Add unit test.

tests/Avalonia.Controls.UnitTests/FlyoutTests.cs

@@ -451,6 +451,28 @@ namespace Avalonia.Controls.UnitTests
                 Assert.Null(popup.Parent);
             }
         }
+        
+        [Fact]
+        public void Should_Reset_Popup_Parent_On_Target_Attach_Following_Detach()
+        {
+            using (CreateServicesWithFocus())
+            {
+                var userControl = new UserControl();
+                var window = PreparedWindow(userControl);
+                window.Show();
+                
+                var flyout = new TestFlyout();
+                flyout.ShowAt(userControl);
+                
+                var popup = Assert.IsType<Popup>(flyout.Popup);
+                Assert.NotNull(popup.Parent);
+                
+                flyout.Hide();
+                
+                flyout.ShowAt(userControl);
+                Assert.NotNull(popup.Parent);
+            }
+        }
 
         [Fact]
         public void ContextFlyout_Can_Be_Set_In_Styles()