When trying to grab the window pointer off the notification in a
windowDidBecomeKey implementation, I kept getting segfaults calling
notification->object(). The second argument of these needs to be a SEL.
https://developer.apple.com/documentation/objectivec/class_addmethod(_:_:_:_:)?language=objc#Discussion
I imagine existing code is getting by by setting the window information
in the delegate's context userdata, which works fine when you only have
one window as you can avoid needing to call notification->object(),
until you want one delegate assigned to two windows, hard to work around.
