[Windows] FlyoutPage: update CollapseStyle at runtime

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.CollapseStyleProperty), MapCollapseStyle);
 #endif
 		}
 
@@ -31,5 +34,28 @@ internal static void MapPrefersPrefersStatusBarHiddenProperty(IFlyoutViewHandler
 			handler.UpdateValue(nameof(PlatformConfiguration.iOSSpecific.Page.PrefersStatusBarHiddenProperty));
 		}
 #endif
+
+#if WINDOWS
+		internal static void MapCollapseStyle(IFlyoutViewHandler handler, IFlyoutView view)
+		{
+			var flyoutLayoutBehavior = (view as FlyoutPage)?.FlyoutLayoutBehavior;
+			if (view is BindableObject bindable && handler.PlatformView is Microsoft.Maui.Platform.RootNavigationView navigationView && flyoutLayoutBehavior is FlyoutLayoutBehavior.Popover)
+			{
+				var collapseStyle = PlatformConfiguration.WindowsSpecific.FlyoutPage.GetCollapseStyle(bindable);
+				switch (collapseStyle)
+				{
+					case PlatformConfiguration.WindowsSpecific.CollapseStyle.Partial:
+						navigationView.FlyoutPaneDisplayMode = Microsoft.UI.Xaml.Controls.NavigationViewPaneDisplayMode.LeftCompact;
+						navigationView.PaneDisplayMode = Microsoft.UI.Xaml.Controls.NavigationViewPaneDisplayMode.LeftCompact;
+						break;
+					case PlatformConfiguration.WindowsSpecific.CollapseStyle.Full:
+					default:
+						navigationView.FlyoutPaneDisplayMode = null;
+						navigationView.PaneDisplayMode = Microsoft.UI.Xaml.Controls.NavigationViewPaneDisplayMode.LeftMinimal;
+						break;
+				}
+			}
+		}
+#endif
 	}
 }

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

@@ -13,7 +13,15 @@ public static class FlyoutPage
 		/// <summary>Bindable property for <see cref="CollapseStyle"/>.</summary>
 		public static readonly BindableProperty CollapseStyleProperty =
 			BindableProperty.CreateAttached("CollapseStyle", typeof(CollapseStyle),
-				typeof(FlyoutPage), CollapseStyle.Full);
+				typeof(FlyoutPage), CollapseStyle.Full, propertyChanged: OnCollapseStylePropertyChanged);
+
+		static void OnCollapseStylePropertyChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			if (bindable is Microsoft.Maui.Controls.FlyoutPage flyoutPage && flyoutPage.Handler is not null)
+			{
+				flyoutPage.Handler.UpdateValue(nameof(CollapseStyleProperty));
+			}
+		}
 
 		/// <include file="../../../../docs/Microsoft.Maui.Controls.PlatformConfiguration.WindowsSpecific/FlyoutPage.xml" path="//Member[@MemberName='GetCollapseStyle'][1]/Docs/*" />
 		public static CollapseStyle GetCollapseStyle(BindableObject element)

