Merge pull request #10285 from AvaloniaUI/fixes/virtualization-fixes

List virtualization fixes

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -427,6 +427,7 @@ namespace Avalonia.Controls.Presenters
 
             Viewport = finalSize;
             Extent = Child!.Bounds.Size.Inflate(Child.Margin);
+            Offset = ScrollViewer.CoerceOffset(Extent, finalSize, Offset);
             _isAnchorElementDirty = true;
 
             return finalSize;

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -403,7 +403,7 @@ namespace Avalonia.Controls
             if (firstIndex == -1)
             {
                 estimatedElementSize = EstimateElementSizeU();
-                firstIndex = (int)(viewportStart / estimatedElementSize);
+                firstIndex = Math.Min((int)(viewportStart / estimatedElementSize), maxIndex);
                 firstIndexU = firstIndex * estimatedElementSize;
             }
 
@@ -411,13 +411,13 @@ namespace Avalonia.Controls
             {
                 if (estimatedElementSize == -1)
                     estimatedElementSize = EstimateElementSizeU();
-                lastIndex = (int)(viewportEnd / estimatedElementSize);
+                lastIndex = Math.Min((int)(viewportEnd / estimatedElementSize), maxIndex);
             }
 
             return new MeasureViewport
             {
-                firstIndex = MathUtilities.Clamp(firstIndex, 0, maxIndex),
+                firstIndex = firstIndex,
-                lastIndex = MathUtilities.Clamp(lastIndex, 0, maxIndex),
+                lastIndex = lastIndex,
                 viewportUStart = viewportStart,
                 viewportUEnd = viewportEnd,
                 startU = firstIndexU,
@@ -1131,6 +1131,7 @@ namespace Avalonia.Controls
                     // The removed range was before the realized elements. Update the first index and
                     // the indexes of the realized elements.
                     _firstIndex -= count;
+                    _startUUnstable = true;
 
                     var newIndex = _firstIndex;
                     for (var i = 0; i < _elements.Count; ++i)

tests/Avalonia.Controls.UnitTests/ScrollViewerTests.cs

@@ -237,6 +237,40 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(1, raised);
         }
 
+        [Fact]
+        public void Reducing_Extent_Should_Constrain_Offset()
+        {
+            var target = new ScrollViewer
+            {
+                Template = new FuncControlTemplate<ScrollViewer>(CreateTemplate),
+            };
+            var root = new TestRoot(target);
+            var raised = 0;
+
+            target.SetValue(ScrollViewer.ExtentProperty, new Size(100, 100));
+            target.SetValue(ScrollViewer.ViewportProperty, new Size(50, 50));
+            target.Offset = new Vector(50, 50);
+
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            target.ScrollChanged += (s, e) =>
+            {
+                Assert.Equal(new Vector(-30, -30), e.ExtentDelta);
+                Assert.Equal(new Vector(-30, -30), e.OffsetDelta);
+                Assert.Equal(default, e.ViewportDelta);
+                ++raised;
+            };
+
+            target.SetValue(ScrollViewer.ExtentProperty, new Size(70, 70));
+
+            Assert.Equal(0, raised);
+
+            root.LayoutManager.ExecuteLayoutPass();
+
+            Assert.Equal(1, raised);
+            Assert.Equal(new Vector(20, 20), target.Offset); 
+        }
+
         private Control CreateTemplate(ScrollViewer control, INameScope scope)
         {
             return new Grid

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Collections.Specialized;
 using System.Linq;
+using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
@@ -278,6 +279,82 @@ namespace Avalonia.Controls.UnitTests
             Assert.Same(focused, target.GetRealizedElements().First());
         }
 
+        [Fact]
+        public void Removing_Range_When_Scrolled_To_End_Updates_Viewport()
+        {
+            using var app = App();
+            var items = new AvaloniaList<string>(Enumerable.Range(0, 100).Select(x => $"Item {x}"));
+            var (target, scroll, itemsControl) = CreateTarget(items: items);
+
+            scroll.Offset = new Vector(0, 900);
+            Layout(target);
+
+            AssertRealizedItems(target, itemsControl, 90, 10);
+
+            items.RemoveRange(0, 80);
+            Layout(target);
+
+            AssertRealizedItems(target, itemsControl, 10, 10);
+            Assert.Equal(new Vector(0, 100), scroll.Offset);
+        }
+
+        [Fact]
+        public void Removing_Range_To_Have_Less_Than_A_Page_Of_Items_When_Scrolled_To_End_Updates_Viewport()
+        {
+            using var app = App();
+            var items = new AvaloniaList<string>(Enumerable.Range(0, 100).Select(x => $"Item {x}"));
+            var (target, scroll, itemsControl) = CreateTarget(items: items);
+
+            scroll.Offset = new Vector(0, 900);
+            Layout(target);
+
+            AssertRealizedItems(target, itemsControl, 90, 10);
+
+            items.RemoveRange(0, 95);
+            Layout(target);
+
+            AssertRealizedItems(target, itemsControl, 0, 5);
+            Assert.Equal(new Vector(0, 0), scroll.Offset);
+        }
+
+        [Fact]
+        public void Resetting_Collection_To_Have_Less_Items_When_Scrolled_To_End_Updates_Viewport()
+        {
+            using var app = App();
+            var items = new ResettingCollection(Enumerable.Range(0, 100).Select(x => $"Item {x}"));
+            var (target, scroll, itemsControl) = CreateTarget(items: items);
+
+            scroll.Offset = new Vector(0, 900);
+            Layout(target);
+
+            AssertRealizedItems(target, itemsControl, 90, 10);
+
+            items.Reset(Enumerable.Range(0, 20).Select(x => $"Item {x}"));
+            Layout(target);
+
+            AssertRealizedItems(target, itemsControl, 10, 10);
+            Assert.Equal(new Vector(0, 100), scroll.Offset);
+        }
+
+        [Fact]
+        public void Resetting_Collection_To_Have_Less_Than_A_Page_Of_Items_When_Scrolled_To_End_Updates_Viewport()
+        {
+            using var app = App();
+            var items = new ResettingCollection(Enumerable.Range(0, 100).Select(x => $"Item {x}"));
+            var (target, scroll, itemsControl) = CreateTarget(items: items);
+
+            scroll.Offset = new Vector(0, 900);
+            Layout(target);
+
+            AssertRealizedItems(target, itemsControl, 90, 10);
+
+            items.Reset(Enumerable.Range(0, 5).Select(x => $"Item {x}"));
+            Layout(target);
+
+            AssertRealizedItems(target, itemsControl, 0, 5);
+            Assert.Equal(new Vector(0, 0), scroll.Offset);
+        }
+
         private static IReadOnlyList<int> GetRealizedIndexes(VirtualizingStackPanel target, ItemsControl itemsControl)
         {
             return target.GetRealizedElements()
@@ -378,5 +455,24 @@ namespace Avalonia.Controls.UnitTests
         }
 
         private static IDisposable App() => UnitTestApplication.Start(TestServices.RealFocus);
+
+        private class ResettingCollection : List<string>, INotifyCollectionChanged
+        {
+            public ResettingCollection(IEnumerable<string> items)
+            {
+                AddRange(items);
+            }
+
+            public void Reset(IEnumerable<string> items)
+            {
+                Clear();
+                AddRange(items);
+                CollectionChanged?.Invoke(
+                    this,
+                    new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
+            }
+
+            public event NotifyCollectionChangedEventHandler? CollectionChanged;
+        }
     }
 }