But it's even better than that: Even if the window is bigger (wider or taller) than the actual screen, the full content (including the stuff which is outside) is grabbed.
One minor issue: In most apps it is not possible to move a window out of the top of the screen, which would enable you to resize it bigger than the actual screen. Reason: The window is blocked by the menu bar. So if you try it with your secondary screen (in dual head mode), it works.

