Added tests to detect scroll jumps.

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -130,6 +130,16 @@ namespace Avalonia.Controls
             set { SetValue(AreVerticalSnapPointsRegularProperty, value); }
         }
 
+        /// <summary>
+        /// Gets the index of the first realized element, or -1 if no elements are realized.
+        /// </summary>
+        public int FirstRealizedIndex => _realizedElements?.FirstIndex ?? -1;
+
+        /// <summary>
+        /// Gets the index of the last realized element, or -1 if no elements are realized.
+        /// </summary>
+        public int LastRealizedIndex => _realizedElements?.LastIndex ?? -1;
+
         protected override Size MeasureOverride(Size availableSize)
         {
             if (!IsEffectivelyVisible)

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -37,7 +37,7 @@ namespace Avalonia.Controls.UnitTests
         {
             using var app = App();
             var items = Enumerable.Range(0, 100).Select(x => new Button { Width = 25, Height = 10});
-            var (target, scroll, itemsControl) = CreateTarget(items: items, useItemTemplate: false);
+            var (target, scroll, itemsControl) = CreateTarget(items: items, itemTemplate: null);
 
             Assert.Equal(1000, scroll.Extent.Height);
 
@@ -252,7 +252,7 @@ namespace Avalonia.Controls.UnitTests
         {
             using var app = App();
             var items = new ObservableCollection<Button>(Enumerable.Range(0, 100).Select(x => new Button { Width = 25, Height = 10 }));
-            var (target, scroll, itemsControl) = CreateTarget(items: items, useItemTemplate: false);
+            var (target, scroll, itemsControl) = CreateTarget(items: items, itemTemplate: null);
 
             Assert.Equal(1000, scroll.Extent.Height);
 
@@ -472,6 +472,122 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(10, raised);
         }
 
+        [Fact]
+        public void Scrolling_Down_With_Larger_Element_Does_Not_Cause_Jump_And_Arrives_At_End()
+        {
+            using var app = App();
+
+            var items = Enumerable.Range(0, 1000).Select(x => new ItemWithHeight(x)).ToList();
+            items[20].Height = 200;
+
+            var itemTemplate = new FuncDataTemplate<ItemWithHeight>((x, _) =>
+                new Canvas
+                {
+                    Width = 100,
+                    [!Canvas.HeightProperty] = new Binding("Height"),
+                });
+
+            var (target, scroll, itemsControl) = CreateTarget(items: items, itemTemplate: itemTemplate);
+
+            var index = target.FirstRealizedIndex;
+
+            // Scroll down to the larger element.
+            while (target.LastRealizedIndex < items.Count - 1)
+            {
+                scroll.LineDown();
+                Layout(target);
+
+                Assert.True(
+                    target.FirstRealizedIndex >= index, 
+                    $"{target.FirstRealizedIndex} is not greater or equal to {index}");
+
+                if (scroll.Offset.Y + scroll.Viewport.Height == scroll.Extent.Height)
+                    Assert.Equal(items.Count - 1, target.LastRealizedIndex);
+
+                index = target.FirstRealizedIndex;
+            }
+        }
+
+        [Fact]
+        public void Scrolling_Up_To_Larger_Element_Does_Not_Cause_Jump()
+        {
+            using var app = App();
+
+            var items = Enumerable.Range(0, 100).Select(x => new ItemWithHeight(x)).ToList();
+            items[20].Height = 200;
+
+            var itemTemplate = new FuncDataTemplate<ItemWithHeight>((x, _) =>
+                new Canvas
+                {
+                    Width = 100,
+                    [!Canvas.HeightProperty] = new Binding("Height"),
+                });
+
+            var (target, scroll, itemsControl) = CreateTarget(items: items, itemTemplate: itemTemplate);
+
+            // Scroll past the larger element.
+            scroll.Offset = new Vector(0, 600);
+            Layout(target);
+
+            // Precondition checks
+            Assert.True(target.FirstRealizedIndex > 20);
+
+            var index = target.FirstRealizedIndex;
+
+            // Scroll up to the top.
+            while (scroll.Offset.Y > 0)
+            {
+                scroll.LineUp();
+                Layout(target);
+
+                Assert.True(target.FirstRealizedIndex <= index, $"{target.FirstRealizedIndex} is not less than {index}");
+                index = target.FirstRealizedIndex;
+            }
+        }
+        
+        [Fact]
+        public void Scrolling_Up_To_Smaller_Element_Does_Not_Cause_Jump()
+        {
+            using var app = App();
+
+            var items = Enumerable.Range(0, 100).Select(x => new ItemWithHeight(x, 30)).ToList();
+            items[20].Height = 25;
+
+            var itemTemplate = new FuncDataTemplate<ItemWithHeight>((x, _) =>
+                new Canvas
+                {
+                    Width = 100,
+                    [!Canvas.HeightProperty] = new Binding("Height"),
+                });
+
+            var (target, scroll, itemsControl) = CreateTarget(items: items, itemTemplate: itemTemplate);
+
+            // Scroll past the larger element.
+            scroll.Offset = new Vector(0, 25 * items[0].Height);
+            Layout(target);
+
+            // Precondition checks
+            Assert.True(target.FirstRealizedIndex > 20);
+
+            var index = target.FirstRealizedIndex;
+
+            // Scroll up to the top.
+            while (scroll.Offset.Y > 0)
+            {
+                scroll.Offset = scroll.Offset - new Vector(0, 5);
+                Layout(target);
+
+                Assert.True(
+                    target.FirstRealizedIndex <= index, 
+                    $"{target.FirstRealizedIndex} is not less than {index}");
+                Assert.True(
+                    index - target.FirstRealizedIndex <= 1,
+                    $"FirstIndex changed from {index} to {target.FirstRealizedIndex}");
+                
+                index = target.FirstRealizedIndex;
+            }
+        }
+
         private static IReadOnlyList<int> GetRealizedIndexes(VirtualizingStackPanel target, ItemsControl itemsControl)
         {
             return target.GetRealizedElements()
@@ -516,7 +632,7 @@ namespace Avalonia.Controls.UnitTests
 
