feat(diff): sync vertical scrolling across the split panes

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

@@ -5,6 +5,8 @@ using System.IO;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Media;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
 using AvaloniaEdit;
 using AvaloniaEdit.Document;
 using AvaloniaEdit.Editing;
@@ -64,6 +66,10 @@ public partial class DiffTextView : UserControl
 
     private DiffLineNumberMargin? _leftMargin, _rightMargin;
 
+    private ScrollViewer? _leftScroll, _rightScroll;
+    private bool _scrollHooked;
+    private bool _syncing;
+
     public DiffTextView()
     {
         InitializeComponent();
@@ -71,6 +77,7 @@ public partial class DiffTextView : UserControl
         _rightTm = RightEditor.InstallTextMate(Registry);
         InstallRenderers();
         Rebuild();
+        HookScrollSync();
     }
 
     protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
@@ -199,6 +206,49 @@ public partial class DiffTextView : UserControl
         RightEditor.TextArea.TextView.InvalidateVisual();
     }
 
+    /// The editors' ScrollViewers only exist once the template has been applied.
+    private void HookScrollSync()
+    {
+        if (_scrollHooked) return;
+        _scrollHooked = true;
+        Dispatcher.UIThread.Post(() =>
+        {
+            _leftScroll = LeftEditor.FindDescendantOfType<ScrollViewer>();
+            _rightScroll = RightEditor.FindDescendantOfType<ScrollViewer>();
+            if (_leftScroll is not null) _leftScroll.ScrollChanged += OnPaneScroll;
+            if (_rightScroll is not null) _rightScroll.ScrollChanged += OnPaneScroll;
+        }, DispatcherPriority.Loaded);
+    }
+
+    private void OnPaneScroll(object? sender, ScrollChangedEventArgs e)
+    {
+        if (_syncing || !IsSplit || _leftScroll is null || _rightScroll is null) return;
+        var fromLeft = ReferenceEquals(sender, _leftScroll);
+        var source = fromLeft ? _leftScroll : _rightScroll;
+        var target = fromLeft ? _rightScroll : _leftScroll;
+        var targetEditor = fromLeft ? RightEditor : LeftEditor;
+        var sourceEditor = fromLeft ? LeftEditor : RightEditor;
+
+        _syncing = true;
+        try
+        {
+            if (WrapLines)
+            {
+                // Line heights differ once lines wrap, so anchor on the top visible line.
+                var topLine = sourceEditor.TextArea.TextView.VisualLines is { Count: > 0 } visualLines
+                    ? visualLines[0].FirstDocumentLine.LineNumber
+                    : (int?)null;
+                if (topLine is { } line && line >= 1 && line <= targetEditor.Document.LineCount)
+                    targetEditor.ScrollToLine(line);
+            }
+            else if (Math.Abs(target.Offset.Y - source.Offset.Y) > 0.5)
+            {
+                target.Offset = new Vector(target.Offset.X, source.Offset.Y);
+            }
+        }
+        finally { _syncing = false; }
+    }
+
     /// <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);