Fix VirtualizingStackPanel offset when scrolling to variable-sized items (#21975)

* Fix VirtualizingStackPanel offset when scrolling to variable-sized items

* Remove the `isScrollIntoView` parameter.

* Tweak now-failing test.

Change the the test to check that the element is outside of the viewport, instead of asserting its exact coordinates - that's the important part.

* Retrigger CI.

Azure Pipelines never queued a build for 3d81ceb.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0169P8BipcMkn44hvhpQ2UNF

---------

Co-authored-by: grokys <grokys@users.noreply.github.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 3 people

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -56,7 +56,7 @@ public class VirtualizingStackPanel : VirtualizingPanel, IScrollSnapPointsInfo
         /// Defines the <see cref="CacheLength"/> property.
         /// </summary>
         public static readonly StyledProperty<double> CacheLengthProperty =
-            AvaloniaProperty.Register<VirtualizingStackPanel, double>(nameof(CacheLength), 0.0, 
+            AvaloniaProperty.Register<VirtualizingStackPanel, double>(nameof(CacheLength), 0.0,
                 validate: v => v is >= 0 and <= 2);
 
         private static readonly AttachedProperty<object?> RecycleKeyProperty =
@@ -80,8 +80,8 @@ public class VirtualizingStackPanel : VirtualizingPanel, IScrollSnapPointsInfo
         private int _focusedIndex = -1;
         private Control? _realizingElement;
         private int _realizingIndex = -1;
-        private double _bufferFactor; 
-        
+        private double _bufferFactor;
+
         private bool _hasReachedStart = false;
         private bool _hasReachedEnd = false;
         private Rect _lastMeasuredExtendedViewport;
@@ -270,7 +270,7 @@ protected override Size ArrangeOverride(Size finalSize)
                             new Rect(0, u, finalSize.Width, sizeU);
 
                         e.Arrange(rect);
-                    
+
                         if (e.IsVisible && _viewport.Intersects(rect))
                         {
                             try
@@ -285,7 +285,7 @@ protected override Size ArrangeOverride(Size finalSize)
                                     e, ex.Message);
                             }
                         }
-                        
+
                         u += orientation == Orientation.Horizontal ? rect.Width : rect.Height;
                     }
                 }
@@ -543,7 +543,7 @@ protected override void OnItemsControlChanged(ItemsControl? oldValue)
             var count = Items.Count;
             var fromControl = from as Control;
 
-            if (count == 0 || 
+            if (count == 0 ||
                 (fromControl is null && direction is not NavigationDirection.First and not NavigationDirection.Last))
                 return null;
 
@@ -644,7 +644,7 @@ protected internal override int IndexFromContainer(Control container)
                 element.BringIntoView();
                 return element;
             }
-            else if (this.GetLayoutRoot() is {} root)
+            else if (this.GetLayoutRoot() is { } root)
             {
                 // Create and measure the element to be brought into view. Store it in a field so that
                 // it can be re-used in the layout pass.
@@ -746,7 +746,7 @@ private MeasureViewport CalculateMeasureViewport(Orientation orientation, IReadO
             }
 
             // Check if the anchor element is not within the currently realized elements.
-            var disjunct = anchorIndex < _realizedElements.FirstIndex || 
+            var disjunct = anchorIndex < _realizedElements.FirstIndex ||
                 anchorIndex > _realizedElements.LastIndex;
 
             return new MeasureViewport
@@ -780,12 +780,12 @@ private Size EstimateDesiredSize(Orientation orientation, int itemCount)
                 // We have an element to scroll to, so we can estimate the desired size based on the
                 // element's position and the remaining elements.
                 var remaining = itemCount - _scrollToIndex - 1;
-                var u = orientation == Orientation.Horizontal ? 
+                var u = orientation == Orientation.Horizontal ?
                     _scrollToElement.Bounds.Right :
                     _scrollToElement.Bounds.Bottom;
                 var sizeU = u + (remaining * _lastEstimatedElementSizeU);
-                return orientation == Orientation.Horizontal ? 
-                    new(sizeU, DesiredSize.Height) : 
+                return orientation == Orientation.Horizontal ?
+                    new(sizeU, DesiredSize.Height) :
                     new(DesiredSize.Width, sizeU);
             }
 
