stamen/modestmaps-js — comment on commit 1e21b70481fbb991cc280127e90eff45de5d1dfd
File:  line 0
By RandomEtc:

Not sure what the root cause is. Momentum scrolling is a bit weird because it's simulating wheel events after you stop touching the touchpad. It seems like the simulated events get mysteriously stopped/cancelled if a new element appears under the mouse. But in other testing I've done it seems like the mouse position behaves as if the pointer has moved, even though it hasn't.

If you scroll slowly you never see the issue, but if you do a two-fingered flick with scroll momentum on you might see stutters and stops. Also Lion's iOS-style elastic-page-scroll-limit behavior is triggered, so perhaps the scroll is being applied at the window level instead of on the map parent where it would be canceled.