Merge pull request #976 from AvaloniaUI/fixes/886-incorrect-virtualized-items

Fix incorrect virtualized items

src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs

@@ -155,8 +155,7 @@ namespace Avalonia.Controls.Presenters
                     case NotifyCollectionChangedAction.Add:
                         CreateAndRemoveContainers();
 
-                        if (e.NewStartingIndex >= FirstIndex &&
-                            e.NewStartingIndex < NextIndex)
+                        if (e.NewStartingIndex < NextIndex)
                         {
                             RecycleContainers();
                         }

tests/Avalonia.Controls.UnitTests/Presenters/ItemsPresenterTests_Virtualization_Simple.cs

@@ -206,6 +206,30 @@ namespace Avalonia.Controls.UnitTests.Presenters
             Assert.Equal(expected, actual);
         }
 
+        [Fact]
+        public void Inserting_Items_Before_Visibile_Containers_Should_Update_Containers()
+        {
+            var target = CreateTarget();
+
+            target.ApplyTemplate();
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            ((ILogicalScrollable)target).Offset = new Vector(0, 5);
+
+            var expected = Enumerable.Range(5, 10).Select(x => $"Item {x}").ToList();
+            var items = (ObservableCollection<string>)target.Items;
+            var actual = target.Panel.Children.Select(x => x.DataContext).ToList();
+
+            Assert.Equal(expected, actual);
+
+            items.Insert(0, "Inserted");
+
+            expected = Enumerable.Range(4, 10).Select(x => $"Item {x}").ToList();
+            actual = target.Panel.Children.Select(x => x.DataContext).ToList();
+            Assert.Equal(expected, actual);
+        }
+
         [Fact]
         public void Removing_First_Materialized_Item_Should_Update_Containers()
         {
@@ -477,7 +501,6 @@ namespace Avalonia.Controls.UnitTests.Presenters
             target.Arrange(new Rect(0, 0, 100, 100));
 
             var expected = Enumerable.Range(0, 6).Select(x => $"Item {x}").ToList();
-            var items = (ObservableCollection<string>)target.Items;
             var actual = target.Panel.Children.Select(x => x.DataContext).ToList();
 
             Assert.Equal(expected, actual);