@@ -901,23 +901,26 @@ private double GetOrEstimateElementU(int index)
             {
                 var first = realized.FirstIndex;
                 var last = realized.LastIndex;
-            
+
                 if (index < first)
                 {
-                    return realized.StartU - ((first - index) * estimatedSize);
+                    // Interpolate between the known panel origin and the first realized item.
+                    // Using the realized items' average size to extrapolate backwards can place
+                    // the target before the panel origin.
+                    return realized.StartU * index / first;
                 }
-            
+
                 if (index > last)
                 {
                     var sizes = realized.SizeU;
                     var realizedSpan = 0.0;
-            
+
                     for (var i = 0; i < sizes.Count; ++i)
                     {
                         var sizeU = sizes[i];
                         realizedSpan += double.IsNaN(sizeU) ? estimatedSize : sizeU;
                     }
-            
+
                     return realized.StartU + realizedSpan + ((index - last - 1) * estimatedSize);
                 }
             }
@@ -940,7 +943,7 @@ private void RealizeElements(
             var viewportEnd = horizontal ? _viewport.Right : _viewport.Bottom;
             var anchorAtEnd = !_hasReachedEnd && index == items.Count - 1 &&
                 MathUtilities.GreaterThanOrClose(viewportEnd, horizontal ? Bounds.Width : Bounds.Height);
-                    
+
             // Reset boundary flags
             _hasReachedStart = false;
             _hasReachedEnd = false;
@@ -956,9 +959,9 @@ private void RealizeElements(
                 _realizingIndex = index;
                 var e = GetOrCreateElement(items, index);
                 _realizingElement = e;
-                
+
                 e.Measure(availableSize);
-                
+
                 var sizeU = horizontal ? e.DesiredSize.Width : e.DesiredSize.Height;
                 var sizeV = horizontal ? e.DesiredSize.Height : e.DesiredSize.Width;
 
@@ -976,10 +979,10 @@ private void RealizeElements(
                 _realizingIndex = -1;
                 _realizingElement = null;
             } while (u < viewport.viewportUEnd && index < items.Count);
-            
+
             // Check if we reached the end of the collection
             _hasReachedEnd = index >= items.Count;
-            
+
             // Store the last index and end U position for the desired size calculation.
             viewport.lastIndex = index - 1;
             viewport.realizedEndU = u;
@@ -994,7 +997,7 @@ private void RealizeElements(
             while (u > viewport.viewportUStart && index >= 0)
             {
                 var e = GetOrCreateElement(items, index);
-                
+
                 e.Measure(availableSize);
                 var sizeU = horizontal ? e.DesiredSize.Width : e.DesiredSize.Height;
                 var sizeV = horizontal ? e.DesiredSize.Height : e.DesiredSize.Width;
@@ -1004,7 +1007,7 @@ private void RealizeElements(
                 viewport.measuredV = Math.Max(viewport.measuredV, sizeV);
                 --index;
             }
-            
+
             // Check if we reached the start of the collection
             _hasReachedStart = index < 0;
 
