[Android] Fix for OnSizeAllocated is not reported for Android AppShell Flyout content. (#30069)

* fix-22045 - Made changes on Android ShellFlyout sizing by adding OnDrawerOpened event and updating Content so OnSizeAllocated is called with correct values when drawer opens.

* fix-22045- Set the contentview frame value when the FlyoutView layout change.

* fix-22045- Changes committed.

Author: BagavathiPerumal

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellFlyoutTemplatedContentRenderer.cs

@@ -419,9 +419,7 @@ bool UpdateContentPadding()
 				// and propagate any margins set
 				if (_contentView?.PlatformView != null)
 				{
-					var dpWidth = _rootView.Context.FromPixels(_contentView.PlatformView.MeasuredWidth);
-					var dpHeight = _rootView.Context.FromPixels(_contentView.PlatformView.MeasuredHeight);
-					_contentView.View.Frame = _contentView.View.ComputeFrame(new Graphics.Rect(0, 0, dpWidth, dpHeight));
+					SetContentViewFrame();
 
 					cl.LeftMargin = (int)_rootView.Context.ToPixels(viewMargin.Left);
 					cl.TopMargin = (int)_rootView.Context.ToPixels(viewMargin.Top);
@@ -434,6 +432,13 @@ bool UpdateContentPadding()
 			return returnValue;
 		}
 
+		void SetContentViewFrame()
+		{
+			var dpWidth = _rootView.Context.FromPixels(_contentView.PlatformView.MeasuredWidth);
+			var dpHeight = _rootView.Context.FromPixels(_contentView.PlatformView.MeasuredHeight);
+			_contentView.View.Frame = _contentView.View.ComputeFrame(new Graphics.Rect(0, 0, dpWidth, dpHeight));
+		}
+
 		void OnFlyoutViewLayoutChanging()
 		{
 			// The second time this fires the non flyout part of the view
@@ -464,6 +469,12 @@ void OnFlyoutViewLayoutChanging()
 				UpdateFooterLayout();
 				UpdateContentPadding();
 			}
+			else if (_contentView?.PlatformView is not null &&
+				((_contentView.PlatformView.MeasuredHeight > 0 && _contentView.View.Frame.Width == 0) ||
+				 (_contentView.PlatformView.MeasuredWidth > 0 && _contentView.View.Frame.Height == 0)))
+			{
+				SetContentViewFrame();
+			}
 		}
 
 		void UpdateVerticalScrollMode()

src/Controls/tests/TestCases.HostApp/Issues/Issue22045.cs

@@ -0,0 +1,86 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 22045, "[Android] OnSizeAllocated not reported for Android AppShell Flyout content", PlatformAffected.Android)]
+public class Issue22045 : Shell
+{
+	public Issue22045()
+	{
+		this.FlyoutBehavior = FlyoutBehavior.Flyout;
+
+		var shellContent = new ShellContent
+		{
+			Title = "Home",
+			Route = "MainPage",
+			ContentTemplate = new DataTemplate(typeof(_22045MainPage))
+		};
+
+		Items.Add(shellContent);
+
+		this.FlyoutContent = new _22045NewContent();
+	}
+
+	public class _22045NewContent : ContentView
+	{
+		Label widthLabel;
+		Label heightLabel;
+
+		public _22045NewContent()
+		{
+			widthLabel = new Label
+			{
+				AutomationId= "WidthLabel",
+				VerticalOptions = LayoutOptions.Center,
+				HorizontalOptions = LayoutOptions.Center
+			};
+
+			heightLabel = new Label
+			{
+				AutomationId = "HeightLabel",
+				VerticalOptions = LayoutOptions.Center,
+				HorizontalOptions = LayoutOptions.Center
+			};
+
+			Content = new VerticalStackLayout
+			{
+				Children =
+				{
+					widthLabel,
+					heightLabel
+				}
+			};
+		}
+		protected override void OnSizeAllocated(double width, double height)
+		{
+			base.OnSizeAllocated(width, height);
+			widthLabel.Text = width.ToString();
+			heightLabel.Text = height.ToString();
+		}
+	}
+
+	public class _22045MainPage : ContentPage
+	{
+		public _22045MainPage()
+		{
+			var button = new Button
+			{
+				Text = "Open Flyout",
+				AutomationId = "OpenFlyoutButton",
+				HorizontalOptions = LayoutOptions.Center,
+				VerticalOptions = LayoutOptions.Center
+			};
+
+			button.Clicked += (s, e) =>
+			{
+				Shell.Current.FlyoutIsPresented = true;
+			};
+
+			Content = new VerticalStackLayout
+			{
+				Children =
+				{
+					button
+				}
+			};
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue22045.cs

@@ -0,0 +1,28 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue22045 : _IssuesUITest
+{
+	public Issue22045(TestDevice testDevice) : base(testDevice)
+	{
+	}
+
+	public override string Issue => "[Android] OnSizeAllocated not reported for Android AppShell Flyout content";
+
+	[Test]
+	[Category(UITestCategories.Shell)]
+	public void GetValidSizeWhenContentViewInFlyoutContent()
+	{
+		App.WaitForElement("OpenFlyoutButton");
+		App.Click("OpenFlyoutButton");
+		var width = App.WaitForElement("WidthLabel").GetText();
+		var height = App.WaitForElement("HeightLabel").GetText();
+		Assert.That(double.TryParse(width, out double w) && w > 0,
+			Is.True, $"Expected positive width, got {width}");
+		Assert.That(double.TryParse(height, out double h) && h > 0,
+			Is.True, $"Expected positive height, got {height}");
+	}
+}