fix(ContentPresenter): Avoid DataContext temporary reset

This change fixes a visible effect of TwoWay binding on ComboBox.SelectedValue being reset to null when being loaded.

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/ContentPresenter/ContentPresenter_Content_DataContext.xaml

+<UserControl
+		x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.ContentPresenterPages.ContentPresenter_Content_DataContext"
+			 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.ContentPresenterPages"
+			 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+			 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+			 mc:Ignorable="d"
+			 d:DesignHeight="300"
+			 d:DesignWidth="400">
+
+	<Grid>
+		<StackPanel DataContext="{x:Null}">
+			<StackPanel Orientation="Horizontal">
+				<TextBlock Text="Empty with DataContext" />
+				<!-- The DataContext is completely ignored if the ContentPresenter doesn't have a ContentTemplate. -->
+				<ContentPresenter DataContext="DataContext" />
+			</StackPanel>
+
+			<StackPanel Orientation="Horizontal">
+				<TextBlock Text="Should be empty:" />
+				<!-- Change "DataContext" to "DataContext2": "DataContext2" -->
+				<!-- The DataContext is taken into account if the ContentPresenter has a ContentTemplate. -->
+				<ContentPresenter x:Name="emptyTestRoot"
+								  x:FieldModifier="public"
+								  DataContext="42">
+					<ContentPresenter.ContentTemplate>
+						<DataTemplate>
+							<TextBlock x:Name="emptyTest"
+									   Text="{Binding}" />
+						</DataTemplate>
+					</ContentPresenter.ContentTemplate>
+				</ContentPresenter>
+			</StackPanel>
+
+			<StackPanel Orientation="Horizontal">
+				<TextBlock Text="Should be 43:" />
+				<!-- Initial: "42" -->
+				<!-- Change "42" to "44": "44" -->
+				<!-- Change "43" to "45": "45" -->
+				<!-- The latest of either DataContext or Content being set takes priority. -->
+				<ContentPresenter x:Name="priorityTestRoot"
+								  x:FieldModifier="public"
+								  DataContext="42"
+								  Content="43">
+					<ContentPresenter.ContentTemplate>
+						<DataTemplate>
+							<TextBlock x:Name="priorityTest"
+									   Text="{Binding}" />
+						</DataTemplate>
+					</ContentPresenter.ContentTemplate>
+				</ContentPresenter>
+			</StackPanel>
+
+			<StackPanel Orientation="Horizontal">
+				<TextBlock Text="Should be 42:" />
+
+				<!-- Initial: "42" -->
+				<!-- Setting Content on ContentPresenter also sets the same value to DataContext locally. -->
+				<ContentPresenter x:Name="MyContentPresenter"
+								  Content="42">
+					<ContentPresenter.ContentTemplate>
+						<DataTemplate>
+							<!-- Empty -->
+							<Border />
+						</DataTemplate>
+					</ContentPresenter.ContentTemplate>
+				</ContentPresenter>
+				<TextBlock x:Name="sameValueTest"
+						   Text="{Binding DataContext, ElementName=MyContentPresenter}" />
+
+			</StackPanel>
+
+			<StackPanel Orientation="Horizontal">
+				<TextBlock Text="Should be DataContext:" />
+
+				<!-- Initial: "DataContext" -->
+				<!-- Change "DataContext" to "DataContext3": "DataContext3" -->
+				<!-- Change "DataContext2" to "DataContext4": "DataContext4" -->
+				<!-- Change "DataContext3" to "DataContext5": "DataContext4" -->
+				<!-- The inherited DataContext takes priority over the local DataContext initially. -->
+				<!-- The inherited DataContext is forever ignored if the local DataContext is changed. -->
+				<Grid x:Name="inheritanceTestRoot"
+					  x:FieldModifier="public"
+					  DataContext="DataContext">
+					<ContentPresenter x:Name="inheritanceTestInner"
+									  x:FieldModifier="public"
+									  DataContext="DataContext2">
+						<ContentPresenter.ContentTemplate>
+							<DataTemplate>
+								<TextBlock x:Name="inheritanceTest"
+										   Text="{Binding}" />
+							</DataTemplate>
+						</ContentPresenter.ContentTemplate>
+					</ContentPresenter>
+				</Grid>
+
+			</StackPanel>
+
+			<StackPanel Orientation="Horizontal">
+				<TextBlock Text="Should be DataContext:" />
+
+				<!-- Initial: "DataContext" -->
+				<!-- Change "DataContext" to "DataContext3": "DataContext3" -->
+				<!-- Change "DataContext2" to "DataContext4": "DataContext4" -->
+				<!-- Change "DataContext3" to "DataContext5": "DataContext4" -->
+				<!-- The inherited DataContext takes priority over the local DataContext initially. -->
+				<!-- The inherited DataContext is forever ignored if the local DataContext is changed. -->
+				<Grid DataContext="DataContext">
+					<ContentPresenter DataContext="DataContext2">
+						<ContentPresenter.ContentTemplate>
+							<DataTemplate>
+								<TextBlock x:Name="sameValueChangingTest"
+										   Text="{Binding}" />
+							</DataTemplate>
+						</ContentPresenter.ContentTemplate>
+					</ContentPresenter>
+				</Grid>
+
+			</StackPanel>
+
+			<StackPanel Orientation="Horizontal">
+				<TextBlock Text="Should be 42:" />
+
+				<!-- Initial: "DataContext" -->
+				<!-- The inherited DataContext takes priority over a null Content. -->
+				<Grid DataContext="42">
+					<ContentPresenter Content="{x:Null}">
+						<ContentPresenter.ContentTemplate>
+							<DataTemplate>
+								<TextBlock x:Name="nullContentChanged"
+										   Text="{Binding}" />
+							</DataTemplate>
+						</ContentPresenter.ContentTemplate>
+					</ContentPresenter>
+				</Grid>
+
+			</StackPanel>
+
+
+		</StackPanel>
+	</Grid>
+</UserControl>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/ContentPresenter/ContentPresenter_Content_DataContext.xaml.cs

