Fix ReactiveUI regression, when DataContext type is different (#15423)

src/Avalonia.ReactiveUI/ReactiveUserControl.cs

@@ -48,14 +48,15 @@ namespace Avalonia.ReactiveUI
 
             if (change.Property == DataContextProperty)
             {
-                if (Object.ReferenceEquals(change.OldValue, ViewModel))
+                if (ReferenceEquals(change.OldValue, ViewModel)
+                    && change.NewValue is null or TViewModel)
                 {
                     SetCurrentValue(ViewModelProperty, change.NewValue);
                 }
             }
             else if (change.Property == ViewModelProperty)
             {
-                if (Object.ReferenceEquals(change.OldValue, DataContext))
+                if (ReferenceEquals(change.OldValue, DataContext))
                 {
                     SetCurrentValue(DataContextProperty, change.NewValue);
                 }

src/Avalonia.ReactiveUI/ReactiveWindow.cs

@@ -48,14 +48,15 @@ namespace Avalonia.ReactiveUI
 
             if (change.Property == DataContextProperty)
             {
-                if (Object.ReferenceEquals(change.OldValue, ViewModel))
+                if (ReferenceEquals(change.OldValue, ViewModel)
+                    && change.NewValue is null or TViewModel)
                 {
                     SetCurrentValue(ViewModelProperty, change.NewValue);
                 }
             }
             else if (change.Property == ViewModelProperty)
             {
-                if (Object.ReferenceEquals(change.OldValue, DataContext))
+                if (ReferenceEquals(change.OldValue, DataContext))
                 {
                     SetCurrentValue(DataContextProperty, change.NewValue);
                 }

tests/Avalonia.ReactiveUI.UnitTests/ReactiveUserControlTest.cs

@@ -134,6 +134,22 @@ namespace Avalonia.ReactiveUI.UnitTests
             Assert.Same(vm2, view.ViewModel);
         }
 
+        // https://github.com/AvaloniaUI/Avalonia/issues/15060
+        [Fact]
+        public void Should_Not_Inherit_DataContext_Of_Wrong_Type()
+        {
+            var view = new ExampleView();
+            var root = new TestRoot(view);
+            
+            Assert.Null(view.DataContext);
+            Assert.Null(view.ViewModel);
+
+            root.DataContext = this;
+
+            Assert.Same(this, view.DataContext);
+            Assert.Null(view.ViewModel);
+        }
+
         [Fact]
         public void Should_Not_Overlap_Change_Notifications()
         {