fix(xLoad): fix unload-reload scenarios when using x:Bind with x:Load

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

@@ -0,0 +1,22 @@
+<Page x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls.xLoad_Back_And_Forth"
+	  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">
+
+	<NavigationView x:Name="NavView" x:FieldModifier="public" PaneDisplayMode="Left">
+		<NavigationView.MenuItemTemplate>
+			<DataTemplate
+				x:DataType="local:xLoadBackAndForthVM">
+				<NavigationViewItem x:Name="nvi" IsSelected="{x:Bind IsSelected, Mode=TwoWay}">
+					<TextBlock
+						x:Name="tb"
+						x:Load="{x:Bind IsSelected, Mode=OneWay}"
+						Text="visible" />
+				</NavigationViewItem>
+			</DataTemplate>
+		</NavigationView.MenuItemTemplate>
+	</NavigationView>
+</Page>

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

@@ -0,0 +1,46 @@
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+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_Back_And_Forth : Page
+	{
+		public xLoad_Back_And_Forth()
+		{
+			this.InitializeComponent();
+			NavView.MenuItemsSource = new List<xLoadBackAndForthVM> { new(), new() };
+		}
+	}
+
+	public class xLoadBackAndForthVM : INotifyPropertyChanged
+	{
+		private bool _isSelected;
+		public event PropertyChangedEventHandler PropertyChanged;
+
+		protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		{
+			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+		}
+
+		protected bool SetField<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
+		{
+			if (EqualityComparer<T>.Default.Equals(field, value)) return false;
+			field = value;
+			OnPropertyChanged(propertyName);
+			return true;
+		}
+
+		public bool IsSelected
+		{
+			get => _isSelected;
+			set => SetField(ref _isSelected, value);
+		}
+	}
+}

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

@@ -487,6 +487,29 @@ public async Task When_xLoad_Visibility_Set()
 			Assert.AreEqual(1, SUT.GetChildren().Count(c => c is ElementStub));
 			Assert.AreEqual(0, SUT.GetChildren().Count(c => c is Border));
 		}
+
+		[TestMethod]
+		[UnoWorkItem("https://github.com/unoplatform/uno/issues/18509")]
+		public async Task When_xLoad_Set_Back_And_Forth()
+		{
+			var SUT = new xLoad_Back_And_Forth();
+			await UITestHelper.Load(SUT);
+
+			Assert.AreEqual("tb", SUT.FindFirstChild<NavigationViewItem>().FindFirstChild<ElementStub>().Name);
+			Assert.IsNull(SUT.FindFirstChild<NavigationViewItem>().FindFirstChild<TextBlock>());
+
+			((List<xLoadBackAndForthVM>)SUT.NavView.MenuItemsSource)[0].IsSelected = true;
+			await UITestHelper.WaitForIdle();
+
+			Assert.AreNotEqual("tb", SUT.FindFirstChild<NavigationViewItem>().FindFirstChild<ElementStub>().Name);
+			Assert.AreEqual("visible", SUT.FindFirstChild<NavigationViewItem>().FindFirstChild<TextBlock>().Text);
+
+			((List<xLoadBackAndForthVM>)SUT.NavView.MenuItemsSource)[0].IsSelected = false;
+			await UITestHelper.WaitForIdle();
+
+			Assert.AreEqual("tb", SUT.FindFirstChild<NavigationViewItem>().FindFirstChild<ElementStub>().Name);
+			Assert.IsNull(SUT.FindFirstChild<NavigationViewItem>().FindFirstChild<TextBlock>());
+		}
 	}
 }
 #endif

src/Uno.UI/UI/Xaml/ElementStub.cs

@@ -194,6 +194,18 @@ private void MaterializeInner()
 				{
 					_isMaterializing = true;
 
+					// Before swapping out the ElementStub, we set the inherited DC locally so that when removed from the
+					// visual tree, the inherited DC is kept intact. This is important in case of setting x:Load with an
+					// x:Bind. The binding we generate from the x:Bind will only listen to property changes while the DC
+					// is present. This is definitely not what happens on WinUI, but considering our implementation of
+					// x:Bind and ElementStub differ completely from WinUI's, this is acceptable for now.
+					// https://github.com/unoplatform/uno/issues/18509
+					if ((this as IDependencyObjectStoreProvider).Store.GetPropertyDetails(DataContextProperty)
+						.CurrentHighestValuePrecedence > DependencyPropertyValuePrecedences.Local)
+					{
+						this.SetValue(DataContextProperty, DataContext, DependencyPropertyValuePrecedences.Local);
+					}
+
 					_content = SwapViews(oldView: (FrameworkElement)this, newViewProvider: ContentBuilder);
 #if ENABLE_LEGACY_TEMPLATED_PARENT_SUPPORT
 					// note: This can be safely removed, once moving away from legacy impl.
@@ -220,6 +232,7 @@ private void Dematerialize()
 
 			if (_content != null)
 			{
+				this.ClearValue(DataContextProperty, DependencyPropertyValuePrecedences.Local);
 				var newView = SwapViews(oldView: (FrameworkElement)_content, newViewProvider: () => this as View);
 				if (newView != null)
 				{