[Windows] Fix for MenuFlyoutItems programmatically added to MenuFlyoutSubItems are not visible

src/Controls/tests/TestCases.HostApp/Issues/Issue26856.cs

@@ -0,0 +1,81 @@
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 26856, "MenuFlyoutItems programmatically added to MenuFlyoutSubItems are not visible", PlatformAffected.UWP)]
+	public class Issue26856 : TestShell
+	{
+		protected override void Init()
+		{
+			// Create MenuBarItem
+			var menuBarItem = new MenuBarItem { Text = "Menu Flyout Item" };
+
+			// Add MenuFlyoutItem to MenuBarItem
+			menuBarItem.Add(new MenuFlyoutItem { Text = "Original Item" });
+
+			// Create MenuFlyoutSubItem
+			var menuFlyoutSubItem = new MenuFlyoutSubItem { Text = "Flyout" };
+
+			// Add MenuFlyoutItem to MenuFlyoutSubItem
+			menuFlyoutSubItem.Add(new MenuFlyoutItem { Text = "Original Sub Item" });
+
+			// Add MenuFlyoutSubItem to MenuBarItem
+			menuBarItem.Add(menuFlyoutSubItem);
+
+			// Add MenuBarItem to MenuBarItems
+			MenuBarItems.Add(menuBarItem);
+
+			var layout = new VerticalStackLayout
+			{
+				Padding = new Thickness(30, 0),
+				Spacing = 25
+			};
+
+			var label = new Label { Text = "Menu Test" };
+			var button = new Button
+			{
+				Text = "Add Flyout Sub Item"
+			};
+			button.AutomationId = "Button";
+
+			button.Clicked += OnButtonClicked;
+
+			// Add elements to layout
+			layout.Children.Add(label);
+			layout.Children.Add(button);
+
+			// Set Content of the Page
+			AddContentPage(new ContentPage { Content = layout });
+			FlyoutBehavior = FlyoutBehavior.Disabled;
+		}
+
+		private void OnButtonClicked(object sender, EventArgs e)
+		{
+			MenuFlyoutSubItem menuFlyoutSubItem = GetSubMenu("Flyout");
+
+			MenuFlyoutItem itemToAdd = new()
+			{
+				Text = "Added Sub Item",
+				IsEnabled = true,
+				Parent = menuFlyoutSubItem
+			};
+			menuFlyoutSubItem.Add(itemToAdd);
+
+		}
+
+		public MenuFlyoutSubItem GetSubMenu(string name)
+		{
+			MenuFlyoutSubItem result = null;
+
+			MenuBarItems.ToList().ForEach(menuBarItem =>
+			{
+				var foundItem = menuBarItem.SingleOrDefault(menuElement => menuElement is MenuFlyoutSubItem subMenu && subMenu.Text == name);
+
+				if (foundItem != null)
+				{
+					result = foundItem as MenuFlyoutSubItem;
+				}
+			});
+
+			return result;
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue26856.cs

@@ -0,0 +1,29 @@
+#if WINDOWS
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue26856 : _IssuesUITest
+	{
+		public override string Issue => "MenuFlyoutItems programmatically added to MenuFlyoutSubItems are not visible";
+
+		public Issue26856(TestDevice device) : base(device)
+		{
+		}
+
+		[Test]
+		[Category(UITestCategories.Page)]
+		public void MenuFlyoutItemShouldVisibleInsideMenuFlyoutSubItems()
+		{
+			App.WaitForElement("Button");
+			App.Tap("Button");
+			App.Tap("Menu Flyout Item");
+			App.WaitForElement("Flyout");
+			App.Tap("Flyout");
+			VerifyScreenshot();
+		}
+	}
+}
+#endif

src/Core/src/Handlers/MenuFlyoutSubItem/MenuFlyoutSubItemHandler.Windows.cs

@@ -89,6 +89,10 @@ void Reset()
 				{
 					items = mfsi.Items;
 				}
+				else if (pvh.PlatformView is MenuBarItem menuBarItem)
+				{
+					items = menuBarItem.Items;
+				}
 
 				if (items is not null)
 				{