Desktop doesn't cover entire screen when rotating monitors.

[Jeremy Phelps]

Sometimes (I can't reliably reproduce the problem) the desktop decides that it doesn't need to cover all of my screens. When this happens, the background only covers part of the screen, and you can't get a right-click menu on any part of the screen that has no background.

Also, windows that are placed over the no-background portion of the desktop leave trails of themselves when you move them.

The attached video shows me right-clicking and dragging a CCSM window around.

The problem seems to present itself when I change monitors' rotation and position relative to each other (which I have to do frequently because the settings never persist across reboots).

If I unplug my monitors and plug them back in one at a time, I can recover the full desktop (but then I have to deal with bug number 1818710).