Added failing test for #1245.

tests/Avalonia.Controls.UnitTests/Primitives/PopupTests.cs

@@ -249,6 +249,37 @@ namespace Avalonia.Controls.UnitTests.Primitives
             }
         }
 
+        [Fact]
+        public void DataContextBeginUpdate_Should_Not_Be_Called_For_Controls_That_Dont_Inherit()
+        {
+            using (CreateServices())
+            {
+                TestControl child;
+                var popup = new Popup
+                {
+                    Child = child = new TestControl(),
+                    DataContext = "foo",
+                };
+
+                var beginCalled = false;
+                child.DataContextBeginUpdate += (s, e) => beginCalled = true;
+
+                // Test for #1245. Here, the child's logical parent is the popup but it's not yet
+                // attached to a visual tree because the popup hasn't been opened.
+                Assert.Same(popup, ((ILogical)child).LogicalParent);
+                Assert.Same(popup, child.InheritanceParent);
+                Assert.Null(child.GetVisualRoot());
+
+                popup.Open();
+
+                // #1245 was caused by the fact that DataContextBeginUpdate was called on `target`
+                // when the PopupRoot was created, even though PopupRoot isn't the
+                // InheritanceParent of child.
+                Assert.False(beginCalled);
+            }
+        }
+
+
         private static IDisposable CreateServices()
         {
             var result = AvaloniaLocator.EnterScope();
@@ -304,5 +335,18 @@ namespace Avalonia.Controls.UnitTests.Primitives
         private class PopupContentControl : ContentControl
         {
         }
+
+        private class TestControl : Decorator
+        {
+            public event EventHandler DataContextBeginUpdate;
+
+            public new IAvaloniaObject InheritanceParent => base.InheritanceParent;
+
+            protected override void OnDataContextBeginUpdate()
+            {
+                DataContextBeginUpdate?.Invoke(this, EventArgs.Empty);
+                base.OnDataContextBeginUpdate();
+            }
+        }
     }
 }