Fri Nov 16, 2012 7:52 am by lax
This happens if the resolution of the game is different from one window to the next.
To correct the issue, open up the in-game graphics/video options and make sure that the windowed mode setting, and the resolution settings, are identical in all windows.
You may need to re-launch the window after changing video settings in the game (e.g. if your Window Layout or window switching breaks as a result). You can just close the window and either re-launch the team (and it will fill in the missing window) or use the individual Slots launch menu (to fill in the missing window manually).