fix(itemsRepeater): Fix regerssion that would prevent nested IR to re…

…nder items after the first page if scrolling slowly

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

@@ -115,6 +115,73 @@ public async Task When_NestedInSVAndOutOfViewportOnInitialLoad_Then_Materialized
 
 			sut.Children.Count.Should().BeGreaterThan(1);
 		}
+
+		[TestMethod]
+		[RunsOnUIThread]
+#if __MACOS__
+		[Ignore("Currently fails on macOS, part of #9282 epic")]
+#endif
+		public async Task When_NestedIRSlowlyChangeViewport_Then_MaterializedNeededItems()
+		{
+			const int viewportHeight = 500;
+
+			var sut = default(ItemsRepeater);
+			var sv = new ScrollViewer
+			{
+				Height = viewportHeight,
+				Content = (sut = new ItemsRepeater()
+				{
+					ItemsSource = Enumerable.Range(0, 10).Select(i => $"Group #{i:D2}"),
+					ItemTemplate = new DataTemplate(() => new StackPanel
+					{
+						Children =
+						{
+							new Border
+							{
+								Background = new SolidColorBrush(Colors.DeepPink),
+								Height = 100,
+								Width = 150,
+								Child = new TextBlock().Apply(tb => tb.SetBinding(TextBlock.TextProperty, new Binding()))
+							},
+							new ItemsRepeater
+							{
+								ItemsSource = Enumerable.Range(0, 50).Select(i => $"Item #{i:D2}"),
+								ItemTemplate = new DataTemplate(() => new Border
+								{
+									Width = 150,
+									Height = 100,
+									Background = new SolidColorBrush(Colors.DeepSkyBlue),
+									Child = new TextBlock().Apply(tb => tb.SetBinding(TextBlock.TextProperty, new Binding()))
+								})
+							}
+						}
+					})
+				})
+			};
+
+			TestServices.WindowHelper.WindowContent = sv;
+			await TestServices.WindowHelper.WaitForIdle();
+
+			sv.ChangeView(null, sv.ExtentHeight/2, null, disableAnimation: true);
+			await TestServices.WindowHelper.WaitForIdle();
+
+			var groupView = sut.Children.Single(g => g.DataContext as string == "Group #05");
+			var groupIr = (ItemsRepeater)((StackPanel)groupView).Children[1];
+
+			var beforeVisibleItems = groupIr.Children.Select(i => i.DataContext?.ToString()).Order().ToArray();
+
+			// Scroll by baby step to not be above the threshold which would cause a complete redraw
+			const int step = 10;
+			for (var i = 0; i < viewportHeight * 5; i += step)
+			{
+				sv.ChangeView(null, sv.VerticalOffset + step, null, disableAnimation: true);
+				await TestServices.WindowHelper.WaitForIdle();
+			}
+
+			var afterVisibleItems = groupIr.Children.Select(i => i.DataContext?.ToString()).Order().ToArray();
+
+			afterVisibleItems.Should().NotContain(beforeVisibleItems);
+		}
 #endif
 
 

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

@@ -89,6 +89,7 @@ protected internal override Size MeasureOverride(VirtualizingLayoutContext conte
 			// Uno workaround [BEGIN]: Keep track of realized items count for viewport invalidation optimization
 			_uno_lastKnownItemsCount = context.ItemCount;
 			_uno_lastKnownRealizedElementsCount = algo.RealizedElementCount;
+			_uno_lastKnownDesiredSize = desiredSize;
 			// Uno workaround [END]
 
 			return desiredSize;
@@ -360,6 +361,7 @@ double GetAverageElementSize(
 		private double _uno_lastKnownAverageElementSize;
 		private int _uno_lastKnownRealizedElementsCount;
 		private int _uno_lastKnownItemsCount;
+		private Size _uno_lastKnownDesiredSize;
 
 		/// <inheritdoc />
 		protected internal override bool IsSignificantViewportChange(Rect oldViewport, Rect newViewport)
@@ -378,18 +380,13 @@ protected internal override bool IsSignificantViewportChange(Rect oldViewport, R
 				// Note2: Depending of the platform (Android), we might be invoked with empty viewport, make sure to consider out-of-bound in such case.
 				// Test case: When_NestedInSVAndOutOfViewportOnInitialLoad_Then_MaterializedEvenWhenScrollingOnMinorAxis
 				const int threshold = 100; // Allows 100px above and after to trigger loading even before IR is visible.
-				var wasOutOfBounds = oldViewport is { Width: 0 } or { Height: 0 }
-					|| MajorEnd(oldViewport) < -threshold
-					|| MajorStart(oldViewport) > MajorSize(oldViewport) + threshold
-					|| MinorEnd(oldViewport) < -threshold
-					|| MinorStart(oldViewport) > MinorSize(oldViewport) + threshold;
 				var isOutOfBounds = newViewport is { Width: 0 } or { Height: 0 }
-					|| MinorEnd(newViewport) < -threshold
-					|| MinorStart(newViewport) > MinorSize(newViewport) + threshold
 					|| MajorEnd(newViewport) < -threshold
-					|| MajorStart(newViewport) > MajorSize(newViewport) + threshold;
+					|| MajorStart(newViewport) > Major(_uno_lastKnownDesiredSize) + threshold
+					|| MinorEnd(newViewport) < -threshold
+					|| MinorStart(newViewport) > Minor(_uno_lastKnownDesiredSize) + threshold;
 
-				return wasOutOfBounds && !isOutOfBounds;
+				return !isOutOfBounds;
 			}
 
 			var size = Math.Max(MajorSize(oldViewport), MajorSize(newViewport));