ScrollView content needs layout when ScrollView.InvalidateMeasure is …

…called (#17639)

### Description of Change

When `ScrollView.Content` changes its alignment (like
`HorizontalOptions` change from `Start` to `End`), then the
`ScrollView`'s layout needs to properly update. It worked on Android and
Mac/iOS, but Windows would not update layout until the window was
resized.

After `HorizontalOptions` changed on the content, Windows would call
`InvalidateMeasure` on the WinUI `ScrollView`, but the child content
didn't get updated. This change makes sure that the content also gets
measured.

This fix is helpful for XAML Hot Reload, so the layout will update as
`HorizontalOptions` changes.

### Issues Fixed

Fixes #14377

src/Controls/tests/DeviceTests/Elements/ScrollView/ScrollViewTests.cs

@@ -60,6 +60,38 @@ await AttachAndRun(scroll, async (handler) =>
 			});
 		}
 
+		[Fact]
+		public async Task TestContentHorizontalOptionsChanged()
+		{
+			var label = new Label
+			{
+				BackgroundColor = Colors.LightBlue,
+				HorizontalOptions = LayoutOptions.Start,
+				Text = "Hello",
+				WidthRequest = 50,
+			};
+
+			var scrollView = new ScrollView
+			{
+				BackgroundColor = Colors.DarkBlue,
+				Content = label,
+				WidthRequest = 300,
+				HeightRequest = 200,
+			};
+
+			SetupBuilder();
+
+			await AttachAndRun(scrollView, async (handler) =>
+			{
+				// Without this delay, the UI didn't render and the bug didn't repro
+				await Task.Delay(100);
+
+				await WaitAssert(() => CloseEnough(scrollView.Content.Frame.Left, 0.0));
+				scrollView.Content.HorizontalOptions = LayoutOptions.End;
+				await WaitAssert(() => CloseEnough(scrollView.Content.Frame.Right, 300.0));
+			});
+		}
+
 		[Theory]
 		[InlineData(ScrollOrientation.Vertical, 100, 300, 0, 100)]
 		[InlineData(ScrollOrientation.Horizontal, 0, 100, 100, 300)]
@@ -188,22 +220,17 @@ static async Task AssertContentSizeChanged(Task<bool> changed)
 
 		static async Task AssertContentSize(Func<Size> actual, Size expected)
 		{
-			await WaitAssert(() => CloseEnough(actual(), expected, 0.2), timeout: 5000, message: $"ContentSize was {actual()}, expected {expected}");
+			await WaitAssert(() => CloseEnough(actual(), expected), timeout: 5000, message: $"ContentSize was {actual()}, expected {expected}");
 		}
 
-		static bool CloseEnough(Size a, Size b, double tolerance)
+		static bool CloseEnough(double a, double b, double tolerance = 0.2)
 		{
-			if (System.Math.Abs(a.Width - b.Width) > tolerance)
-			{
-				return false;
-			}
-
-			if (System.Math.Abs(a.Height - b.Height) > tolerance)
-			{
-				return false;
-			}
+			return System.Math.Abs(a - b) <= tolerance;
+		}
 
-			return true;
+		static bool CloseEnough(Size a, Size b, double tolerance = 0.2)
+		{
+			return CloseEnough(a.Width, b.Width, tolerance) && CloseEnough(a.Height, b.Height, tolerance);
 		}
 
 		static Task<bool> WatchContentSizeChanged(ScrollView scrollView)

src/Core/src/Handlers/ScrollView/ScrollViewHandler.Windows.cs

@@ -20,6 +20,16 @@ protected override ScrollViewer CreatePlatformView()
 			return new ScrollViewer();
 		}
 
+		internal static void MapInvalidateMeasure(IScrollViewHandler handler, IView view, object? args)
+		{
+			handler.PlatformView.InvalidateMeasure(view);
+
+			if (handler.PlatformView.Content is FrameworkElement content)
+			{
+				content.InvalidateMeasure();
+			}
+		}
+
 		protected override void ConnectHandler(ScrollViewer platformView)
 		{
 			base.ConnectHandler(platformView);

src/Core/src/Handlers/ScrollView/ScrollViewHandler.cs

@@ -29,7 +29,10 @@ public partial class ScrollViewHandler : IScrollViewHandler
 
 		public static CommandMapper<IScrollView, IScrollViewHandler> CommandMapper = new(ViewCommandMapper)
 		{
-			[nameof(IScrollView.RequestScrollTo)] = MapRequestScrollTo
+			[nameof(IScrollView.RequestScrollTo)] = MapRequestScrollTo,
+#if WINDOWS
+			[nameof(IView.InvalidateMeasure)] = MapInvalidateMeasure,
+#endif
 		};
 
 		public ScrollViewHandler() : base(Mapper, CommandMapper)