Fix ListBox crash when scrolling after item removal (#21838)

* Add regression test for ListBox removal scroll crash

* Fix virtualized arrange after collection changes

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -251,6 +251,13 @@ namespace Avalonia.Controls
                 var orientation = Orientation;
                 var u = _realizedElements!.StartU;
 
+                // Collection changes before the realized range make its exact position unstable
+                // until the next measure. ScrollIntoView can intentionally defer that measure
+                // while waiting for an updated viewport, so use the same position estimate used
+                // when realizing an element instead of arranging the range at NaN.
+                if (double.IsNaN(u))
+                    u = GetOrEstimateElementU(_realizedElements.FirstIndex);
+
                 for (var i = 0; i < _realizedElements.Count; ++i)
                 {
                     var e = _realizedElements.Elements[i];

tests/Avalonia.Controls.UnitTests/ListBoxVirtualizationIssueTests.cs

@@ -11,6 +11,31 @@ namespace Avalonia.Controls.UnitTests;
 
 public class ListBoxVirtualizationIssueTests : ScopedTestBase
 {
+    [Fact]
+    public void Removing_First_Item_After_Scrolling_To_End_Should_Allow_Scrolling_To_Start()
+    {
+        using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+        {
+            var items = new ObservableCollection<int>(Enumerable.Range(0, 100));
+            var target = new ListBox
+            {
+                Template = new FuncControlTemplate(CreateListBoxTemplate),
+                ItemsSource = items,
+                ItemTemplate = new FuncDataTemplate<int>((_, _) => new TextBlock { Height = 50 }),
+                ItemsPanel = new FuncTemplate<Panel?>(() => new VirtualizingStackPanel()),
+            };
+
+            Prepare(target);
+            target.ScrollIntoView(99);
+
+            items.RemoveAt(0);
+            target.ScrollIntoView(0);
+
+            var firstContainer = Assert.IsType<ListBoxItem>(target.ContainerFromIndex(0));
+            Assert.Equal(1, firstContainer.Content);
+        }
+    }
+
     [Fact]
     public void Replaced_ItemsSource_Should_Not_Show_Old_Selected_Item_When_Scrolled_Back()
     {