Called via the event queue whenever the visibility of a window has changed, either through a call to the window's show, through the showing/hiding of one of the window's ancestors, or through the activating or deactivating of the window or its ancestor in a container (e.g., via delete-child). The method's argument indicates whether the window is now visible or not.
This method is not called when the window is initially created; it is called only after a change from the window's initial visibility. Furthermore, if a show notification event is queued for the window and it reverts its visibility before the event is dispatched, then the dispatch is cancelled.