+using Windows.UI.Xaml.Controls;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.ContentPresenterPages
+{
+	public sealed partial class ContentPresenter_Content_DataContext : UserControl
+	{
+		public ContentPresenter_Content_DataContext()
+		{
+			this.InitializeComponent();
+		}
+	}
+}

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

@@ -12,6 +12,7 @@ using Windows.UI.Xaml.Controls;
 using Windows.UI.Xaml.Media;
 using static Private.Infrastructure.TestServices;
 using System.Collections.ObjectModel;
+using Windows.UI.Xaml.Data;
 
 #if NETFX_CORE
 using Uno.UI.Extensions;
@@ -611,6 +612,68 @@ namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls
 		}
 #endif
 
+		[TestMethod]
+		public void When_SelectedItem_TwoWay_Binding()
+		{
+			var itemsControl = new ItemsControl()
+			{
+				ItemsPanel = new ItemsPanelTemplate(() => new StackPanel()),
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var comboBox = new ComboBox();
+					comboBox.Name = "combo";
+
+					comboBox.SetBinding(ComboBox.ItemsSourceProperty, new Binding { Path = new("Numbers") });
+					comboBox.SetBinding(
+						ComboBox.SelectedItemProperty,
+						new Binding { Path = new("SelectedNumber"), Mode = BindingMode.TwoWay });
+
+					return comboBox;
+				})
+			};
+
+			var test = new[] {
+				new TwoWayBindingItem()
+			};
+
+			WindowHelper.WindowContent = itemsControl;
+
+			itemsControl.ItemsSource = test;
+
+			var comboBox = itemsControl.FindName("combo") as ComboBox;
+
+			Assert.AreEqual(3, test[0].SelectedNumber);
+			Assert.AreEqual(3, comboBox.SelectedItem);
+		}
+
+		public class TwoWayBindingItem : System.ComponentModel.INotifyPropertyChanged
+        {
+			private int _selectedNumber;
+
+			public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
+
+			public TwoWayBindingItem()
+			{
+				Numbers = new List<int> { 1, 2, 3, 4 };
+				SelectedNumber = 3;
+			}
+
+			public List<int> Numbers { get; }
+
+			public int SelectedNumber
+			{
+				get
+				{
+					return _selectedNumber;
+				}
+				set
+				{
+					_selectedNumber = value;
+					PropertyChanged?.Invoke(this, new(nameof(Item)));
+				}
+			}
+		}
+
 		public class ItemModel
 		{
 			public string Text { get; set; }

