ScrollView content needs layout when ScrollView.InvalidateMeasure is called

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

@@ -60,6 +60,38 @@ public async Task TestContentSizeChangedVertical(ScrollOrientation orientation)
 			});
 		}
 
+		[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)