[Windows] Fixed Unstable order of Flyout Items with conditional visibility

src/Controls/src/Core/Handlers/Shell/Windows/ShellView.cs

@@ -122,20 +122,34 @@ internal void UpdateMenuItemSource()
 			{
 				_flyoutGrouping = newGrouping;
 				var newItems = IterateItems(newGrouping).ToList();
+				var newItemsSet = new HashSet<object>(newItems);
+				var flyoutItemsSet = new HashSet<object>(FlyoutItems);
 
-				foreach (var item in newItems)
+				for (int index = 0; index < newItems.Count; index++)
 				{
-					if (!FlyoutItems.Contains(item))
+					var item = newItems[index];
+
+					if (!flyoutItemsSet.Contains(item))
 					{
-						FlyoutItems.Add(item);
+						// Use Insert when within bounds, otherwise Add
+						if (index < FlyoutItems.Count)
+						{
+							FlyoutItems.Insert(index, item);
+						}
+						else
+						{
+							FlyoutItems.Add(item);
+						}
 					}
 				}
 
 				for (var i = FlyoutItems.Count - 1; i >= 0; i--)
 				{
 					var item = FlyoutItems[i];
-					if (!newItems.Contains(item))
+					if (!newItemsSet.Contains(item))
+					{
 						FlyoutItems.RemoveAt(i);
+					}
 				}
 			}
 

src/Controls/tests/TestCases.HostApp/Issues/Issue23834.cs

@@ -0,0 +1,54 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 23834, "Flyout Item misbehavior", PlatformAffected.UWP)]
+public class Issue23834 : TestShell
+{
+	protected override void Init()
+	{
+		var shellContent = new ShellContent()
+		{
+			ContentTemplate = new DataTemplate(typeof(Issue23834SamplePage))
+		};
+
+		this.Items.Add(new FlyoutItem()
+		{
+			Title = "Flyout Item 1",
+			IsVisible = false,
+			Items =
+			{
+				shellContent
+			}
+		});
+
+		this.Items.Add(new FlyoutItem()
+		{
+			Title = "Flyout Item 2",
+			Items =
+			{
+				shellContent
+			}
+		});
+	}
+}
+
+public class Issue23834SamplePage : ContentPage
+{
+	public Issue23834SamplePage()
+	{
+		var layout = new StackLayout();
+		var changeButton = new Button
+		{
+			Text = "Change First Item's IsVisible",
+			AutomationId = "button"
+		};
+
+		changeButton.Clicked += (s, e) =>
+		{
+			Shell.Current.Items[0].IsVisible = true;
+		};
+
+		layout.Children.Add(changeButton);
+
+		Content = layout;
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue23834.cs

@@ -0,0 +1,21 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+public class Issue23834 : _IssuesUITest
+{
+	public Issue23834(TestDevice device) : base(device) { }
+
+	public override string Issue => "Flyout Item misbehavior";
+
+	[Test]
+	[Category(UITestCategories.FlyoutPage)]
+	public void Issue23834FlyoutMisbehavior()
+	{
+		App.WaitForElement("button");
+		App.Tap("button");
+		App.ShowFlyout();
+		VerifyScreenshot();
+	}
+}