@@ -1039,7 +1042,7 @@ private Control GetOrCreateElement(IReadOnlyList<object?> items, int index)
         {
             return _realizedElements?.GetElement(index);
         }
-        
+
         private static Control? GetRealizedElement(
             int index,
             ref int specialIndex,
@@ -1118,7 +1121,7 @@ private void RecycleElement(Control element, int index)
         {
             Debug.Assert(ItemsControl is not null);
             Debug.Assert(ItemContainerGenerator is not null);
-            
+
             _scrollAnchorProvider?.UnregisterAnchorCandidate(element);
 
             var recycleKey = element.GetValue(RecycleKeyProperty);
@@ -1153,7 +1156,7 @@ private void RecycleElementOnItemRemoved(Control element)
             _scrollAnchorProvider?.UnregisterAnchorCandidate(element);
 
             var recycleKey = element.GetValue(RecycleKeyProperty);
-            
+
             if (recycleKey is null)
             {
                 ItemContainerGenerator!.ClearItemContainer(element);
@@ -1181,7 +1184,7 @@ private void RecycleFocusedElement()
             _focusedElement = null;
             _focusedIndex = -1;
         }
-        
+
         private void RecycleScrollToElement()
         {
             if (_scrollToElement != null)
@@ -1191,7 +1194,7 @@ private void RecycleScrollToElement()
             _scrollToElement = null;
             _scrollToIndex = -1;
         }
-        
+
         private void PushToRecyclePool(object recycleKey, Control element)
         {
             _recyclePool ??= new();
@@ -1211,7 +1214,7 @@ private void UpdateElementIndex(Control element, int oldIndex, int newIndex)
 
             ItemContainerGenerator.ItemContainerIndexChanged(element, oldIndex, newIndex);
         }
-        
+
         private Rect CalculateExtendedViewport(bool vertical, double viewportSize, double bufferSize)
         {
 
@@ -1378,7 +1381,7 @@ private void OnEffectiveViewportChanged(object? sender, EffectiveViewportChanged
         private void OnItemsControlPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if (_focusedElement is not null &&
-                e.Property == KeyboardNavigation.TabOnceActiveElementProperty && 
+                e.Property == KeyboardNavigation.TabOnceActiveElementProperty &&
                 e.GetOldValue<IInputElement?>() == _focusedElement)
             {
                 // TabOnceActiveElement has moved away from _focusedElement so we can recycle it.
@@ -1392,15 +1395,15 @@ private void OnCacheLengthChanged(AvaloniaPropertyChangedEventArgs e)
         {
             var newValue = e.GetNewValue<double>();
             _bufferFactor = newValue;
-    
+
             // Force a recalculation of the extended viewport on the next layout pass
             InvalidateMeasure();
         }
-        
+
         /// <inheritdoc/>
         public IReadOnlyList<double> GetIrregularSnapPoints(Orientation orientation, SnapPointsAlignment snapPointsAlignment)
         {
-            if(_realizedElements == null)
+            if (_realizedElements == null)
                 return new List<double>();
 
             return new VirtualizingSnapPointsList(_realizedElements, ItemsControl?.ItemsSource?.Count() ?? 0, orientation, Orientation, snapPointsAlignment, EstimateElementSizeU());

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
@@ -1683,15 +1683,15 @@ public void Focused_Container_Is_Positioned_Correctly_when_Container_Size_Change
         }
 
         [Fact]
-        public void Focused_Container_Is_Positioned_Correctly_When_Scrolled_Past_Items_With_Different_Heights()
+        public void Focused_Container_Is_Positioned_Outside_Viewport_When_Scrolled_Past_Items_With_Different_Heights()
         {
             using var app = App();
 
             var items = Enumerable.Range(0, 20)
                 .Select(x => new ItemWithHeight(x, x < 10 ? 10 : 50))
                 .ToList();
 
-            var (target, _, _) = CreateTarget(items: items, itemTemplate: CanvasWithHeightTemplate);
+            var (target, scroll, _) = CreateTarget(items: items, itemTemplate: CanvasWithHeightTemplate);
 
             var focused = Assert.IsType<ContentPresenter>(target.ContainerFromIndex(5));
             focused.Focusable = true;
@@ -1702,18 +1702,19 @@ public void Focused_Container_Is_Positioned_Correctly_When_Scrolled_Past_Items_W
 
             Assert.True(target.FirstRealizedIndex > 5);
 