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

@@ -5,6 +5,7 @@ using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using Private.Infrastructure;
+using Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.ContentPresenterPages;
 using Windows.Foundation;
 using Windows.UI;
 using Windows.UI.Xaml;
@@ -35,6 +36,113 @@ public class Given_ContentPresenter
 		Assert.AreEqual(HorizontalAlignment.Stretch, border.HorizontalAlignment);
 	}
 
+	[TestMethod]
+	public void When_Binding_And_DataContext_Same_As_Content()
+	{
+		var SUT = new ContentPresenter();
+		var dataContextChangedCount = 0;
+
+		SUT.DataContextChanged += (s, e) => dataContextChangedCount++;
+
+		SUT.DataContext = new object();
+		SUT.Content = SUT.DataContext;
+
+		TestServices.WindowHelper.WindowContent = SUT;
+
+		// This test ensures that the ContentPresenter does not reset
+		// the DataContext to null and then back to the content and have
+		// two-way bindings propagating the null value back to the source.
+		Assert.AreEqual(1, dataContextChangedCount);
+	}
+
+	[TestMethod]
+	public void When_Content_Presenter_Empty()
+	{
+		var sut = new ContentPresenter_Content_DataContext();
+
+		TestServices.WindowHelper.WindowContent = sut;
+
+		Assert.AreEqual("", GetTextBlockText(sut, "emptyTest"));
+
+		sut.emptyTestRoot.DataContext = "43";
+
+		Assert.AreEqual("43", GetTextBlockText(sut, "emptyTest"));
+	}
+
+	[TestMethod]
+	public void When_Content_Presenter_Priority()
+	{
+		var sut = new ContentPresenter_Content_DataContext();
+
+		TestServices.WindowHelper.WindowContent = sut;
+
+		Assert.AreEqual("43", GetTextBlockText(sut, "priorityTest"));
+
+		sut.priorityTestRoot.DataContext = "44";
+		Assert.AreEqual("44", GetTextBlockText(sut, "priorityTest"));
+
+		sut.priorityTestRoot.Content = "45";
+		Assert.AreEqual("45", GetTextBlockText(sut, "priorityTest"));
+
+		sut.priorityTestRoot.DataContext = "46";
+		Assert.AreEqual("46", GetTextBlockText(sut, "priorityTest"));
+	}
+
+	[TestMethod]
+	public void When_Content_Presenter_SameValue()
+	{
+		var sut = new ContentPresenter_Content_DataContext();
+
+		TestServices.WindowHelper.WindowContent = sut;
+
+		Assert.AreEqual("42", GetTextBlockText(sut, "sameValueTest"));
+	}
+
+	[TestMethod]
+	public void When_Content_Presenter_Inheritance()
+	{
+		var sut = new ContentPresenter_Content_DataContext();
+
+		TestServices.WindowHelper.WindowContent = sut;
+
+		Assert.AreEqual("DataContext", GetTextBlockText(sut, "inheritanceTest"));
+
+		sut.inheritanceTestRoot.DataContext = "46";
+		Assert.AreEqual("46", GetTextBlockText(sut, "inheritanceTest"));
+
+		sut.inheritanceTestRoot.DataContext = "47";
+		Assert.AreEqual("47", GetTextBlockText(sut, "inheritanceTest"));
+
+		sut.inheritanceTestInner.DataContext = "48";
+		Assert.AreEqual("48", GetTextBlockText(sut, "inheritanceTest"));
+
+		sut.inheritanceTestRoot.DataContext = "49";
+		Assert.AreEqual("48", GetTextBlockText(sut, "inheritanceTest"));
+	}
+
+	[TestMethod]
+	public void When_Content_Presenter_SameValue_Changing()
+	{
+		var sut = new ContentPresenter_Content_DataContext();
+
+		TestServices.WindowHelper.WindowContent = sut;
+
+		Assert.AreEqual("DataContext", GetTextBlockText(sut, "sameValueChangingTest"));
+	}
+
+	[TestMethod]
+	public void When_Content_Presenter_Null_Content_Changed()
+	{
+		var sut = new ContentPresenter_Content_DataContext();
+
+		TestServices.WindowHelper.WindowContent = sut;
+
+		Assert.AreEqual("42", GetTextBlockText(sut, "nullContentChanged"));
+	}
+
+	static string GetTextBlockText(FrameworkElement sut, string v)
+		=> (sut.FindName(v) as TextBlock)?.Text ?? "";
+
 	public static IEnumerable<object[]> GetAlignments()
 	{
 		var configurations = new List<AlignmentTestConfiguration>();

