This bug was reported on the debian BTS 4 years ago by Elrond, it still applies. When a window is waiting for placement, it is displayed as nine squares that follows the cursor, a click then place it. If while the window is not placed, the used switch VT (ctrl+alt+Fx) and switch back to the X server, a copy of the nine squares will be left on the screen. It will be cleared by moving some windows over it for instance.
Mass closure: This bug has been untouched for more than six years, and is not obviously still valid. Please reopen this bug or file a new report if you continue to experience issues with current releases.
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