fix(ui): auto-scroll the lists sidebar while dragging near its top/bottom edge

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Islands/ListsIslandView.axaml

@@ -255,7 +255,7 @@
     </Border>
 
     <!-- ── Scrollable body ── -->
-    <ScrollViewer>
+    <ScrollViewer x:Name="BodyScroll" DragDrop.AllowDrop="True">
       <StackPanel Margin="6,0,6,4">
 
         <!-- SMART LISTS section -->

src/ClaudeDo.Ui/Views/Islands/ListsIslandView.axaml.cs

@@ -5,6 +5,7 @@ using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.Threading;
 using Avalonia.VisualTree;
 using ClaudeDo.Ui.ViewModels;
 using ClaudeDo.Ui.ViewModels.Islands;
@@ -13,6 +14,23 @@ using ClaudeDo.Ui.Views.Modals;
 
 namespace ClaudeDo.Ui.Views.Islands;
 
+/// <summary>Edge-band scroll step for dragging inside the lists sidebar.</summary>
+public static class ListsAutoScroll
+{
+    private const double Band = 40;
+    private const double MaxStep = 14;
+
+    /// <summary>Per-tick scroll delta for a pointer at <paramref name="y"/> in a viewport of <paramref name="height"/>: negative near the top, positive near the bottom, 0 outside the bands.</summary>
+    public static double Delta(double y, double height)
+    {
+        var band = Math.Min(Band, height / 2);
+        if (band <= 0) return 0;
+        if (y < band) return -MaxStep * Math.Min(1, (band - y) / band);
+        if (y > height - band) return MaxStep * Math.Min(1, (y - (height - band)) / band);
+        return 0;
+    }
+}
+
 public partial class ListsIslandView : UserControl
 {
     private static readonly DataFormat<string> ListRowFormat =
@@ -33,12 +51,42 @@ public partial class ListsIslandView : UserControl
         _pendingFolderDragPress = null;
     }
 
+    private DispatcherTimer? _autoScrollTimer;
+    private double _autoScrollDelta;
+
+    /// <summary>Keeps the sidebar scrolling while the drag pointer sits in an edge band; DragOver alone may not repeat on a still pointer.</summary>
+    private void OnBodyDragOver(object? sender, DragEventArgs e)
+    {
+        // Gaps and section labels are only drop targets so DragOver reaches us — they accept nothing.
+        if (FindListItemBorder(e.Source as Visual) is null) e.DragEffects = DragDropEffects.None;
+
+        _autoScrollDelta = ListsAutoScroll.Delta(e.GetPosition(BodyScroll).Y, BodyScroll.Bounds.Height);
+        if (_autoScrollDelta == 0) { StopAutoScroll(); return; }
+
+        _autoScrollTimer ??= new DispatcherTimer(TimeSpan.FromMilliseconds(16), DispatcherPriority.Background, (_, _) =>
+        {
+            var maxY = Math.Max(0, BodyScroll.Extent.Height - BodyScroll.Viewport.Height);
+            var y = Math.Clamp(BodyScroll.Offset.Y + _autoScrollDelta, 0, maxY);
+            BodyScroll.Offset = new Vector(BodyScroll.Offset.X, y);
+        });
+        _autoScrollTimer.Start();
+    }
+
+    private void StopAutoScroll()
+    {
+        _autoScrollDelta = 0;
+        _autoScrollTimer?.Stop();
+    }
+
     public ListsIslandView()
     {
         InitializeComponent();
         AddHandler(PointerPressedEvent, OnTunnelPointerPressed, RoutingStrategies.Tunnel);
         AddHandler(PointerMovedEvent, OnTunnelPointerMoved, RoutingStrategies.Tunnel);
         AddHandler(PointerReleasedEvent, (_, _) => CancelPendingFolderDrag(), RoutingStrategies.Tunnel);
+        BodyScroll.AddHandler(DragDrop.DragOverEvent, OnBodyDragOver, RoutingStrategies.Bubble, handledEventsToo: true);
+        BodyScroll.AddHandler(DragDrop.DragLeaveEvent, (_, _) => StopAutoScroll(), RoutingStrategies.Bubble, handledEventsToo: true);
+        BodyScroll.AddHandler(DragDrop.DropEvent, (_, _) => StopAutoScroll(), RoutingStrategies.Bubble, handledEventsToo: true);
         DataContextChanged += (_, _) =>
         {
             if (DataContext is ListsIslandViewModel vm)
@@ -92,6 +140,7 @@ public partial class ListsIslandView : UserControl
         }
         finally
         {
+            StopAutoScroll();
             vm.ClearDropHints();
         }
     }
@@ -115,6 +164,7 @@ public partial class ListsIslandView : UserControl
         }
         finally
         {
+            StopAutoScroll();
             vm.ClearDropHints();
         }
     }

tests/ClaudeDo.Ui.Tests/ListsAutoScrollTests.cs

@@ -0,0 +1,40 @@
+using ClaudeDo.Ui.Views.Islands;
+using Xunit;
+
+namespace ClaudeDo.Ui.Tests;
+
+public class ListsAutoScrollTests
+{
+    [Fact]
+    public void Middle_of_viewport_does_not_scroll() =>
+        Assert.Equal(0, ListsAutoScroll.Delta(300, 600));
+
+    [Fact]
+    public void Top_band_scrolls_up_and_speeds_up_towards_the_edge()
+    {
+        var near = ListsAutoScroll.Delta(30, 600);
+        var edge = ListsAutoScroll.Delta(0, 600);
+        Assert.True(near < 0);
+        Assert.True(edge < near);
+    }
+
+    [Fact]
+    public void Bottom_band_scrolls_down_and_speeds_up_towards_the_edge()
+    {
+        var near = ListsAutoScroll.Delta(570, 600);
+        var edge = ListsAutoScroll.Delta(600, 600);
+        Assert.True(near > 0);
+        Assert.True(edge > near);
+    }
+
+    [Fact]
+    public void Pointer_outside_the_viewport_is_clamped_to_max_step() =>
+        Assert.Equal(ListsAutoScroll.Delta(0, 600), ListsAutoScroll.Delta(-50, 600));
+
+    [Fact]
+    public void Tiny_or_empty_viewport_is_safe()
+    {
+        Assert.Equal(0, ListsAutoScroll.Delta(0, 0));
+        Assert.True(ListsAutoScroll.Delta(5, 20) < 0);
+    }
+}