src/Uno.UI.Tests/Windows_UI_XAML_Controls/ComboBoxTests/Given_ComboBox.cs

@@ -8,7 +8,8 @@ using Windows.UI.Xaml.Controls;
 using Windows.UI.Xaml.Controls.Primitives;
 using Windows.UI.Xaml;
 using Uno.UI.Tests.App.Xaml;
-using System.Windows.Data;
+using Uno.UI.Xaml;
+using Windows.UI.Xaml.Data;
 
 namespace Uno.UI.Tests.ComboBoxTests
 {
@@ -121,5 +122,78 @@ namespace Uno.UI.Tests.ComboBoxTests
 			comboBox.Items.RemoveAt(2);
 			Assert.AreEqual<int>(-1, comboBox.SelectedIndex);
 		}
+
+		[TestMethod]
+		public void When_SelectedItem_TwoWay_Binding()
+		{
+			var itemsControl = new ItemsControl()
+			{
+				ItemsPanel = new ItemsPanelTemplate(() => new StackPanel()),
+				ItemTemplate = new DataTemplate(() =>
+				{
+					var comboBox = new ComboBox();
+					comboBox.Name = "combo";
+
+					comboBox.SetBinding(
+						ComboBox.ItemsSourceProperty,
+						new Binding { Path = new("Numbers")
+					});
+					comboBox.SetBinding(
+						ComboBox.SelectedItemProperty,
+						new Binding { Path = new("SelectedNumber"), Mode = BindingMode.TwoWay });
+
+					return comboBox;
+				})
+			};
+
+			var test = new[] {
+				new TwoWayBindingItem()
+			};
+
+			itemsControl.ForceLoaded();
+
+			itemsControl.ItemsSource = test;
+
+			var comboBox = itemsControl.FindName("combo") as ComboBox;
+
+			Assert.AreEqual(3, test[0].SelectedNumber);
+			Assert.AreEqual(3, comboBox.SelectedItem);
+		}
+
+		public class TwoWayBindingItem : System.ComponentModel.INotifyPropertyChanged
+		{
+			private int _selectedNumber;
+
+			public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
+
+			public TwoWayBindingItem()
+			{
+				Numbers = new List<int> { 1, 2, 3, 4 };
+				SelectedNumber = 3;
+			}
+
+			public List<int> Numbers { get; }
+
+			public int SelectedNumber
+			{
+				get
+				{
+					return _selectedNumber;
+				}
+				set
+				{
+					_selectedNumber = value;
+					PropertyChanged?.Invoke(this, new(nameof(SelectedNumber)));
+				}
+			}
+		}
+
+		public class ItemModel
+		{
+			public string Text { get; set; }
+
+			public override string ToString() => Text;
+		}
+
 	}
 }

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

@@ -820,11 +820,18 @@ namespace Windows.UI.Xaml.Controls
 			{
 				_firstLoadResetDone = true;
 
+				// This test avoids the ContentPresenter from resetting
+				// the DataContext to null (or the inherited value) and then back to
+				// the content and have two-way bindings propagating the null value
+				// back to the source.
+				if (!ReferenceEquals(DataContext, Content))
+				{
 					// On first load UWP clears the local value of a ContentPresenter.
 					// The reason for this behavior is unknown.
 					this.ClearValue(DataContextProperty, DependencyPropertyValuePrecedences.Local);
 
 					TrySetDataContextFromContent(Content);
+				}
 
 				return true;
 			}