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

Those divs serve different purposes. The new _zoomDiv is the d3 technique of having an offscreen div element to simulate a scroll event and normalize the wheel delta value. The mouseDiv in my fix is a shield that sits on top of the map parent to give continuous mouse scroll events, otherwise mouse wheel momentum seems to get interrupted by DOM changes. Note that the `MM.addEvent` call for wheel events is listening on map.parent in the current version - that's what causes the glitches.

Basically it should be possible to "throw" the zoom of the map and let mouse wheel momentum do its thing, just like it does if you scroll a long web page.