Gtk.Widget::visibility-notify-event

function callback(widget, event:Gdk.EventVisibility, ):Boolean;

The ::visibility-notify-event will be emitted when the @widget's window is obscured or unobscured.

To receive this signal the Gdk.Window associated to the widget needs to enable the #GDK_VISIBILITY_NOTIFY_MASK mask.

widget: instance of Gtk.Widget that is emitting the signal
event: the Gdk.EventVisibility which triggered this signal.
Returns: true to stop other handlers from being invoked for the event. false to propagate the event further.