A window spanning two monitors is blank on one display on Mac OS X

When placing a window between two screens, the window will only be drawn on the screen that has the most important part of the window on it. This is not specific to DisplayLink screens and implies that a window can only be on one screen at a time. 

It is caused by a concept introduced in OS X 10.9 (Mavericks) of "displays as spaces". To disable this feature go to the Mission Control preference and disable "Displays have separate Spaces". This however will also disable other features like the menu bar on all screens.

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