Listview.ScrollIntoView for non-virtualizing panels

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Controls/ListView/ListView_BringIntoView.xaml

@@ -5,6 +5,7 @@
     xmlns:local="using:SamplesApp.Windows_UI_Xaml_Controls.ListView"
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+	xmlns:muxc="using:Microsoft.UI.Xaml.Controls"
     mc:Ignorable="d"
     d:DesignHeight="300"
     d:DesignWidth="400">
@@ -16,12 +17,18 @@
 			</StackPanel>
 		</DataTemplate>
 	</UserControl.Resources>
-
-    <Grid>
-		<ListView x:Name="myList"
-				  ItemsSource="{Binding [SampleItemsMed]}"
-				  ItemTemplate="{StaticResource ListItemTemplate}"/>
-		<Button Content="Bring into view"
-				Click="BringIntoView" />		
-    </Grid>
+
+	<StackPanel>
+		<StackPanel Orientation="Horizontal">
+			<muxc:NumberBox x:Name="nb" Minimum="0" SmallChange="1" />
+			<StackPanel>
+				<TextBlock Text="Leading" />
+				<CheckBox x:Name="chkBox" />
+			</StackPanel>
+			<Button Content="Bring into view" Click="BringIntoView" />
+		</StackPanel>
+		<Grid Height="400">
+			<ListView x:Name="myList" ItemTemplate="{StaticResource ListItemTemplate}" />
+	    </Grid>
+	</StackPanel>
 </UserControl>

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Controls/ListView/ListView_BringIntoView.xaml.cs

@@ -1,4 +1,6 @@
-using Uno.UI.Samples.Controls;
+using System;
+using System.Linq;
+using Uno.UI.Samples.Controls;
 using Microsoft.UI.Xaml.Controls;
 using SamplesApp.Windows_UI_Xaml_Controls.Models;
 using Microsoft.UI.Xaml;
@@ -14,13 +16,18 @@ public sealed partial class ListView_BringIntoView : UserControl
 		public ListView_BringIntoView()
 		{
 			this.InitializeComponent();
+
+			var random = new Random(42);
+			// using newlines to vary the height of each item
+			myList.ItemsSource = Enumerable.Range(0, 100).Select(i => $"item {i}" + new string('\n', random.Next(0, 5))).ToArray();
 		}
 
 		public void BringIntoView(object sender, RoutedEventArgs e)
 		{
 			var list = (Microsoft.UI.Xaml.Controls.ListViewBase)this.myList;
 
-			list.ScrollIntoView(list.SelectedItem, ScrollIntoViewAlignment.Leading);
+			var alignment = (chkBox.IsChecked ?? false) ? ScrollIntoViewAlignment.Leading : ScrollIntoViewAlignment.Default;
+			list.ScrollIntoView(list.ItemFromIndex((int)nb.Value), alignment);
 		}
 	}
 }

