[iOS] Fix ScrollView not expanding to full width in landscape when HorizontalOptions=FillAndExpand

src/Controls/tests/TestCases.HostApp/Issues/Issue20042.cs

@@ -0,0 +1,58 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 20042, "[iOS] ScrollView HorizontalOptions=StartAndExpand does not expand to fill screen width in landscape", PlatformAffected.iOS)]
+public class Issue20042 : ContentPage
+{
+	public Issue20042()
+	{
+		var scrollView = new ScrollView
+		{
+			Orientation = ScrollOrientation.Vertical,
+			HorizontalOptions = LayoutOptions.StartAndExpand,
+			VerticalOptions = LayoutOptions.Fill,
+		};
+
+		var innerGrid = new Grid
+		{
+			BackgroundColor = Colors.LightBlue,
+			AutomationId = "InnerGrid",
+		};
+
+		var label = new Label
+		{
+			Text = "ScrollView with HorizontalOptions=StartAndExpand\nIn landscape, this blue area must fill the full screen width.",
+			HorizontalTextAlignment = TextAlignment.Center,
+			VerticalTextAlignment = TextAlignment.Center,
+			Margin = new Thickness(16),
+		};
+
+		innerGrid.Add(label);
+		scrollView.Content = innerGrid;
+
+		// A reference element that always fills the full page width — used to compare against InnerGrid.
+		var referenceBar = new BoxView
+		{
+			Color = Colors.Red,
+			HeightRequest = 4,
+			HorizontalOptions = LayoutOptions.Fill,
+			AutomationId = "ReferenceBar",
+		};
+
+		Content = new Grid
+		{
+			RowDefinitions = new RowDefinitionCollection
+			{
+				new RowDefinition { Height = GridLength.Auto },
+				new RowDefinition { Height = GridLength.Star },
+			},
+			Children =
+			{
+				referenceBar,
+				scrollView,
+			}
+		};
+
+		Grid.SetRow(referenceBar, 0);
+		Grid.SetRow(scrollView, 1);
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue20042.cs

@@ -0,0 +1,48 @@
+#if IOS || ANDROID //SetOrientation is only supported on iOS and Android; orientation change is not available on Windows and MacCatalyst.
+
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue20042 : _IssuesUITest
+{
+	public Issue20042(TestDevice device) : base(device) { }
+
+	public override string Issue => "[iOS] ScrollView HorizontalOptions=StartAndExpand does not expand to fill screen width in landscape";
+
+	[Test]
+	[Category(UITestCategories.ScrollView)]
+	public void ScrollViewShouldFillScreenWidthInLandscape()
+	{
+		App.WaitForElement("InnerGrid");
+
+		var portraitGridRect = App.WaitForElement("InnerGrid").GetRect();
+		var portraitRefRect = App.WaitForElement("ReferenceBar").GetRect();
+
+		Assert.That(portraitGridRect.Width, Is.EqualTo(portraitRefRect.Width).Within(5),
+			$"Portrait: InnerGrid width ({portraitGridRect.Width}) should match full page width ({portraitRefRect.Width})");
+
+		try
+		{
+			App.SetOrientationLandscape();
+			App.WaitForElement("InnerGrid");
+
+			var landscapeGridRect = App.WaitForElement("InnerGrid").GetRect();
+			var landscapeRefRect = App.WaitForElement("ReferenceBar").GetRect();
+
+			Assert.That(landscapeGridRect.Width, Is.EqualTo(landscapeRefRect.Width).Within(5),
+				$"Landscape: InnerGrid width ({landscapeGridRect.Width}) should match full screen width ({landscapeRefRect.Width})");
+
+			Assert.That(landscapeGridRect.Width, Is.GreaterThan(portraitGridRect.Width),
+				"Landscape width should be greater than portrait width");
+		}
+		finally
+		{
+			App.SetOrientationPortrait();
+		}
+	}
+}
+
+#endif

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

@@ -163,6 +163,17 @@ Size ICrossPlatformLayout.CrossPlatformMeasure(double widthConstraint, double he
 			var width = contentSize.Width <= widthConstraint ? contentSize.Width : widthConstraint;
 			var height = contentSize.Height <= heightConstraint ? contentSize.Height : heightConstraint;
 
+			// Expand the ScrollView to fill the available constraint if the content requests it (e.g., Fill alignment).
+			if (scrollView.PresentedContent is { } presentedContent)
+			{
+				var fillWidth = double.IsInfinity(widthConstraint) ? width : widthConstraint;
+				var fillHeight = double.IsInfinity(heightConstraint) ? height : heightConstraint;
+				var adjustedSize = new Size(width, height).AdjustForFill(
+					new Rect(0, 0, fillWidth, fillHeight), presentedContent);
+				width = adjustedSize.Width;
+				height = adjustedSize.Height;
+			}
+
 			width = ViewHandlerExtensions.ResolveConstraints(width, scrollView.Width, scrollView.MinimumWidth, scrollView.MaximumWidth);
 			height = ViewHandlerExtensions.ResolveConstraints(height, scrollView.Height, scrollView.MinimumHeight, scrollView.MaximumHeight);