Restore tab selection during drag

src/Dock.Avalonia/Internal/ItemDragHelper.cs

@@ -41,6 +41,10 @@ internal class ItemDragHelper
     private const double AutoScrollSpeed = 200.0; // pixels per second
     private const double AutoScrollTimerInterval = 16.0; // ~60fps
     private PointerEventArgs? _pressedArgs;
+    private SelectingItemsControl? _selectingItemsControl;
+    private int _selectedIndexOnPress = -1;
+    private object? _selectedItemOnPress;
+    private bool _selectionRestored;
 
     public ItemDragHelper(
         Control owner,
@@ -76,6 +80,7 @@ public void Detach()
         _owner.RemoveHandler(InputElement.PointerCaptureLostEvent, PointerCaptureLost);
 
         StopAutoScroll();
+        ResetSelectionTracking();
     }
 
     private void PointerPressed(object? sender, PointerPressedEventArgs e)
@@ -90,6 +95,7 @@ private void PointerPressed(object? sender, PointerPressedEventArgs e)
             _targetIndex = -1;
             _itemsControl = itemsControl;
             _draggedContainer = _owner;
+            CaptureSelection(itemsControl);
 
             if (_draggedContainer is not null)
             {
@@ -174,6 +180,7 @@ private void Released()
         _itemsControl = null;
 
         _draggedContainer = null;
+        ResetSelectionTracking();
     }
 
     private void AddTransforms(ItemsControl? itemsControl)
@@ -448,6 +455,7 @@ private void PointerMoved(object? sender, PointerEventArgs e)
             if (!IsPositionWithinDragBounds(position, _itemsControl))
             {
                 _dragStarted = false;
+                RestoreSelectionIfNeeded();
                 Released();
                 _captured = false;
                 _dragOutside?.Invoke(_pressedArgs, e);
@@ -469,6 +477,7 @@ private void PointerMoved(object? sender, PointerEventArgs e)
                     if (Math.Abs(diff.X) > horizontalDragThreshold)
                     {
                         _dragStarted = true;
+                        RestoreSelectionIfNeeded();
                     }
                     else
                     {
@@ -480,6 +489,7 @@ private void PointerMoved(object? sender, PointerEventArgs e)
                     if (Math.Abs(diff.Y) > verticalDragThreshold)
                     {
                         _dragStarted = true;
+                        RestoreSelectionIfNeeded();
                     }
                     else
                     {
@@ -564,6 +574,71 @@ private void PointerMoved(object? sender, PointerEventArgs e)
         }
     }
 
+    private void CaptureSelection(ItemsControl itemsControl)
+    {
+        if (itemsControl is SelectingItemsControl selectingItemsControl)
+        {
+            _selectingItemsControl = selectingItemsControl;
+            _selectedIndexOnPress = selectingItemsControl.SelectedIndex;
+            _selectedItemOnPress = selectingItemsControl.SelectedItem;
+            _selectionRestored = false;
+        }
+        else
+        {
+            ResetSelectionTracking();
+        }
+    }
+
+    private void RestoreSelectionIfNeeded()
+    {
+        if (_selectionRestored || _selectingItemsControl is null)
+        {
+            return;
+        }
+
+        var selectingItemsControl = _selectingItemsControl;
+
+        if (_selectedItemOnPress is not null)
+        {
+            if (!Equals(selectingItemsControl.SelectedItem, _selectedItemOnPress))
+            {
+                if (selectingItemsControl.Items is IList list && list.Contains(_selectedItemOnPress))
+                {
+                    selectingItemsControl.SelectedItem = _selectedItemOnPress;
+                    _selectionRestored = true;
+                    return;
+                }
+            }
+            else
+            {
+                _selectionRestored = true;
+                return;
+            }
+        }
+
+        if (_selectedIndexOnPress >= 0 &&
+            selectingItemsControl.Items is { } items &&
+            _selectedIndexOnPress < items.Count)
+        {
+            if (selectingItemsControl.SelectedIndex != _selectedIndexOnPress)
+            {
+                selectingItemsControl.SelectedIndex = _selectedIndexOnPress;
+            }
+            _selectionRestored = true;
+            return;
+        }
+
+        _selectionRestored = true;
+    }
+
+    private void ResetSelectionTracking()
+    {
+        _selectingItemsControl = null;
+        _selectedIndexOnPress = -1;
+        _selectedItemOnPress = null;
+        _selectionRestored = false;
+    }
+
     private void SetDraggingPseudoClasses(Control control, bool isDragging)
     {
         if (isDragging)
@@ -583,4 +658,3 @@ private void SetTranslateTransform(Control control, double x, double y)
         control.RenderTransform = transformBuilder.Build();
     }
 }
-

