Ensure zero-based arrangement rectangle

Fixes #18513

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

@@ -35,5 +35,118 @@ public async Task ScrollViewContentSizeSet()
 				});
 			});
 		}
+
+		[Fact]
+		public async Task ContentSizeExpandsToViewport()
+		{
+			EnsureHandlerCreated(builder => { builder.ConfigureMauiHandlers(handlers => { handlers.AddHandler<Entry, EntryHandler>(); }); });
+
+			var scrollView = new ScrollView();
+
+			var entry = new Entry() { Text = "In a ScrollView", HeightRequest = 10 };
+
+
+			static CoreGraphics.CGSize getViewportSize(UIScrollView scrollView)
+			{
+				return scrollView.AdjustedContentInset.InsetRect(scrollView.Bounds).Size;
+			};
+
+			var scrollViewHandler = await InvokeOnMainThreadAsync(() =>
+			{
+				return CreateHandlerAsync<ScrollViewHandler>(scrollView);
+			});
+
+			await InvokeOnMainThreadAsync(async () =>
+			{
+				await scrollViewHandler.PlatformView.AttachAndRun(async () =>
+				{
+					var uiScrollView = scrollViewHandler.PlatformView;
+
+					uiScrollView.ContentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentBehavior.Always;
+					var parent = uiScrollView.Superview;
+					uiScrollView.Bounds = parent.Bounds;
+					uiScrollView.Center = parent.Center;
+
+					scrollView.Content = entry;
+
+					parent.SetNeedsLayout();
+					parent.LayoutIfNeeded();
+
+					await Task.Yield();
+
+					var contentSize = uiScrollView.ContentSize;
+					var viewportSize = getViewportSize(uiScrollView);
+
+					Assert.Equal(viewportSize.Height, contentSize.Height);
+					Assert.Equal(viewportSize.Width, contentSize.Width);
+				});
+			});
+		}
+
+		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

@@ -103,9 +103,11 @@ public static void MapOrientation(IScrollViewHandler handler, IScrollView scroll
 			// without having to re-layout the ScrollView
 
 			var fullContentSize = scrollView.PresentedContent?.DesiredSize ?? Size.Zero;
-			var viewportBounds = uiScrollView.Bounds;
-			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);
 		}
 
@@ -289,6 +291,11 @@ static void SetContentSizeForOrientation(UIScrollView uiScrollView, double viewp
 			uiScrollView.ContentSize = contentSize;
 		}
 
+		static CGSize GetViewportSize(UIScrollView platformScrollView)
+		{
+			return platformScrollView.AdjustedContentInset.InsetRect(platformScrollView.Bounds).Size;
+		}
+
 		Size ICrossPlatformLayout.CrossPlatformMeasure(double widthConstraint, double heightConstraint)
 		{
 			var scrollView = VirtualView;
@@ -301,17 +308,18 @@ Size ICrossPlatformLayout.CrossPlatformMeasure(double widthConstraint, double he
 				return Size.Zero;
 			}
 
-			var scrollViewBounds = platformScrollView.Bounds;
+			var viewportSize = GetViewportSize(platformScrollView);
+
 			var padding = scrollView.Padding;
 
 			if (widthConstraint == 0)
 			{
-				widthConstraint = scrollViewBounds.Width;
+				widthConstraint = viewportSize.Width;
 			}
 
 			if (heightConstraint == 0)
 			{
-				heightConstraint = scrollViewBounds.Height;
+				heightConstraint = viewportSize.Height;
 			}
 
 			// Account for the ScrollView Padding before measuring the content
@@ -334,9 +342,15 @@ 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 = platformScrollView.Bounds;
-			var viewportHeight = viewportBounds.Height;
-			var viewportWidth = viewportBounds.Width;
+			var viewportSize = GetViewportSize(platformScrollView);
+
+			// Get a Rect for doing the CrossPlatformArrange of the Content
+			var viewportRect = new Rect(Point.Zero, viewportSize.ToSize());
+
+			var contentSize = crossPlatformLayout.CrossPlatformArrange(viewportRect);
+
+			var viewportHeight = viewportSize.Height;
+			var viewportWidth = viewportSize.Width;
 			SetContentSizeForOrientation(platformScrollView, viewportWidth, viewportHeight, scrollView.Orientation, contentSize);
 
 			var container = GetContentView(platformScrollView);
@@ -351,8 +365,8 @@ Size ICrossPlatformLayout.CrossPlatformArrange(Rect bounds)
 				var containerBounds = contentSize;
 
 				container.Bounds = new CGRect(0, 0,
-					Math.Max(containerBounds.Width, scrollViewBounds.Width),
-					Math.Max(containerBounds.Height, scrollViewBounds.Height));
+					Math.Max(containerBounds.Width, viewportSize.Width),
+					Math.Max(containerBounds.Height, viewportSize.Height));
 
 				container.Center = new CGPoint(container.Bounds.GetMidX(), container.Bounds.GetMidY());
 			}