[Windows] FlyoutPage: Fix CollapsedPaneWidth mapping and updates

src/Controls/src/Core/FlyoutPage/FlyoutPage.Mapper.cs

@@ -12,6 +12,9 @@ public partial class FlyoutPage
 #if IOS
 			FlyoutViewHandler.Mapper.ReplaceMapping<IFlyoutView, IFlyoutViewHandler>(nameof(PlatformConfiguration.iOSSpecific.Page.PrefersHomeIndicatorAutoHiddenProperty), MapPrefersHomeIndicatorAutoHiddenProperty);
 			FlyoutViewHandler.Mapper.ReplaceMapping<IFlyoutView, IFlyoutViewHandler>(nameof(PlatformConfiguration.iOSSpecific.Page.PrefersStatusBarHiddenProperty), MapPrefersPrefersStatusBarHiddenProperty);
+#endif
+#if WINDOWS
+			FlyoutViewHandler.Mapper.ReplaceMapping<IFlyoutView, IFlyoutViewHandler>(nameof(PlatformConfiguration.WindowsSpecific.FlyoutPage.CollapsedPaneWidthProperty), MapCollapsedPaneWidth);
 #endif
 		}
 
@@ -31,5 +34,16 @@ internal static void MapPrefersPrefersStatusBarHiddenProperty(IFlyoutViewHandler
 			handler.UpdateValue(nameof(PlatformConfiguration.iOSSpecific.Page.PrefersStatusBarHiddenProperty));
 		}
 #endif
+
+#if WINDOWS
+		internal static void MapCollapsedPaneWidth(IFlyoutViewHandler handler, IFlyoutView view)
+		{
+			if (view is BindableObject bindable && handler.PlatformView is Microsoft.Maui.Platform.RootNavigationView navigationView)
+			{
+				var collapsedPaneWidth = PlatformConfiguration.WindowsSpecific.FlyoutPage.GetCollapsedPaneWidth(bindable);
+				navigationView.CompactPaneLength = collapsedPaneWidth;
+			}
+		}
+#endif
 	}
-}
+}

src/Controls/src/Core/PlatformConfiguration/WindowsSpecific/FlyoutPage.cs

@@ -67,7 +67,7 @@ public static IPlatformElementConfiguration<Windows, FormsElement> UsePartialCol
 		/// <summary>Bindable property for attached property <c>CollapsedPaneWidth</c>.</summary>
 		public static readonly BindableProperty CollapsedPaneWidthProperty =
 			BindableProperty.CreateAttached("CollapsedPaneWidth", typeof(double),
-				typeof(FlyoutPage), 48d, validateValue: (bindable, value) => (double)value >= 0);
+				typeof(FlyoutPage), 48d, validateValue: (bindable, value) => (double)value >= 0, propertyChanged : OnCollapsedPaneWidthChanged);
 
 		/// <summary>Gets the width of the collapsed flyout pane on Windows.</summary>
 		/// <param name="element">The element to get the collapsed pane width from.</param>
@@ -77,6 +77,14 @@ public static double GetCollapsedPaneWidth(BindableObject element)
 			return (double)element.GetValue(CollapsedPaneWidthProperty);
 		}
 
+		static void OnCollapsedPaneWidthChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			if (bindable is Microsoft.Maui.Controls.FlyoutPage flyoutPage && flyoutPage.Handler is not null)
+			{
+				flyoutPage.Handler.UpdateValue(nameof(CollapsedPaneWidthProperty));
+			}
+		}
+
 		/// <summary>Sets the width of the collapsed flyout pane on Windows.</summary>
 		/// <param name="element">The element to set the collapsed pane width on.</param>
 		/// <param name="collapsedPaneWidth">The collapsed pane width in device-independent units.</param>

src/Controls/tests/TestCases.HostApp/Issues/Issue33785.cs

@@ -0,0 +1,76 @@
+using Microsoft.Maui.Controls.PlatformConfiguration;
+using Microsoft.Maui.Controls.PlatformConfiguration.WindowsSpecific;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 33785, "[Windows] FlyoutPage CollapsedPaneWidth Not Working", PlatformAffected.UWP)]
+public class Issue33785 : TestFlyoutPage
+{
+	Microsoft.Maui.Controls.Label _label;
+	protected override void Init()
+	{
+		this.On<Microsoft.Maui.Controls.PlatformConfiguration.Windows>().SetCollapseStyle(CollapseStyle.Partial);
+		this.On<Microsoft.Maui.Controls.PlatformConfiguration.Windows>().CollapsedPaneWidth(50);
+
+		// Set the flyout page properties
+		FlyoutLayoutBehavior = FlyoutLayoutBehavior.Popover;
+
+		// Create the flyout content
+		var flyoutPage = new ContentPage
+		{
+			Title = "Master",
+			BackgroundColor = Colors.Blue
+		};
+
+		var page1Button = new Button
+		{
+			Text = "Change",
+			AutomationId = "FlyoutItem",
+			HorizontalOptions = LayoutOptions.Start,
+			VerticalOptions = LayoutOptions.Center
+		};
+		page1Button.Clicked += (s, e) =>
+		{
+			this.On<Microsoft.Maui.Controls.PlatformConfiguration.Windows>().CollapsedPaneWidth(100);
+			_label.Text = "CollapsedPaneWidth set to 100";
+		};
+
+		flyoutPage.Content = new VerticalStackLayout
+		{
+			Children = { page1Button }
+		};
+
+		// Create the detail content
+		var detailPage = new ContentPage
+		{
+			Title = "Detail",
+			BackgroundColor = Colors.LightYellow
+		};
+
+		_label = new Microsoft.Maui.Controls.Label
+		{
+			Text = "Test for CollapsedPaneWidth",
+			AutomationId = "CollapsedPaneLabel",
+			HorizontalOptions = LayoutOptions.Center,
+			HorizontalTextAlignment = TextAlignment.Center,
+		};
+
+		detailPage.Content = new VerticalStackLayout
+		{
+			Children = {
+				new Microsoft.Maui.Controls.Label
+				{
+					Text = "Welcome to .NET MAUI!",
+					TextColor = Colors.Black,
+					HorizontalOptions = LayoutOptions.Center,
+					VerticalOptions = LayoutOptions.Center
+				},
+				_label
+			}
+		};
+
+		// Set the flyout and detail pages
+		Flyout = flyoutPage;
+		Detail = detailPage;
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue33785.cs

@@ -0,0 +1,23 @@
+#if WINDOWS		//CollapsedPaneWidth is Windows Specific API
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+public class Issue33785 : _IssuesUITest
+{
+	public Issue33785(TestDevice device) : base(device) { }
+
+	public override string Issue => "[Windows] FlyoutPage CollapsedPaneWidth Not Working";
+	[Test]
+	[Category(UITestCategories.FlyoutPage)]
+	public void VerifyFlyoutPageCollapsedPaneWidth()
+	{
+		App.WaitForElement("CollapsedPaneLabel");
+		App.TapFlyoutPageIcon();
+		App.Tap("FlyoutItem");
+		App.TapFlyoutPageIcon();
+		VerifyScreenshot();
+	}
+}
+#endif