test: add tests for ElementStub changes

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Controls/xLoad_Visibility.xaml

@@ -0,0 +1,12 @@
+<Page x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls.xLoad_Visibility"
+	  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+	  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+	  xmlns:local="using:Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls"
+	  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+	  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+	  mc:Ignorable="d">
+
+	<Border x:Name="LazyLoadedBorder" x:Load="False" Visibility="Visible">
+		<TextBlock Text="Content created when it becomes visible" />
+	</Border>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Controls/xLoad_Visibility.xaml.cs

@@ -0,0 +1,17 @@
+using Microsoft.UI.Xaml.Controls;
+
+// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=234238
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls
+{
+	/// <summary>
+	/// An empty page that can be used on its own or navigated to within a Frame.
+	/// </summary>
+	public sealed partial class xLoad_Visibility : Page
+	{
+		public xLoad_Visibility()
+		{
+			this.InitializeComponent();
+		}
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_xLoad.cs

@@ -2,6 +2,7 @@
 #if !WINAPPSDK
 using System;
 using System.Diagnostics;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using Private.Infrastructure;
@@ -446,6 +447,17 @@ private async Task AssertIsNullAsync<T>(Func<T> getter, TimeSpan? timeout = null
 
 			Assert.IsNull(getter());
 		}
+
+		[TestMethod]
+		public async Task When_xLoad_Visibility_Set()
+		{
+			var SUT = new xLoad_Visibility();
+			TestServices.WindowHelper.WindowContent = SUT;
+			await TestServices.WindowHelper.WaitForIdle();
+
+			Assert.AreEqual(1, SUT.GetChildren().Count(c => c is ElementStub));
+			Assert.AreEqual(0, SUT.GetChildren().Count(c => c is Border));
+		}
 	}
 }
 #endif

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Media/Given_VisualTreeHelper.cs

@@ -16,6 +16,7 @@
 using Microsoft.UI.Xaml.Media;
 using FluentAssertions;
 using Uno.Extensions;
+using Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls;
 using static Private.Infrastructure.TestServices;
 
 namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Media
@@ -149,6 +150,24 @@ public async Task When_HitTestTranslatedElement()
 #if !UNO_HAS_MANAGED_POINTERS
 		[Ignore("Root visual tree elements are not configured properly to use managed hit testing.")]
 #endif
+#if __MACOS__
+		[Ignore("Currently fails on macOS, part of #9282 epic")]
+#endif
+		public async Task When_ElementStub_Not_Counted()
+		{
+			var SUT = new xLoad_Visibility();
+			WindowHelper.WindowContent = SUT;
+			await WindowHelper.WaitForIdle();
+
+			Assert.AreEqual(0, VisualTreeHelper.GetChildrenCount(SUT));
+			Assert.IsNull(VisualTreeHelper.GetChild(SUT, 0));
+		}
+
+		[TestMethod]
+		[RunsOnUIThread]
+#if !UNO_HAS_MANAGED_POINTERS
+		[Ignore("Root visual tree elements are not configured properly to use managed hit testing.")]
+#endif
 #if __MACOS__
 		[Ignore("Currently fails on macOS, part of #9282 epic")]
 #endif