fix(reg): Patch regresssion unoplatform#16176

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

@@ -454,7 +454,10 @@ public async Task When_ItemSignificantlyHigher_Then_VirtualizeProperly()
 				{
 					Width = 100,
 					Margin = new Thickness(10),
-					Child = new TextBlock().Apply(tb => tb.SetBinding(TextBlock.TextProperty, new Binding()))
+					Child = new ItemsControl
+					{
+						ItemTemplate = new DataTemplate(() => new TextBlock().Apply(tb => tb.SetBinding(TextBlock.TextProperty, new Binding())))
+					}.Apply(tb => tb.SetBinding(ItemsControl.ItemsSourceProperty, new Binding { Path = nameof(MyItem.Lines) }))
 				}
 				.Apply(b => b.SetBinding(FrameworkElement.HeightProperty, new Binding { Path = nameof(MyItem.Height) }))
 				.Apply(b => b.SetBinding(FrameworkElement.BackgroundProperty, new Binding { Path = nameof(MyItem.Color) }))),
@@ -480,16 +483,53 @@ public async Task When_ItemSignificantlyHigher_Then_VirtualizeProperly()
 			sut.MaterializedItems.Should().NotContain(sut.Source[0]); // Confirm that first items has been removed!
 			sut.MaterializedItems.Should().NotContain(sut.Source[1]);
 
+			sut.Scroller.ChangeView(null, 2940, null, disableAnimation: true); // Then scroll enough for first items to be DE-materialized
+			await TestServices.WindowHelper.WaitForIdle();
+
 			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
 		}
 
+		[TestMethod]
+		[RunsOnUIThread]
+#if __MACOS__
+		[Ignore("Currently fails on macOS, part of #9282 epic")]
+#elif !__SKIA__
+		[Ignore("Fails due to async native scrolling.")]
+#endif
+		public async Task When_UnloadReload_Then_MaterializeItemsForCurrentViewport()
+		{
+			var sut = SUT.Create(30, new Size(100, 500));
+
+			await sut.Load();
+
+			// Only few first items should have been materialized so far
+			sut.MaterializedItems.Should().Contain(sut.Source[0]);
+			sut.MaterializedItems.Should().Contain(sut.Source[5]);
+			sut.MaterializedItems.Should().NotContain(sut.Source[10]);
+			sut.MaterializedItems.Should().NotContain(sut.Source[15]);
+
+			await sut.Unload();
+			await sut.Load();
+
+			// Confirm that first items has been re-materialized
+			sut.MaterializedItems.Should().Contain(sut.Source[0]);
+			sut.MaterializedItems.Should().Contain(sut.Source[5]);
+			sut.MaterializedItems.Should().NotContain(sut.Source[10]);
+			sut.MaterializedItems.Should().NotContain(sut.Source[15]);
+
+			// Item 0 should be at offset 0
+			sut.MaterializedElements.OrderBy(e => e.DataContext).First().LayoutSlot.Y.Should().Be(0, "Item #0 should be at the origin of the IR (negative offset means we are in trouble!)");
+		}
+
 		private record MyItem(int Id, double Height, Color Color)
 		{
 			public string Title => $"Item {Id}";
+
+			public string[] Lines { get; } = Enumerable.Range(0, (int)(Height / 10)).Select(i => $"Line {i:D3}").ToArray();
 		}
 
 #nullable enable
@@ -540,7 +580,9 @@ private record SUT<T>(Border Root, ScrollViewer Scroller, ItemsRepeater Repeater
 		{
 			public int Materialized => Repeater.Children.Count(elt => elt.ActualOffset.X >= 0);
 
-			public IEnumerable<T> MaterializedItems => Repeater.Children.Where(elt => elt.ActualOffset.X >= 0).Select(elt => (T)elt.DataContext);
+			public IEnumerable<T> MaterializedItems => MaterializedElements.Select(elt => (T)elt.DataContext);
+
+			public IEnumerable<UIElement> MaterializedElements => Repeater.Children.Where(elt => elt.ActualOffset.X >= 0);
 
 			public async ValueTask Load()
 			{

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

@@ -144,8 +144,12 @@ FlowLayoutAnchorInfo GetAnchorForRealizationRect(Size availableSize, Virtualizin
 					realizationWindowOffsetInExtent + MajorSize(realizationRect) >= 0 && realizationWindowOffsetInExtent <= majorSize)
 				{
 					anchorIndex = (int)(realizationWindowOffsetInExtent / averageElementSize);
-					offset = anchorIndex * averageElementSize + MajorStart(lastExtent);
+					// Uno workaround [BEGIN]: Make sure items at index 0 is always at offset 0
 					anchorIndex = Math.Max(0, Math.Min(itemsCount - 1, anchorIndex));
+					// Uno workaround [END]
+
+					offset = anchorIndex * averageElementSize + MajorStart(lastExtent);
+					//anchorIndex = Math.Max(0, Math.Min(itemsCount - 1, anchorIndex)); // Line moved before computation of the offset for uno
 				}
 			}