fix: Don't propagate DataContext via Border.Child

Restore the Uno 2.x behaviour whereby Border.ChildProperty doesn't propagate inherited values (including DataContext) to the property value. The inheritance is handled by the visual tree in the normal case (since Child is always a view).

This restores the ability for the framework to call SetParent() to simulate a different inheritance hierarchy than the actual visual tree configuration, eg for AppBarButton using the native style.

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/FramePages/Page_With_AppBarButton_Visibility_Bound.xaml

@@ -0,0 +1,22 @@
+<Page
+    x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.FramePages.Page_With_AppBarButton_Visibility_Bound"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.FramePages"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d"
+    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+
+	<Grid>
+		<CommandBar Content="Page with AppBarButton">
+			<CommandBar.PrimaryCommands>
+				<AppBarButton Visibility="{Binding ButtonVisibility, FallbackValue=Collapsed}">
+					<TextBlock x:Name="innerTextBlock"
+							   x:FieldModifier="public"
+							   Text="{Binding ButtonText}" />
+				</AppBarButton>
+			</CommandBar.PrimaryCommands>
+		</CommandBar>
+	</Grid>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/FramePages/Page_With_AppBarButton_Visibility_Bound.xaml.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using Windows.UI.Xaml.Data;
+using Windows.UI.Xaml.Input;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+
+// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=234238
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.FramePages
+{
+	/// <summary>
+	/// An empty page that can be used on its own or navigated to within a Frame.
+	/// </summary>
+	public sealed partial class Page_With_AppBarButton_Visibility_Bound : Page
+	{
+		public Page_With_AppBarButton_Visibility_Bound()
+		{
+			this.InitializeComponent();
+		}
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_AppBarButton.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Uno.UI.RuntimeTests.Helpers;
+using Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.FramePages;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using static Private.Infrastructure.TestServices;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls
+{
+	[TestClass]
+	[RunsOnUIThread]
+	public class Given_AppBarButton
+	{
+		[TestMethod]
+		public async Task Check_DataContext_Propagation()
+		{
+			using (StyleHelper.UseNativeFrameNavigation())
+			{
+				var frame = new Frame();
+				WindowHelper.WindowContent = frame;
+				await WindowHelper.WaitForIdle();
+				frame.Navigate(typeof(Page_With_AppBarButton_Visibility_Bound));
+				await WindowHelper.WaitForIdle();
+				var page = frame.Content as Page_With_AppBarButton_Visibility_Bound;
+				Assert.IsNotNull(page);
+				page.DataContext = new MyContext();
+				await WindowHelper.WaitForIdle();
+				var tb = page.innerTextBlock;
+				Assert.IsNotNull(tb);
+				Assert.AreEqual("Archaeopteryx", tb.Text);
+				Assert.IsTrue(tb.ActualWidth > 0);
+				Assert.IsTrue(tb.ActualHeight > 0);
+			}
+		}
+
+		private class MyContext
+		{
+			public Visibility ButtonVisibility => Visibility.Visible;
+			public string ButtonText => "Archaeopteryx";
+		}
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_Border.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Data;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls
+{
+	[TestClass]
+	[RunsOnUIThread]
+	public class Given_Border
+	{
+		[TestMethod]
+		public void Check_DataContext_Propagation()
+		{
+			var tb = new TextBlock();
+			tb.SetBinding(TextBlock.TextProperty, new Binding { Path = new PropertyPath("TestText") });
+			var SUT = new Border
+			{
+				Child = tb
+			};
+
+			var root = new Grid
+			{
+				DataContext = new MyContext()
+			};
+
+			root.Children.Add(SUT);
+
+			Assert.AreEqual("Vampire squid", tb.Text);
+		}
+
+		private class MyContext
+		{
+			public string TestText => "Vampire squid";
+		}
+	}
+}

src/Uno.UI/UI/Xaml/Controls/Border/Border.cs

@@ -75,6 +75,10 @@ public virtual UIElement Child
 				typeof(Border),
 				new FrameworkPropertyMetadata(
 					null,
+					// Since this is a view, inheritance is handled through the visual tree, rather than via the property. We explicitly
+					// disable the property-based propagation here to support the case where the Parent property is overridden to simulate
+					// a different inheritance hierarchy, as is done for some controls with native styles.
+					FrameworkPropertyMetadataOptions.ValueDoesNotInheritDataContext,
 					(s, e) => ((Border)s)?.OnChildChanged((UIElement)e.OldValue, (UIElement)e.NewValue)
 				)
 			);