-            var firstIndex = target.FirstRealizedIndex;
-            var firstRealized = Assert.IsType<ContentPresenter>(target.ContainerFromIndex(firstIndex));
-            var realized = target.GetRealizedElements()
-                .Where(x => x is not null)
-                .Cast<Control>()
-                .ToList();
+            var firstRealized = Assert.IsType<ContentPresenter>(
+                target.ContainerFromIndex(target.FirstRealizedIndex));
+            focused = Assert.IsType<ContentPresenter>(target.ContainerFromIndex(5));
 
-            var estimatedSize = realized.Average(x => x.DesiredSize.Height);
-            var expectedTop = firstRealized.Bounds.Top - ((firstIndex - 5) * estimatedSize);
+            // The focused container's position is estimated, as it's outside the realized range.
+            // The estimate must never place it before the panel origin...
+            Assert.True(focused.Bounds.Top >= 0);
 
-            focused = Assert.IsType<ContentPresenter>(target.ContainerFromIndex(5));
-            Assert.Equal(expectedTop, focused.Bounds.Top, 3);
+            // ...must keep it above the realized range rather than overlapping it...
+            Assert.True(focused.Bounds.Bottom <= firstRealized.Bounds.Top);
+
+            // ...and must keep it out of the viewport, so it can't appear as a ghost item.
+            Assert.True(focused.Bounds.Bottom <= scroll.Offset.Y);
         }
 
         [Theory]
@@ -1765,6 +1766,40 @@ public void Focused_Container_Is_Positioned_Correctly_when_Container_Size_Change
             Assert.Equal(new Rect(0, 140, 100, 20), container.Bounds);
         }
 
+        [Theory]
+        [InlineData(25, Orientation.Vertical)]
+        [InlineData(99, Orientation.Vertical)]
+        [InlineData(25, Orientation.Horizontal)]
+        [InlineData(99, Orientation.Horizontal)]
+        public void ScrollIntoView_With_Variable_Size_Items_Keeps_Target_In_Viewport(int targetIndex, Orientation orientation)
+        {
+            using var app = App();
+
+            var firstHalfSize = targetIndex < 60 ? 20 : 40;
+            var secondHalfSize = targetIndex < 60 ? 40 : 20;
+            var horizontal = orientation == Orientation.Horizontal;
+            IEnumerable<object> items = horizontal ?
+                Enumerable.Range(0, 100).Select(x => new ItemWithWidth(x, x < 50 ? firstHalfSize : secondHalfSize)) :
+                Enumerable.Range(0, 100).Select(x => new ItemWithHeight(x, x < 50 ? firstHalfSize : secondHalfSize));
+            Optional<IDataTemplate?> itemTemplate = horizontal ? CanvasWithWidthTemplate : CanvasWithHeightTemplate;
+            var (target, scroll, _) = CreateTarget(items: items, itemTemplate: itemTemplate, orientation: orientation);
+
+            target.ScrollIntoView(60);
+            target.ScrollIntoView(targetIndex);
+
+            var container = Assert.IsType<ContentPresenter>(target.ContainerFromIndex(targetIndex));
+            var message = $"Bounds={container.Bounds}, Offset={scroll.Offset}, Viewport={scroll.Viewport}, Extent={scroll.Extent}";
+
+            var containerStart = horizontal ? container.Bounds.Left : container.Bounds.Top;
+            var containerEnd = horizontal ? container.Bounds.Right : container.Bounds.Bottom;
+            var viewportStart = horizontal ? scroll.Offset.X : scroll.Offset.Y;
+            var viewportEnd = viewportStart + (horizontal ? scroll.Viewport.Width : scroll.Viewport.Height);
+
+            Assert.True(containerStart > 0, message);
+            Assert.True(containerStart >= viewportStart, message);
+            Assert.True(containerEnd <= viewportEnd, message);
+        }
+
         [Fact]
         public void When_Vertical_Calculates_ViewPort_At_Start_Of_List()
         {