Checks whether the given event pops open the system menu in the top-left corner of the window (Windows only). If the window's system menu is opened, #t is returned, otherwise #f is returned.
![[previous]](../icons/left.gif){kind=icon}
![[up]](../icons/up.gif){kind=icon}
![[next]](../icons/right.gif){kind=icon}
![[index]](../icons/index.gif){kind=icon}