Unfortunately this is hard for me to test myself because I have yet to get a Magic Mouse of my own, but I've been told by my testers who do have a magic mouse that momentum scrolling isn't working in my app. I've not subclassed NSScrollView, but scrollview's document view is all custom. I have not overridden scrollWheel: anywhere, either, and yet momentum apparently just isn't working. I'm not even sure where to begin. I thought it'd just send scrollWheel events and things would take care of themselves. (Scrolling with a wheel or on the MBP trackpad works as expected.) Obviously I must somehow be doing something that's stopping it, but I don't even know where to begin. Thoughts?
I figured this out awhile ago and the problem was that on scroll, I was doing a lot of fancy view manipulation somewhat like how on the iPhone the UITableView adds and removes views as they scroll on and off screen. This worked great for performance - but the more I got into OSX programming, the more I realized this was wrong for OSX (but the right idea of iPhone).
Anyway, what's really going on, it seems, is that when you do something like a wheel scroll, the scroll event is sent to the view that's under the mouse cursor and then it ripples down the responders/views until it gets somewhere that handles it. Normally this isn't a problem, but with momentum scrolling the OS is really just sending ever smaller scrollWheel events to the view that was under the cursor at the time the momentum started. That means if the view is removed during the course of scrolling (because it scrolls off screen or something), it breaks the chain and the momentum stops because the view that's still getting the scrollWheel messages is no longer in the view hierarchy.
The "simple" fix is to not remove the view that got the last scrollWheel event - even if it's off screen. The better fix (and the one I went with) is to not try to use NSViews like they are UIViews and instead just draw the content using drawRect. :) Not only is that about a billion times faster, it Just Works(tm) with momentum scrolling because it's how OSX expects things to be done.
Repeat after me: OSX is not iPhoneOS.. :P