External dialog flickering

When I click "Show CustomDialog Externally" under the Dialogs menu in the MahApps.Metro Demo application, the dialog will shown but I can see a "lag". The content is ok, but the border is shown the upper side of the screen, and then go to the right position.

After 5 seconds, the dialog is closed, but there are some flickering effect on the background.

Thanks for your collaboration!
