fix(itemsRepeater): Fix an issue where IR might re-position first ite…

…ms if items below are significantly higher

src/SamplesApp/UITests.Shared/ItemExclusions.props

@@ -17,10 +17,6 @@
 		<Compile Remove="$(MSBuildThisFileDirectory)Microsoft_UI_Xaml_Controls\PagerControlTests\**" />
 		<None Include="$(MSBuildThisFileDirectory)Microsoft_UI_Xaml_Controls\PagerControlTests\**" />
 
-		<Page Remove="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\Repeater\**" />
-		<Compile Remove="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\Repeater\**" />
-		<None Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\Repeater\**" />
-
 		<Page Remove="$(MSBuildThisFileDirectory)Lottie\**" />
 		<Compile Remove="$(MSBuildThisFileDirectory)Lottie\**" />
 		<None Include="$(MSBuildThisFileDirectory)Lottie\**" />

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Repeater/Given_ItemsRepeater.cs

@@ -430,13 +430,82 @@ public async Task When_ClearItemsWhileUnloaded_Then_MaterializeItems()
 			sut.Materialized.Should().Be(0);
 		}
 
-		private record SUT(Border Root, ScrollViewer Scroller, ItemsRepeater Repeater, ObservableCollection<string> Source)
+
+		[TestMethod]
+		[RunsOnUIThread]
+#if __MACOS__
+		[Ignore("Currently fails on macOS, part of #9282 epic")]
+#endif
+		public async Task When_ItemSignificantlyHigher_Then_VirtualizeProperly()
 		{
-			public static SUT Create(int itemsCount = 3, Size? viewport = default)
+			var sut = SUT.Create(
+				new ObservableCollection<MyItem>
+				{
+					new (0, 200, Colors.FromARGB("#FF0000")),
+					new (1, 400, Colors.FromARGB("#FF8000")),
+					new (2, 200, Colors.FromARGB("#FFFF00")),
+					new (3, 5000, Colors.FromARGB("#008000")),
+					new (4, 100, Colors.FromARGB("#0000FF")),
+					new (5, 100, Colors.FromARGB("#A000C0"))
+				},
+				new DataTemplate(() => new Border
+				{
+					Width = 100,
+					Margin = new Thickness(10),
+					Child = new TextBlock().Apply(tb => tb.SetBinding(TextBlock.TextProperty, new Binding()))
+				}
+				.Apply(b => b.SetBinding(FrameworkElement.HeightProperty, new Binding{Path = nameof(MyItem.Height)}))
+				.Apply(b => b.SetBinding(FrameworkElement.BackgroundProperty, new Binding { Path = nameof(MyItem.Color) }))),
+				new Size(100, 500)
+			);
+
+			await sut.Load();
+			sut.Scroller.ViewChanged += (s, e) => Console.WriteLine($"Vertical: {sut.Scroller.VerticalOffset}");
+
+			var originalEstimatedExtent = sut.Scroller.ExtentHeight;
+
+			sut.Scroller.ChangeView(null, 800, null, disableAnimation: true); // First scroll enough to get item #3 to be materialized
+			await TestServices.WindowHelper.WaitForIdle();
+
+			sut.MaterializedItems.Should().Contain(sut.Source[3]); // Confirm that item has been materialized!
+			sut.Scroller.ExtentHeight.Should().BeGreaterThan(originalEstimatedExtent); // Confirm that the extent has increased due to item #3
+
+			var item3OriginalVerticalOffset = sut.Repeater.Children.First(elt => ReferenceEquals(elt.DataContext, sut.Source[3])).ActualOffset.Y;
+
+			sut.Scroller.ChangeView(null, 1500, null, disableAnimation: true); // Then scroll enough for first items to be DE-materialized
+			await TestServices.WindowHelper.WaitForIdle();
+
+			sut.MaterializedItems.Should().NotContain(sut.Source[0]); // Confirm that first items has been removed!
+			sut.MaterializedItems.Should().NotContain(sut.Source[1]);
+
+			var item3UpdatedVerticalOffset = sut.Repeater.Children.First(elt => ReferenceEquals(elt.DataContext, sut.Source[3])).ActualOffset.Y;
+
+			item3UpdatedVerticalOffset.Should().Be(item3OriginalVerticalOffset); // Confirm that item #3 has not been moved down
+			var result = await UITestHelper.ScreenShot(sut.Repeater);
+			ImageAssert.HasColorAt(result, 10, 10, Colors.FromARGB("#008000")); // For safety also check it's effectively the item 3 that is visible
+		}
+
+		private record MyItem(int Id, double Height, Color Color)
+		{
+			public string Title => $"Item {Id}";
+		}
+
+#nullable enable
+		private static class SUT
+		{
+			public static SUT<T> Create<T>(ObservableCollection<T> source, DataTemplate? itemTemplate = null, Size? viewport = default)
 			{
+				itemTemplate ??= new DataTemplate(() => new Border
+				{
+					Width = 100,
+					Height = 100,
+					Background = new SolidColorBrush(Colors.DeepSkyBlue),
+					Margin = new Thickness(10),
+					Child = new TextBlock().Apply(tb => tb.SetBinding(TextBlock.TextProperty, new Binding()))
+				});
+
 				var repeater = default(ItemsRepeater);
 				var scroller = default(ScrollViewer);
-				var source = new ObservableCollection<string>(Enumerable.Range(0, itemsCount).Select(i => $"Item #{i}"));
 				var root = new Border
 				{
 					BorderThickness = new Thickness(5),
@@ -447,14 +516,7 @@ public static SUT Create(int itemsCount = 3, Size? viewport = default)
 						{
 							ItemsSource = source,
 							Layout = new StackLayout(),
-							ItemTemplate = new DataTemplate(() => new Border
-							{
-								Width = 100,
-								Height = 100,
-								Background = new SolidColorBrush(Colors.DeepSkyBlue),
-								Margin = new Thickness(10),
-								Child = new TextBlock().Apply(tb => tb.SetBinding(TextBlock.TextProperty, new Binding()))
-							})
+							ItemTemplate = itemTemplate
 						})
 					})
 				};
@@ -468,9 +530,15 @@ public static SUT Create(int itemsCount = 3, Size? viewport = default)
 				return new(root, scroller, repeater, source);
 			}
 
