Comment 2 for bug 229298

I encountered a similar problem (See attached screenshot).

For me it occurred just after configuring my second monitor (which I configured using nvidia-settings) and without my xorg.conf saved, and I was getting some other funny effects as well (like windows opening on the border between my screens, toolbars spanning both screens, etc).

However, after fiddling with my settings and restarting X the problem seemed to go away.

If you're still encountering this bug, it might be worth posting your xorg.conf and/or more information on your setup.