Detecting a change in the active display relies on watching for the NSWorkspaceActiveDisplayDidChangeNotification
message which is not documented by Apple. While this message has been around at least since OS X 10.9, because it is undocumented, we cannot be positive that Apple won't remove it in a future OS X update. Because this watcher works by listening for posted messages, should Apple remove this notification, your callback function will no longer receive messages about this change -- it won't crash or change behavior in any other way. This documentation will be updated if this status changes.