Most of AppKit in fact don't use the -[mouseDragged] and -[mouseUp]
events for tracking.
Really? Outside of things like buttons, that's an unfortunate choice,
isn't it? As the "Cocoa Event-Handling Guide" points out, a class
that handles dragging in this manner is "more difficult to extend
without the subclass reimplementing all the dragging code".
You'd think that extending AppKit classes would be a fairly common
thing to want to do...
