[Windows] Fix unwanted corner radius in Shell flyout items

src/Controls/tests/TestCases.HostApp/Issues/Issue17414.cs

@@ -0,0 +1,35 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 17414, "Default styling for controls does not work", PlatformAffected.UWP)]
+public partial class Issue17414Shell : Shell
+{
+	public Issue17414Shell()
+	{
+		FlyoutBehavior = FlyoutBehavior.Flyout;
+		ItemTemplate = new DataTemplate(() =>
+		{
+			return new Label
+			{
+				Margin = new Thickness(20),
+				Text = "Home"
+			};
+		});
+
+		var detailContentPage = new ContentPage
+		{
+			BackgroundColor = Colors.LightGreen,
+			Content = new Label
+			{
+				AutomationId = "MainPageLabel",
+				Text = "Verify that the flyout content has no default corner radius",
+				HorizontalOptions = LayoutOptions.Center,
+				VerticalOptions = LayoutOptions.Center
+			}
+		};
+
+		Items.Add(new ShellContent
+		{
+			Content = detailContentPage
+		});
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue17414.cs

@@ -0,0 +1,24 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue17414 : _IssuesUITest
+{
+    public override string Issue => "Default styling for controls does not work";
+
+    public Issue17414(TestDevice device)
+    : base(device)
+    { }
+
+    [Test]
+    [Category(UITestCategories.FlyoutPage)]
+    [Category(UITestCategories.Shell)]
+    public void VerifyFlyoutContentHasNoDefaultCornerRadius()
+    {
+        App.WaitForElement("MainPageLabel");
+        App.TapShellFlyoutIcon();
+        VerifyScreenshot();
+    }
+}

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

@@ -134,6 +134,8 @@ protected override void OnApplyTemplate()
 			// It causes the content to not be flush up against the title bar
 			PaneContentGrid.Margin = new WThickness(0, 0, 0, 0);
 			UpdateMenuItemsContainerHeight();
+			RootSplitView.CornerRadius = new UI.Xaml.CornerRadius(0);
+			CornerRadius = new UI.Xaml.CornerRadius(0);
 
 			// On WinAppSDK 1.7, it seems that if the PaneContentGrid width is not explicitly set,
 			// it takes on the desired width of its child.