Border has a 1 pixel thickness even when it's thickness property is set to 0 - fix

src/Controls/samples/Controls.Sample.UITests/Issues/Issue20156.xaml

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue20156"
+             Title="Issue20156">
+    <VerticalStackLayout
+        Padding="30,0"
+        BackgroundColor="Red"
+        Spacing="0">
+
+        <Border StrokeThickness="0"
+            BackgroundColor="Blue"
+            HeightRequest="100"
+            StrokeShape="RoundRectangle 0"/>
+
+        <Border StrokeThickness="0"
+            BackgroundColor="Blue"
+            HeightRequest="100"
+            StrokeShape="RoundRectangle 0"/>
+
+        <Border StrokeThickness="0"
+            BackgroundColor="Blue"
+            HeightRequest="100"
+            StrokeShape="RoundRectangle 0"/>
+
+        <Button Text="WaitForStubControl" AutomationId="WaitForStubControl"/>
+
+    </VerticalStackLayout>
+</ContentPage>

src/Controls/samples/Controls.Sample.UITests/Issues/Issue20156.xaml.cs

@@ -0,0 +1,14 @@
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+namespace Maui.Controls.Sample.Issues;
+
+[XamlCompilation(XamlCompilationOptions.Compile)]
+[Issue(IssueTracker.Github, 20156, "Border has a 1 pixel thickness even when it's thickness property is set to 0", PlatformAffected.All)]
+
+public partial class Issue20156 : ContentPage
+{
+	public Issue20156()
+	{
+		InitializeComponent();
+	}
+}

src/Controls/src/Core/Border/Border.cs

@@ -308,11 +308,15 @@ protected override void OnPropertyChanged([CallerMemberName] string? propertyNam
 			base.OnPropertyChanged(propertyName);
 
 			if (propertyName == HeightProperty.PropertyName ||
-				propertyName == WidthProperty.PropertyName ||
-				propertyName == StrokeShapeProperty.PropertyName)
+				propertyName == WidthProperty.PropertyName)
+			{
 				Handler?.UpdateValue(nameof(IBorderStroke.Shape));
-			else if (propertyName == StrokeThicknessProperty.PropertyName)
+			}
+			else if (propertyName == StrokeThicknessProperty.PropertyName ||
+					 propertyName == StrokeShapeProperty.PropertyName)
+			{
 				UpdateStrokeShape();
+			}
 			else if (propertyName == StrokeDashArrayProperty.PropertyName)
 				Handler?.UpdateValue(nameof(IBorderStroke.StrokeDashPattern));
 		}

src/Controls/tests/UITests/Tests/Issues/Issue20156.cs

@@ -0,0 +1,23 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.AppiumTests.Issues;
+
+public class Issue20156 : _IssuesUITest
+{
+	public override string Issue => "Border has a 1 pixel thickness even when it's thickness property is set to 0";
+
+	public Issue20156(TestDevice device)
+		: base(device)
+	{ }
+
+    [Test]
+	public void BorderShouldHaveNoThickness()
+	{
+		_ = App.WaitForElement("WaitForStubControl");
+
+		// The test passes if there's no gap between the borders
+		VerifyScreenshot();
+	}
+}