--Reposted conversation from email thread--
Check out the notification menu on pinterest new/you/messages has some interesting behavior, "new" and "you" never trigger page scrolling but "messages" does when you get to the end, it might be useful to take a look at what the difference in these sections is. This is relevant to the Winter TOC, Echo notifications, and possibly task recommendations. I'd like to see if we can emulate this behavior in all of our scrolling flyouts, it feels much more polished than the page suddenly scrolling when you get to the bottom of a scrolled list in a flyout.
If you don't have an account on pinterest I can demo for you, I'd record a video, but it really is more of a feeling, that video won't really convey. Since we're seeing more and more uses of this element in the interface, it might make sense for one of the 2 teams working on it to componentize something that we can use consistently.
When you're scrolling a list in the flyout, when you get to the end (on e.g. the "you" section), and just keep scrolling, you will never scroll the page no matter how much you scroll (as long as your mouse is still over the flyout)
FWIW, for Echo, it seems like pretty good behavior in Firefox 30.0. When I get to the end and keep scrolling, the first scroll or two at the end do nothing, but if I continue it will start scrolling the page.
I don't have time to work on this right now, though. I recommend you file an enhancement request in Bugzilla or float it on the design list.