public void set_monitor (int monitor_num)
Informs GTK+ on which monitor a menu should be popped up.
This function should be called from a MenuPositionFunc if the menu should not appear on the same monitor as the pointer. This information can’t be reliably inferred from the coordinates returned by a MenuPositionFunc, since, for very long menus, these coordinates may extend beyond the monitor boundaries or even the screen boundaries.
the number of the monitor on which the menu should be popped up