It repros on the phabricator.com version. I'm *not* talking about the center-button-hold scrolling system, i.e., autoscroll, but regular mouse wheel scrolling. The problem seems to depend on text metrics, which is why it's hard to repro for some people.

I am. I don't see how T4987 might help: the problem is the overflow handling. The problem seems to arise when there's just a little bit of *vertical* play in that element; analysis of the DOM element indicates that setting overflow-x is also setting overflow-y to auto.

I'm using the normal GTK2 build of Firefox 30 on Linux; I can easily repro on https://secure.phabricator.com/D9871 --- put mouse over the "create table" in that diff and try to scroll up and down. It may be easier to repro if you try several zoom levels.