(define-gboxed-variant-cstruct event "GdkEvent"
  (type event-type)
  (window (g:object window))
  (send-event (:boolean :int8))
  (:variant type
            ...
            ((:visibility-notify) event-visibility
             (state visibility-state))
  ... ))  

Generated when the window visibility status has changed. The gdk:event-visibility structure has been deprecated since version 3.12 and should not be used in newly written code. Modern composited windowing systems with pervasive transparency make it impossible to track the visibility of a window reliably, so this event can not be guaranteed to provide useful information.