tests/Dock.Avalonia.HeadlessTests/DocumentTabStripSelectionRestoreTests.cs

@@ -0,0 +1,113 @@
+using Avalonia;
+using Avalonia.Collections;
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using Avalonia.Input;
+using Dock.Avalonia.Controls;
+using Xunit;
+
+namespace Dock.Avalonia.HeadlessTests;
+
+public class DocumentTabStripSelectionRestoreTests
+{
+    [AvaloniaFact]
+    public void DragOutside_RestoresSelection()
+    {
+        var (window, tabStrip) = CreateTabStrip();
+        try
+        {
+            tabStrip.SelectedIndex = 0;
+            var originalIndex = tabStrip.SelectedIndex;
+
+            var tabItem = GetTabItem(tabStrip, 1);
+
+            var pointer = new Pointer(1, PointerType.Mouse, true);
+            var pressed = CreatePressedArgs(tabItem, tabStrip, new Point(5, 5), pointer);
+            tabItem.RaiseEvent(pressed);
+
+            tabStrip.SelectedIndex = 1;
+            Assert.Equal(1, tabStrip.SelectedIndex);
+
+            var moved = CreateMovedArgs(tabItem, tabStrip, new Point(-50, 5), pointer);
+            tabItem.RaiseEvent(moved);
+
+            Assert.Equal(originalIndex, tabStrip.SelectedIndex);
+        }
+        finally
+        {
+            window.Close();
+        }
+    }
+
+    [AvaloniaFact]
+    public void DragThreshold_RestoresSelection()
+    {
+        var (window, tabStrip) = CreateTabStrip();
+        try
+        {
+            tabStrip.SelectedIndex = 0;
+            var originalIndex = tabStrip.SelectedIndex;
+
+            var tabItem = GetTabItem(tabStrip, 1);
+
+            var pointer = new Pointer(2, PointerType.Mouse, true);
+            var pressed = CreatePressedArgs(tabItem, tabStrip, new Point(5, 5), pointer);
+            tabItem.RaiseEvent(pressed);
+
+            tabStrip.SelectedIndex = 1;
+            Assert.Equal(1, tabStrip.SelectedIndex);
+
+            var moved = CreateMovedArgs(tabItem, tabStrip, new Point(20, 5), pointer);
+            tabItem.RaiseEvent(moved);
+
+            Assert.Equal(originalIndex, tabStrip.SelectedIndex);
+        }
+        finally
+        {
+            window.Close();
+        }
+    }
+
+    private static (Window window, DocumentTabStrip tabStrip) CreateTabStrip()
+    {
+        var tabStrip = new DocumentTabStrip
+        {
+            Width = 400,
+            Height = 32,
+            ItemsSource = new AvaloniaList<string> { "Doc1", "Doc2" }
+        };
+
+        var window = new Window
+        {
+            Width = 400,
+            Height = 200,
+            Content = tabStrip
+        };
+
+        window.Show();
+        tabStrip.ApplyTemplate();
+        window.UpdateLayout();
+        tabStrip.UpdateLayout();
+
+        return (window, tabStrip);
+    }
+
+    private static DocumentTabStripItem GetTabItem(DocumentTabStrip tabStrip, int index)
+    {
+        var tabItem = tabStrip.ContainerFromIndex(index) as DocumentTabStripItem;
+        Assert.NotNull(tabItem);
+        return tabItem!;
+    }
+
+    private static PointerPressedEventArgs CreatePressedArgs(Control source, Visual rootVisual, Point position, Pointer pointer)
+    {
+        var properties = new PointerPointProperties(RawInputModifiers.LeftMouseButton, PointerUpdateKind.LeftButtonPressed);
+        return new PointerPressedEventArgs(source, pointer, rootVisual, position, 0, properties, KeyModifiers.None, 1);
+    }
+
+    private static PointerEventArgs CreateMovedArgs(Control source, Visual rootVisual, Point position, Pointer pointer)
+    {
+        var properties = new PointerPointProperties(RawInputModifiers.LeftMouseButton, PointerUpdateKind.Other);
+        return new PointerEventArgs(InputElement.PointerMovedEvent, source, pointer, rootVisual, position, 0, properties, KeyModifiers.None);
+    }
+}