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_ListViewBase.cs

@@ -4483,6 +4483,82 @@ void SetSelectedItem(object item)
 				else { throw new NotImplementedException(); }
 			}
 		}
+
+		[TestMethod]
+		[RunsOnUIThread]
+#if __MACOS__
+		[Ignore("NotImplemented ListViewBase.ScrollIntoView")]
+#elif __ANDROID__ || __IOS__
+		[Ignore("This test is for managed ListViewBase.")]
+#endif
+		public async Task When_ScrollIntoView_No_Virtualization()
+		{
+			var source = Enumerable.Range(0, 100).ToArray();
+			var lv = new ListView { Width = 400, Height = 200, ItemsSource = source };
+
+			await UITestHelper.Load(lv);
+
+			lv.ScrollIntoView(source[50]);
+			await WindowHelper.WaitForIdle();
+
+			var sv = lv.FindFirstDescendant<ScrollViewer>();
+			var container = (ContentControl)lv.ContainerFromIndex(50);
+
+			var offset = container.TransformToVisual(lv).TransformPoint(default);
+			var (offsetStart, vpExtent) = (offset.Y, sv.ViewportHeight);
+
+			Assert.IsTrue(0 <= offsetStart && offsetStart + container.ActualHeight <= vpExtent, $"Container#50 should be within viewport: 0 <= {offsetStart} <= {vpExtent}");
+		}
+
+#if HAS_UNO
+		[TestMethod]
+		[RunsOnUIThread]
+#if __MACOS__
+		[Ignore("NotImplemented ListViewBase.ScrollIntoView")]
+#else
+		[Ignore("https://github.com/unoplatform/uno/issues/16246")]
+#endif
+		public async Task When_ScrollIntoView_Containers_With_Varying_Heights()
+		{
+			var random = new Random(42);
+			var lv = new ListView
+			{
+				Height = 400,
+				ItemsSource = Enumerable.Range(0, 100).Select(i => $"item {i}" + new string('\n', random.Next(0, 5))).ToArray(),
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var tb = new TextBlock();
+					tb.SetBinding(TextBlock.TextProperty, new Binding());
+
+					return new StackPanel
+					{
+						Padding = new Thickness(16),
+						Children =
+						{
+							tb
+						}
+					};
+				})
+			};
+
+			await UITestHelper.Load(lv);
+
+			var sv = lv.FindFirstDescendant<ScrollViewer>();
+			var i = 7049.5;
+			while (i > 0)
+			{
+				sv.ScrollToVerticalOffset(i);
+				await WindowHelper.WaitForIdle();
+				i -= 400;
+				// try i -= 800, it will also fail the test even though visually, item 0 is visible, but the list view is interally corrupted
+			}
+
+			sv.ScrollToVerticalOffset(i);
+			await WindowHelper.WaitForIdle();
+
+			Assert.IsNotNull(lv.ContainerFromItem(0));
+		}
+#endif
 	}
 
 	public partial class Given_ListViewBase // data class, data-context, view-model, template-selector

src/Uno.UI/UI/Xaml/Controls/ListViewBase/ListViewBase.managed.cs

@@ -87,13 +87,54 @@ private void TryLoadMoreItems()
 
 		public void ScrollIntoView(object item, ScrollIntoViewAlignment alignment)
 		{
-			if (VirtualizingPanel?.GetLayouter() is { } layouter)
+			if (ContainerFromItem(item) is UIElement element)
+			{
+				// The container we want to jump to is already materialized, so just jump to it.
+				// This means we're in a non-virtualizing panel or in a virtualizing panel where the container we want is materialized for some reason (e.g. partially in view)
+				ScrollIntoViewFastPath(element, alignment);
+			}
+			else if (VirtualizingPanel?.GetLayouter() is { } layouter)
 			{
 				layouter.ScrollIntoView(item, alignment);
 			}
-			else if (this.Log().IsEnabled(LogLevel.Warning))
+		}
+
+		private void ScrollIntoViewFastPath(UIElement element, ScrollIntoViewAlignment alignment)
+		{
+			if (ScrollViewer is { } sv && sv.Presenter is { } presenter)
 			{
-				this.Log().LogWarning($"{nameof(ScrollIntoView)} not supported when using non-virtualizing panels.");
+				var offsetXY = element.TransformToVisual(presenter).TransformPoint(Point.Zero);
+
+				var (newOffset, elementLength, presenterOffset, presenterViewportLength) =
+					ItemsPanelRoot.PhysicalOrientation is Orientation.Vertical
+						? (offsetXY.Y, element.ActualSize.Y, presenter.VerticalOffset, presenter.ViewportHeight)
+						: (offsetXY.X, element.ActualSize.X, presenter.HorizontalOffset, presenter.ViewportWidth);
+
+				if (presenterOffset < newOffset && newOffset + elementLength < presenterOffset + presenterViewportLength)
+				{
+					// if the element is within the visible viewport, do nothing.
+					return;
+				}
+
+				// If we use the above offset directly, the item we want to jump to will be the start of the viewport, i.e. leading
+				if (alignment is ScrollIntoViewAlignment.Default)
+				{
+					if (presenterOffset < newOffset)
+					{
+						// scroll one "viewport page" less: this brings the element's start right after the viewport's length ends
+						// we then scroll again by elementLength so that the end of the element is the end of the viewport
+						newOffset += (-presenterViewportLength) + elementLength;
+					}
+				}
+
+				if (ItemsPanelRoot.PhysicalOrientation is Orientation.Vertical)
+				{
+					sv.ScrollToVerticalOffset(newOffset);
+				}
+				else
+				{
+					sv.ScrollToHorizontalOffset(newOffset);
+				}
 			}
 		}
 #endif