Double BringIntoView in VirtualizingStackPanel (#14419)

This fixes a case when elements have different widths and the BringIntoView wants to scroll horizontally due to custom TargetRect

Co-authored-by: Max Katz <maxkatz6@outlook.com>

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -430,6 +430,12 @@ namespace Avalonia.Controls
                     root.LayoutManager.ExecuteLayoutPass();
                 }
 
+                // During the previous BringIntoView, the scroll width extent might have been out of date if
+                // elements have different widths. Because of that, the ScrollViewer might not scroll to the correct offset.
+                // After the previous BringIntoView, Y offset should be correct and an extra layout pass has been executed,
+                // hence the width extent should be correct now, and we can try to scroll again.
+                scrollToElement.BringIntoView();
+
                 _scrollToElement = null;
                 _scrollToIndex = -1;
                 return scrollToElement;

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -1116,6 +1116,44 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(container.IsVisible);
         }
 
+        [Fact]
+        public void ScrollIntoView_With_TargetRect_Outside_Viewport_Should_Scroll_To_Item()
+        {
+            using var app = App();
+            var items = Enumerable.Range(0, 101).Select(x => new ItemWithHeight(x, x * 100 + 1));
+            var itemTemplate = new FuncDataTemplate<ItemWithHeight>((x, _) =>
+                new Border
+                {
+                    Height = 10,
+                    [!Layoutable.WidthProperty] = new Binding("Height"),
+                });
+            var (target, scroll, itemsControl) = CreateTarget(
+                items: items,
+                itemTemplate: itemTemplate,
+                styles: new[]
+                {
+                    new Style(x => x.OfType<ScrollViewer>())
+                    {
+                        Setters =
+                        {
+                            new Setter(ScrollViewer.HorizontalScrollBarVisibilityProperty, ScrollBarVisibility.Visible),
+                        }
+                    }
+                });
+            itemsControl.ContainerPrepared += (_, ev) =>
+            {
+                ev.Container.AddHandler(Control.RequestBringIntoViewEvent, (_, e) =>
+                {
+                    var dataContext = e.TargetObject.DataContext as ItemWithHeight;
+                    e.TargetRect = new Rect(dataContext.Height - 50, 0, 50, 10);
+                });
+            };
+
+            target.ScrollIntoView(100);
+
+            Assert.Equal(9901, scroll.Offset.X);
+        }
+
         private static IReadOnlyList<int> GetRealizedIndexes(VirtualizingStackPanel target, ItemsControl itemsControl)
         {
             return target.GetRealizedElements()