When we are clipping (due to fullscreen), certain operations are bugged (such as creating another window on top, grabbing the window during a size-move operation, changing mouse capture, etc) so we must temporarily unclip first.
Seems unclipping it temporarily during such operation is enough to fix this.
This probably wasn't very obvious in practice because most times "fullscreen" means a single non-resizeable window with no menus and so on (e.g. fullscreen games), but that's not always the case.
-- v2: winex11.drv: Correctly handle clipping for fullscreen windows.