src/Controls/tests/TestCases.HostApp/Issues/Issue18200.cs

@@ -0,0 +1,79 @@
+using Microsoft.Maui.Controls.PlatformConfiguration;
+using  Microsoft.Maui.Controls.PlatformConfiguration.WindowsSpecific;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 18200, "Flyout Page SetCollapseStyle doesn't have any change", PlatformAffected.UWP)]
+public class Issue18200 : TestFlyoutPage
+{
+	Button _button;
+	protected override void Init()
+    {
+		this.On<Microsoft.Maui.Controls.PlatformConfiguration.Windows>().SetCollapseStyle(CollapseStyle.Partial);
+
+        // 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 = "Page1",
+			AutomationId = "FlyoutItem",
+            HorizontalOptions = LayoutOptions.Start,
+            VerticalOptions = LayoutOptions.Center
+        };
+
+        flyoutPage.Content = new VerticalStackLayout
+        {
+            Children = { page1Button }
+        };
+
+        // Create the detail content
+        var detailPage = new ContentPage
+        {
+            Title = "Detail",
+            BackgroundColor = Colors.LightYellow
+        };
+
+        _button = new Button
+        {
+            Text = "Change Collapse Style",
+            AutomationId = "CollapseStyleButton",
+        };
+        _button.Clicked += OnCollapseStyleValueChanged;
+
+        detailPage.Content = new VerticalStackLayout
+        {
+            Children = {
+                new Microsoft.Maui.Controls.Label
+                {
+                    Text = "Welcome to .NET MAUI!",
+                    TextColor = Colors.Black,
+                    HorizontalOptions = LayoutOptions.Center,
+                    VerticalOptions = LayoutOptions.Center
+                },
+                _button
+            }
+        };
+
+        // Set the flyout and detail pages
+        Flyout = flyoutPage;
+        Detail = detailPage;
+    }
+
+    void OnCollapseStyleValueChanged(object sender, EventArgs e)
+    {
+		var currentCollapseStyle = this.On<Microsoft.Maui.Controls.PlatformConfiguration.Windows>().GetCollapseStyle();
+		var newCollapseStyle = currentCollapseStyle == CollapseStyle.Full 
+			? CollapseStyle.Partial 
+			: CollapseStyle.Full;
+
+		this.On<Microsoft.Maui.Controls.PlatformConfiguration.Windows>().SetCollapseStyle(newCollapseStyle);
+    }
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue18200.cs

@@ -0,0 +1,29 @@
+# if WINDOWS  // It's a Windows specific API issue, so restricting the other platforms
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue18200 : _IssuesUITest
+{
+	public Issue18200(TestDevice device)
+		: base(device)
+	{ }
+
+	public override string Issue => "Flyout Page SetCollapseStyle doesn't have any change";
+
+	[Test]
+	[Category(UITestCategories.FlyoutPage)]
+	public void VerifyFlyoutCollapseStyleBehaviorChanges()
+	{
+		App.WaitForElement("CollapseStyleButton");
+		App.Tap("CollapseStyleButton");
+		App.TapFlyoutPageIcon();
+		App.TapFlyoutPageIcon(); // Close the flyout
+		App.WaitForNoElement("FlyoutItem");
+		App.Tap("CollapseStyleButton");
+		App.WaitForElement("FlyoutItem");
+	}
+}
+#endif

src/Core/src/Platform/Windows/MauiNavigationView.cs

@@ -182,13 +182,14 @@ internal void UpdatePaneDisplayModeFromFlyoutBehavior(FlyoutBehavior flyoutBehav
 			{
 				case FlyoutBehavior.Flyout:
 					IsPaneToggleButtonVisible = true;
+					var flyoutMode = FlyoutPaneDisplayMode ?? NavigationViewPaneDisplayMode.LeftMinimal;
 					// WinUI bug: Setting PaneDisplayMode to the same value and updating SelectedItem during navigation
 					// causes the selection and selected item indicator to not update correctly.
 					// Workaround: Only set PaneDisplayMode when the value actually changes.
 					// Related: https://github.com/microsoft/microsoft-ui-xaml/issues/9812
-					if (PaneDisplayMode != NavigationViewPaneDisplayMode.LeftMinimal)
+					if (PaneDisplayMode != flyoutMode)
 					{
-						PaneDisplayMode = NavigationViewPaneDisplayMode.LeftMinimal;
+						PaneDisplayMode = flyoutMode;
 					}
 					break;
 				case FlyoutBehavior.Locked:
@@ -371,6 +372,11 @@ internal static readonly DependencyProperty PaneToggleButtonWidthProperty
 				new PropertyMetadata(DefaultPaneToggleButtonWidth, OnPaneToggleButtonSizeChanged));
 		private NavigationViewPaneDisplayMode? _pinPaneDisplayModeTo;
 
+		// Stores the preferred pane display mode for FlyoutBehavior.Flyout, set by CollapseStyle on Windows.
+		// When null, the default LeftMinimal is used. This ensures CollapseStyle.Partial (LeftCompact) survives
+		// layout-driven FlyoutBehavior re-evaluations (e.g. size/orientation changes).
+		internal NavigationViewPaneDisplayMode? FlyoutPaneDisplayMode { get; set; }
+
 		internal double PaneToggleButtonWidth
 		{
 			get => (double)GetValue(PaneToggleButtonWidthProperty);