMate;
 using ClaudeDo.Ui.ViewModels.Modals;
 using TextMateSharp.Grammars;
@@ -57,6 +61,8 @@ public partial class DiffTextView : UserControl
 
     private AlignedDiff _aligned = AlignedDiff.Empty;
 
+    private DiffLineNumberMargin? _leftMargin, _rightMargin;
+
     public DiffTextView()
     {
         InitializeComponent();
@@ -104,6 +110,7 @@ public partial class DiffTextView : UserControl
             _rightRows = Array.Empty<RowInfo?>();
         }
 
+        RebuildMargins();
         ApplyWrap();
         ApplyGrammar(File?.Path);
     }
@@ -144,7 +151,98 @@ public partial class DiffTextView : UserControl
         _rightTm?.SetGrammar(scope);
     }
 
+    /// The margin's column layout depends on split vs unified, so it is rebuilt rather than
+    /// reconfigured whenever the layout changes.
+    private void RebuildMargins()
+    {
+        if (_leftMargin is not null) LeftEditor.TextArea.LeftMargins.Remove(_leftMargin);
+        if (_rightMargin is not null) RightEditor.TextArea.LeftMargins.Remove(_rightMargin);
+
+        var foreground = Brush("TextFaintBrush", Color.Parse("#80FFFFFF"));
+        var typeface = new Typeface(LeftEditor.FontFamily);
+
+        _leftMargin = new DiffLineNumberMargin(LeftRow, showOld: true, showNew: !IsSplit,
+                                               foreground, typeface, LeftEditor.FontSize);
+        LeftEditor.TextArea.LeftMargins.Insert(0, _leftMargin);
+
+        _rightMargin = new DiffLineNumberMargin(RightRow, showOld: false, showNew: true,
+                                                foreground, typeface, RightEditor.FontSize);
+        RightEditor.TextArea.LeftMargins.Insert(0, _rightMargin);
+    }
+
+    private IBrush Brush(string key, Color fallback) =>
+        this.TryGetResource(key, ActualThemeVariant, out var value) && value is IBrush brush
+            ? brush
+            : new SolidColorBrush(fallback);
+
     /// <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);
+
+    /// <summary>Draws one or two gutter columns of diff line numbers from the row lookup.</summary>
+    private sealed class DiffLineNumberMargin : AbstractMargin
+    {
+        private const double ColumnWidth = 44;
+        private const double Gap = 6;
+
+        private readonly Func<int, RowInfo?> _rows;
+        private readonly bool _showOld;
+        private readonly bool _showNew;
+        private readonly IBrush _foreground;
+        private readonly Typeface _typeface;
+        private readonly double _fontSize;
+
+        public DiffLineNumberMargin(Func<int, RowInfo?> rows, bool showOld, bool showNew,
+                                    IBrush foreground, Typeface typeface, double fontSize)
+        {
+            _rows = rows;
+            _showOld = showOld;
+            _showNew = showNew;
+            _foreground = foreground;
+            _typeface = typeface;
+            _fontSize = fontSize;
+        }
+
+        private double Columns => (_showOld ? 1 : 0) + (_showNew ? 1 : 0);
+
+        protected override Size MeasureOverride(Size availableSize) =>
+            new(Columns * ColumnWidth + Gap, 0);
+
+        protected override void OnTextViewChanged(TextView? oldTextView, TextView? newTextView)
+        {
+            if (oldTextView is not null) oldTextView.VisualLinesChanged -= OnVisualLinesChanged;
+            base.OnTextViewChanged(oldTextView, newTextView);
+            if (newTextView is not null) newTextView.VisualLinesChanged += OnVisualLinesChanged;
+            InvalidateVisual();
+        }
+
+        private void OnVisualLinesChanged(object? sender, EventArgs e) => InvalidateVisual();
+
+        public override void Render(DrawingContext context)
+        {
+            var textView = TextView;
+            if (textView is null || !textView.VisualLinesValid) return;
+
+            foreach (var visualLine in textView.VisualLines)
+            {
+                var lineNumber = visualLine.FirstDocumentLine.LineNumber;
+                if (_rows(lineNumber) is not { } row) continue;
+
+                var y = visualLine.VisualTop - textView.ScrollOffset.Y;
+                var column = 0;
+                if (_showOld) DrawNumber(context, row.OldNo, column++, y);
+                if (_showNew) DrawNumber(context, row.NewNo, column, y);
+            }
+        }
+
+        private void DrawNumber(DrawingContext context, int? value, int column, double y)
+        {
+            if (value is null) return;
+            var text = new FormattedText(value.Value.ToString(CultureInfo.InvariantCulture),
+                CultureInfo.InvariantCulture, FlowDirection.LeftToRight, _typeface, _fontSize, _foreground);
+            // Right-align inside the column so the digits line up across rows.
+            var x = (column + 1) * ColumnWidth - text.Width - Gap;
+            context.DrawText(text, new Point(x, y));
+        }
+    }
 }