Fix for border color of the RadioButton is visible in Windows

src/Controls/tests/TestCases.HostApp/Issues/Issue28153.cs

@@ -0,0 +1,30 @@
+namespace Controls.TestCases.HostApp.Issues;
+
+[Issue(IssueTracker.Github, 28153, "The border color of the RadioButton is visible in Windows only", PlatformAffected.UWP)]
+class Issue28153 : ContentPage
+{
+	public Issue28153()
+	{
+		VerticalStackLayout verticalStackLayout = new VerticalStackLayout();
+
+		RadioButton radioButtonWithoutBorder = new RadioButton
+		{
+			AutomationId = "RadioButtonWithoutBorder",
+			Content = "RadioButton",
+			BorderColor = Colors.Red
+		};
+
+		RadioButton radioButtonWithBorder = new RadioButton
+		{
+			AutomationId = "RadioButtonWithBorder",
+			Content = "RadioButton",
+			BorderColor = Colors.Red,
+			BorderWidth = 3
+		};
+
+		verticalStackLayout.Children.Add(radioButtonWithoutBorder);
+		verticalStackLayout.Children.Add(radioButtonWithBorder);
+
+		Content = verticalStackLayout;
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue28153.cs

@@ -0,0 +1,21 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+internal class Issue28153 : _IssuesUITest
+{
+	public Issue28153(TestDevice device) : base(device) { }
+
+	public override string Issue => "The border color of the RadioButton is visible in Windows only";
+
+	[Test]
+	[Category(UITestCategories.RadioButton)]
+	public void ValidateRadioButtonNoBorderColorWhenNoBorderWidth()
+	{
+		App.WaitForElement("RadioButtonWithoutBorder");
+		VerifyScreenshot();
+	}
+}
+

src/Core/src/Platform/Windows/RadioButtonExtensions.cs

@@ -82,7 +82,8 @@ static void UpdateColors(ResourceDictionary resource, string[] keys, WBrush? bru
 
 		public static void UpdateStrokeThickness(this RadioButton nativeRadioButton, IRadioButton radioButton)
 		{
-			nativeRadioButton.BorderThickness = radioButton.StrokeThickness <= 0 ? WinUIHelpers.CreateThickness(3) : WinUIHelpers.CreateThickness(radioButton.StrokeThickness);
+			// Ensure stroke thickness is non-negative; a negative value causes an exception, so it defaults to zero if unset or invalid.
+			nativeRadioButton.BorderThickness = radioButton.StrokeThickness < 0 ? WinUIHelpers.CreateThickness(0) : WinUIHelpers.CreateThickness(radioButton.StrokeThickness);
 		}
 
 		public static void UpdateCornerRadius(this RadioButton nativeRadioButton, IRadioButton radioButton)