Called via the event queue whenever the enable state of a window has changed, either through a call to the window's enable method, or through the enabling/disabling of one of the window's ancestors. The method's argument indicates whether the window is now enabled 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 enable state. Furthermore, if a enable notification event is queued for the window and it reverts its enabled state before the event is dispatched, then the dispatch is cancelled.
If the enable state of a window's ancestor changes while the window is not active (e.g., because it was removed with delete-child), then no enable events are queued for the inactive window. But if the window is later re-activated into an enable state that is different from the window's state when it was de-activated, then an enable event is immediately queued.