fix(ComboBox): updated selection not scrolled into view

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

@@ -1410,6 +1410,61 @@ public async Task When_ComboBox_Popup_Dismissed()
 			Assert.IsTrue(dropDownClosedFired, "DropDownClosed event was not fired");
 		}
 
+		[TestMethod]
+		public Task When_ComboBox_ScrollIntoView_SelectedItem() => When_ComboBox_ScrollIntoView_Selection(viaIndex: false);
+
+		[TestMethod]
+		public Task When_ComboBox_ScrollIntoView_SelectedIndex() => When_ComboBox_ScrollIntoView_Selection(viaIndex: true);
+
+		private async Task When_ComboBox_ScrollIntoView_Selection(bool viaIndex)
+		{
+			var source = ( // 12x20=240 items, enough to overflow
+				from a in "qwerasdfzxcv"
+				from b in Enumerable.Range(0, 20)
+				select string.Concat(a, b)
+			).ToArray();
+			var sut = new ComboBox
+			{
+				ItemsSource = source,
+			};
+			await UITestHelper.Load(sut, x => x.IsLoaded);
+
+			var popup = sut.FindFirstDescendantOrThrow<Popup>("Popup");
+
+			// open the popup
+			sut.IsDropDownOpen = true;
+			await UITestHelper.WaitFor(() => popup.IsOpen, timeoutMS: 2000, "timed out waiting on the popup to open");
+
+			// wait until the dropdown panel is ready
+			await UITestHelper.WaitFor(() => sut.ItemsPanelRoot is { }, timeoutMS: 2000, "timed out waiting on the ItemsPanelRoot");
+
+			// set selection
+			if (viaIndex)
+			{
+				sut.SelectedIndex = source.Length - 3;
+			}
+			else
+			{
+				sut.SelectedItem = source[^3];
+			}
+			await UITestHelper.WaitForIdle();
+
+			// sanity check
+			Assert.AreEqual(source.Length - 3, sut.SelectedIndex, "SelectedIndex should be the 3rd last");
+			Assert.AreEqual(source[^3], sut.SelectedItem, "SelectedItem should be the 3rd last");
+
+			var sv = sut.ItemsPanelRoot.FindFirstAncestorOrThrow<ScrollViewer>();
+			var cbi = sut.ContainerFromIndex(sut.SelectedIndex) as FrameworkElement;
+			Assert.IsNotNull(cbi, "Selected container should not be null");
+
+			var cbiAbsRect = new Rect(cbi.ActualOffset.X, cbi.ActualOffset.Y, cbi.ActualWidth, cbi.ActualHeight);
+			var viewportAbsRect = new Rect(sv.HorizontalOffset, sv.VerticalOffset, sv.ViewportWidth, sv.ViewportHeight);
+			var intersection = viewportAbsRect;
+			intersection.Intersect(cbiAbsRect);
+
+			Assert.IsTrue(cbiAbsRect == intersection, $"Selected container should be fully within viewport: CBI={PrettyPrint.FormatRect(cbiAbsRect)}, VP={PrettyPrint.FormatRect(viewportAbsRect)}");
+		}
+
 		public sealed class TwoWayBindingClearViewModel : IDisposable
 		{
 			public enum Themes

src/Uno.UI/Extensions/ViewExtensions.visual-tree.cs

@@ -233,6 +233,19 @@ internal static bool TryGetDpValue<T>(object owner, string property, [NotNullWhe
 		.OfType<T>()
 		.FirstOrDefault();
 
+	/// <summary>
+	/// Returns the first ancestor of a specified type.
+	/// </summary>
+	/// <typeparam name="T">The type of ancestor to find.</typeparam>
+	/// <param name="reference">Any node of the visual tree</param>
+	/// <remarks>First Counting from the <paramref name="reference"/> and not from the root of tree.</remarks>
+	/// <exception cref="Exception">If the specified node could not be found.</exception>
+	public static T? FindFirstAncestorOrThrow<T>(this _View? reference) => EnumerateAncestors(reference)
+		.OfType<T>()
+		.FirstOrDefault() ??
+		throw new Exception($"Unable to find element: {typeof(T).Name}");
+
+
 	/// <summary>
 	/// Returns the first ancestor of a specified type that satisfies the <paramref name="predicate"/>.
 	/// </summary>

src/Uno.UI/UI/Xaml/Controls/ComboBox/ComboBox.custom.cs

@@ -485,6 +485,74 @@ internal Brush LightDismissOverlayBackground
 		set { SetValue(LightDismissOverlayBackgroundProperty, value); }
 	}
 
