Arrange content based on arrangement bounds rather than viewport bounds

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

@@ -83,5 +83,71 @@ await scrollViewHandler.PlatformView.AttachAndRun(async () =>
 				});
 			});
 		}
+
+		internal class TestStackLayout : VerticalStackLayout 
+		{
+			public Rect LastArrangeBounds { get; set; }
+
+			protected override Size ArrangeOverride(Rect bounds)
+			{
+				LastArrangeBounds = bounds;
+				return base.ArrangeOverride(bounds);
+			}
+		}
+
+		[Fact]
+		public async Task ContentChangeDoesNotResetScrollPosition()
+		{
+			var topLabel = new Label
+			{
+				WidthRequest = 100,
+				HeightRequest = 5000,
+				Text = "Hello",
+				BackgroundColor = Colors.LightBlue
+			};
+
+			var bottomLabel = new Label { Text = "Howdy" };
+
+			var layout = new TestStackLayout
+			{
+				topLabel,
+				bottomLabel
+			};
+
+			var scroll = new ScrollView
+			{
+				Content = layout,
+				HeightRequest = 400
+			};
+
+			var topLabelHandler = await CreateHandlerAsync<LabelHandler>(topLabel);
+			var bottomLabelHandler = await CreateHandlerAsync<LabelHandler>(bottomLabel);
+			var layoutHandler = await CreateHandlerAsync<LayoutHandler>(layout);
+			var scrollHandler = await CreateHandlerAsync<ScrollViewHandler>(scroll);
+
+			await AttachAndRun(scroll, async (handler) =>
+			{
+				var platformView = scrollHandler.PlatformView;
+
+				// Scroll down by 5000
+				scrollHandler.VirtualView.RequestScrollTo(0, 5000, true);
+
+				// Give it time to update
+				await Task.Delay(100);
+
+				// Verify that the content layout didn't pick up any incorrect offsets
+				// The arrangement for the actual _content_ should always start at Y=0 because
+				// of the ContentView shim. If we ever stop using the ContentView shim for 
+				// the iOS ScrollView implementation, this test will likely become invalid.
+				Assert.Equal(0, layout.LastArrangeBounds.Top);
+
+				// Change the text of the bottom label; this _should_ have no effect on scrolling
+				bottomLabel.Text = "Changed";
+				await Task.Delay(100);
+
+				// The content should still be arranged at Y=0
+				Assert.Equal(0, layout.LastArrangeBounds.Top);
+			});
+		}
 	}
 }

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

@@ -98,9 +98,9 @@ public static void MapOrientation(IScrollViewHandler handler, IScrollView scroll
 
 			var fullContentSize = scrollView.PresentedContent?.DesiredSize ?? Size.Zero;
 
-			var viewportBounds = GetViewportBounds(uiScrollView);
-			var viewportWidth = viewportBounds.Width;
-			var viewportHeight = viewportBounds.Height;
+			var viewportSize = GetViewportSize(uiScrollView);
+			var viewportWidth = viewportSize.Width;
+			var viewportHeight = viewportSize.Height;
 
 			SetContentSizeForOrientation(uiScrollView, viewportWidth, viewportHeight, scrollView.Orientation, fullContentSize);
 		}
@@ -123,7 +123,7 @@ public static void MapRequestScrollTo(IScrollViewHandler handler, IScrollView sc
 				var availableScrollWidth = uiScrollView.ContentSize.Width - uiScrollView.Frame.Width;
 				var minScrollHorizontal = Math.Min(request.HorizontalOffset, availableScrollWidth);
 				var minScrollVertical = Math.Min(request.VerticalOffset, availableScrollHeight);
-				uiScrollView.SetContentOffset(new CGPoint(minScrollHorizontal, minScrollVertical), !request.Instant);
+				uiScrollView.SetContentOffset(new CoreGraphics.CGPoint(minScrollHorizontal, minScrollVertical), !request.Instant);
 
 				if (request.Instant)
 				{
@@ -181,15 +181,16 @@ static void InsertContentView(UIScrollView platformScrollView, IScrollView scrol
 				return;
 			}
 
