When using a multiple-screen display, each screen displaying a different part of the desktop, and the main screen is located on the larger one, the windows managers has some troubles displaying stacked dialog windows. For instance, the  the windows switcher appears as a black area (see attached screen capture), and the screensaver display dialog doesn't appear at all, leading to potential password leak when typing blindly...

When the main screen is located on the laptop screen, when both screen display the same content, or when only the external screen is used, the problem doesn't appear.