+			public static SUT<string> Create(int itemsCount = 3, Size? viewport = default)
+				=> Create(new ObservableCollection<string>(Enumerable.Range(0, itemsCount).Select(i => $"Item #{i}")), viewport: viewport);
+		}
+
+		private record SUT<T>(Border Root, ScrollViewer Scroller, ItemsRepeater Repeater, ObservableCollection<T> Source)
+		{
 			public int Materialized => Repeater.Children.Count(elt => elt.ActualOffset.X >= 0);
 
-			public IEnumerable<string> MaterializedItems => Repeater.Children.Where(elt => elt.ActualOffset.X >= 0).Select(elt => elt.DataContext?.ToString());
+			public IEnumerable<T> MaterializedItems => Repeater.Children.Where(elt => elt.ActualOffset.X >= 0).Select(elt => (T)elt.DataContext);
 
 			public async ValueTask Load()
 			{

src/Uno.UI/Microsoft/UI/Xaml/Controls/Repeater/StackLayout.cs

@@ -176,7 +176,11 @@ Rect GetExtent(
 				if (firstRealized != null)
 				{
 					MUX_ASSERT(lastRealized != null);
-					SetMajorStart(ref extent, (float)(MajorStart(firstRealizedLayoutBounds) - firstRealizedItemIndex * averageElementSize));
+					var firstRealizedMajor = (float)(MajorStart(firstRealizedLayoutBounds) - firstRealizedItemIndex * averageElementSize);
+					// Uno workaround [BEGIN]: Make sure to not move items above the viewport. This can be the case if an items is significantly higher than previous items (will increase the average items size)
+					firstRealizedMajor = Math.Max(0.0f, firstRealizedMajor);
+					// Uno workaround [END]
+					SetMajorStart(ref extent, firstRealizedMajor);
 					var remainingItems = itemsCount - lastRealizedItemIndex - 1;
 					SetMajorSize(ref extent, MajorEnd(lastRealizedLayoutBounds) - MajorStart(extent) + (float)(remainingItems * averageElementSize));
 				}