+	internal void ScrollIntoView(object item, ScrollIntoViewAlignment alignment = ScrollIntoViewAlignment.Default)
+	{
+		if (ItemsPanelRoot is null)
+		{
+			// Not ready.
+			return;
+		}
+
+		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)
+		{
+#if __APPLE_UIKIT__ || __ANDROID__
+			// TODO
+#else
+			layouter.ScrollIntoView(item, alignment);
+#endif
+		}
+	}
+
+	private void ScrollIntoViewFastPath(UIElement element, ScrollIntoViewAlignment alignment)
+	{
+		if (ScrollViewer is { } sv && sv.Presenter is { } presenter)
+		{
+			var offsetXY = element.TransformToVisual(presenter).TransformPoint(
+#if __SKIA__ // Skia correctly doesn't include the offsets in TransformToVisual
+				new Point(presenter.HorizontalOffset, presenter.VerticalOffset)
+#else
+				Point.Zero
+#endif
+				);
+
+			var orientation = ItemsPanelRoot?.PhysicalOrientation ?? Orientation.Vertical;
+
+			var (elementOffset, elementLength, presenterOffset, presenterViewportLength) =
+				orientation is Orientation.Vertical
+					? (offsetXY.Y, element.ActualSize.Y, presenter.VerticalOffset, presenter.ViewportHeight)
+					: (offsetXY.X, element.ActualSize.X, presenter.HorizontalOffset, presenter.ViewportWidth);
+
+			if (presenterOffset <= elementOffset && elementOffset + 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.
+			// For the default alignment, we move the element to either of the viewport ends (i.e. to the top or the bottom of the
+			// viewport. To move to the bottom, we 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.
+			var newOffset = alignment is ScrollIntoViewAlignment.Default && presenterOffset < elementOffset
+				? elementOffset - presenterViewportLength + elementLength
+				: elementOffset;
+
+			if (orientation is Orientation.Vertical)
+			{
+				sv.ScrollToVerticalOffset(newOffset);
+			}
+			else
+			{
+				sv.ScrollToHorizontalOffset(newOffset);
+			}
+		}
+	}
+
 	internal static DependencyProperty LightDismissOverlayBackgroundProperty { get; } =
 		DependencyProperty.Register("LightDismissOverlayBackground", typeof(Brush), typeof(ComboBox), new FrameworkPropertyMetadata(null));
 

src/Uno.UI/UI/Xaml/Controls/Primitives/Selector.cs

@@ -170,10 +170,9 @@ internal virtual void OnSelectedItemChanged(object oldSelectedItem, object selec
 				isSelectionUnset = true;
 			}
 
-			var newIndex = -1;
 			if (!_changingSelectedIndex)
 			{
-				newIndex = IndexFromItem(selectedItem);
+				var newIndex = IndexFromItem(selectedItem);
 				if (SelectedIndex != newIndex)
 				{
 					SelectedIndex = newIndex;
@@ -191,22 +190,9 @@ internal virtual void OnSelectedItemChanged(object oldSelectedItem, object selec
 			}
 
 #if !IS_UNIT_TESTS
-			if (newIndex != -1 && IsInLiveTree)
+			if (SelectedIndex != -1 && IsInLiveTree)
 			{
-				if (this is ListViewBase lvb
-#if __APPLE_UIKIT__
-					// workaround to prevent scrolling when it is not ready
-					// without this, the ios TabView could render blank if the selection happens too early.
-					&& ContainerFromIndex(newIndex) is FrameworkElement { IsLoaded: true }
-#endif
-				)
-				{
-#if __APPLE_UIKIT__ || __ANDROID__
-					lvb.InstantScrollToIndex(newIndex);
-#else
-					lvb.ScrollIntoView(selectedItem);
-#endif
-				}
+				ScrollSelectionIntoView();
 			}
 #endif
 
@@ -222,6 +208,34 @@ internal virtual void OnSelectedItemChanged(object oldSelectedItem, object selec
 			}
 		}
 
+		private void ScrollSelectionIntoView()
+		{
+			if (SelectedIndex == -1 || !IsInLiveTree) return;
+
+			if (this is ListViewBase lvb)
+			{
+#if __APPLE_UIKIT__
+				// workaround to prevent scrolling when it is not ready
+				// without this, the ios TabView could render blank if the selection happens too early.
+				if (ContainerFromIndex(SelectedIndex) is not FrameworkElement { IsLoaded: true }) return;
+#endif
+
+#if __APPLE_UIKIT__ || __ANDROID__
+				lvb.InstantScrollToIndex(SelectedIndex);
+#else
+				lvb.ScrollIntoView(SelectedItem);
+#endif
+			}
+			else if (this is ComboBox cb)
+			{
+#if __APPLE_UIKIT__ || __ANDROID__
+				// TODO
+#else
+				cb.ScrollIntoView(SelectedItem);
+#endif
+			}
+		}
+
 		internal void TryUpdateSelectorItemIsSelected(object item, bool isSelected)
 		{
 			if (item is SelectorItem si)