-			var contentContainer = new ContentView
+			var contentContainer = new ContentView()
 			{
 				View = scrollView.PresentedContent,
-				Tag = ContentPanelTag,
-				// This is where we normally would inject the cross-platform ScrollView's layout logic; instead, we're injecting the
-				// methods from this handler so it can make some adjustments for things like Padding before the default logic is invoked
-				CrossPlatformLayout = crossPlatformLayout
+				Tag = ContentPanelTag
 			};
 
+			// This is where we normally would inject the cross-platform ScrollView's layout logic; instead, we're injecting the
+			// methods from this handler so it can make some adjustments for things like Padding before the default logic is invoked
+			contentContainer.CrossPlatformLayout = crossPlatformLayout;
+
 			platformScrollView.ClearSubviews();
 			contentContainer.AddSubview(platformContent);
 			platformScrollView.AddSubview(contentContainer);
@@ -284,9 +285,9 @@ static void SetContentSizeForOrientation(UIScrollView uiScrollView, double viewp
 			uiScrollView.ContentSize = contentSize;
 		}
 
-		static CGRect GetViewportBounds(UIScrollView platformScrollView)
+		static CGSize GetViewportSize(UIScrollView platformScrollView)
 		{
-			return platformScrollView.AdjustedContentInset.InsetRect(platformScrollView.Bounds);
+			return platformScrollView.AdjustedContentInset.InsetRect(platformScrollView.Bounds).Size;
 		}
 
 		Size ICrossPlatformLayout.CrossPlatformMeasure(double widthConstraint, double heightConstraint)
@@ -301,18 +302,18 @@ Size ICrossPlatformLayout.CrossPlatformMeasure(double widthConstraint, double he
 				return Size.Zero;
 			}
 
-			var viewPortBounds = GetViewportBounds(platformScrollView);
+			var viewportSize = GetViewportSize(platformScrollView);
 
 			var padding = scrollView.Padding;
 
 			if (widthConstraint == 0)
 			{
-				widthConstraint = viewPortBounds.Width;
+				widthConstraint = viewportSize.Width;
 			}
 
 			if (heightConstraint == 0)
 			{
-				heightConstraint = viewPortBounds.Height;
+				heightConstraint = viewportSize.Height;
 			}
 
 			// Account for the ScrollView Padding before measuring the content
@@ -333,26 +334,29 @@ Size ICrossPlatformLayout.CrossPlatformArrange(Rect bounds)
 
 			// The UIScrollView's bounds are available, so we can use them to make sure the ContentSize makes sense
 			// for the ScrollView orientation
-			var viewportBounds = GetViewportBounds(platformScrollView);
+			var viewportSize = GetViewportSize(platformScrollView);
+
+			// Get a Rect for doing the CrossPlatformArrange of the Content
+			var viewportRect = new Rect(Graphics.Point.Zero, viewportSize.ToSize());
 
-			var contentSize = crossPlatformLayout.CrossPlatformArrange(viewportBounds.ToRectangle());
+			var contentSize = crossPlatformLayout.CrossPlatformArrange(viewportRect);
 
-			var viewportHeight = viewportBounds.Height;
-			var viewportWidth = viewportBounds.Width;
+			var viewportHeight = viewportSize.Height;
+			var viewportWidth = viewportSize.Width;
 			SetContentSizeForOrientation(platformScrollView, viewportWidth, viewportHeight, scrollView.Orientation, contentSize);
 
 			var container = GetContentView(platformScrollView);
 
-			if (container != null)
+			if (container?.Superview is UIScrollView uiScrollView)
 			{
 				// Ensure the container is at least the size of the UIScrollView itself, so that the 
 				// cross-platform layout logic makes sense and the contents don't arrange outside the 
 				// container. (Everything will look correct if they do, but hit testing won't work properly.)
 				var containerBounds = contentSize;
 
 				container.Bounds = new CGRect(0, 0,
-					Math.Max(containerBounds.Width, viewportBounds.Width),
-					Math.Max(containerBounds.Height, viewportBounds.Height));
+					Math.Max(containerBounds.Width, viewportSize.Width),
+					Math.Max(containerBounds.Height, viewportSize.Height));
 
 				container.Center = new CGPoint(container.Bounds.GetMidX(), container.Bounds.GetMidY());
 			}