         private static (VirtualizingStackPanel, ScrollViewer, ItemsControl) CreateTarget(
             IEnumerable<object>? items = null,
-            bool useItemTemplate = true,
+            Optional<IDataTemplate?> itemTemplate = default,
             IEnumerable<Style>? styles = null)
         {
             var target = new VirtualizingStackPanel();
@@ -530,6 +646,7 @@ namespace Avalonia.Controls.UnitTests
 
             var scroll = new ScrollViewer 
             { 
+                Name = "PART_ScrollViewer",
                 Content = presenter,
                 Template = ScrollViewerTemplate(),
             };
@@ -537,13 +654,11 @@ namespace Avalonia.Controls.UnitTests
             var itemsControl = new ItemsControl
             {
                 ItemsSource = items,
-                Template = new FuncControlTemplate<ItemsControl>((_, _) => scroll),
+                Template = new FuncControlTemplate<ItemsControl>((_, ns) => scroll.RegisterInNameScope(ns)),
                 ItemsPanel = new FuncTemplate<Panel?>(() => target),
+                ItemTemplate = itemTemplate.GetValueOrDefault(DefaultItemTemplate()),
             };
 
-            if (useItemTemplate)
-                itemsControl.ItemTemplate = new FuncDataTemplate<object>((x, _) => new Canvas { Width = 100, Height = 10 });
-
             var root = new TestRoot(true, itemsControl);
             root.ClientSize = new(100, 100);
 
@@ -555,6 +670,11 @@ namespace Avalonia.Controls.UnitTests
             return (target, scroll, itemsControl);
         }
 
+        private static IDataTemplate DefaultItemTemplate()
+        {
+            return new FuncDataTemplate<object>((x, _) => new Canvas { Width = 100, Height = 10 });
+        }
+
         private static void Layout(Control target)
         {
             var root = (ILayoutRoot?)target.GetVisualRoot();
@@ -578,6 +698,18 @@ namespace Avalonia.Controls.UnitTests
 
         private static IDisposable App() => UnitTestApplication.Start(TestServices.RealFocus);
 
+        private class ItemWithHeight
+        {
+            public ItemWithHeight(int index, double height = 10)
+            {
+                Caption = $"Item {index}";
+                Height = height;
+            }
+            
+            public string Caption { get; set; }
+            public double Height { get; set; }
+        }
+
         private class ResettingCollection : List<string>, INotifyCollectionChanged
         {
             public ResettingCollection(IEnumerable<string> items)