The Widget.VisibilityNotifyEvent event utilizes this delegate:

Event data is passed via the Gtk.VisibilityNotifyEventArgs parameter.

To attach a Gtk.VisibilityNotifyEventHandler to an event, add the VisibilityNotifyEventHandler instance to the event. The methods referenced by the VisibilityNotifyEventHandler instance are invoked whenever the event is raised, until the VisibilityNotifyEventHandler is removed from the event.