Does this repro here? Can you be more specific about your Browser/OS? Offhand I am not able to reproduce the issue on Mac OS X / Firefox 30.

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.

(I can't repro on Firefox 30 on OSX either.)

The rule is "overflow-x: auto", meaning we intend for the area to be scrollable if the width of the diff is wider than the viewport. If you zoom in and scroll left and right, then focus is correctly given to the box and you have to move the mouse out to continue scrolling down. Browsers that are displaying it naturally should not stop or trigger left and right scrolling in that area. Can I ask how old your install is?

Firefox does seem to be bad at re-engaging the up/down scrolling though, Chrome and Safari pick back up immediately.

haha Firefox on my Ubuntu VM is 16.0.0. Guess I have some updates to install.

I can't repro this even on Ubuntu 12.10 / Firefox 29. My mousewheel always horizontally scrolls even over the overflow-x areas:

2ad501873ae4415fbc2748026f25d87daf916a72 might have fixed this, depending on where your install is.
T4987 was the original bug.

You are at Facebook I gather?

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.

Maybe you could make a screencast of the issue on this install? I'm highly skeptical that overflow is the problem, merely the symptom.

Our general understanding is the Facebook install is not kept up to date, and cherry picked for fixes. @peng.li.315213 might have more specifics as to where it is currently from HEAD.

(I think his main account is @sowedance.)

haha ok, sorry.

Our HEAD is 4a6d2e9 (D8792) but we do have the fix for T4987 in. I am not quite sure but I think the issue here is that when you are holding the mouse wheel and then move the mouse to scroll up/down (only supported by FF I guess). For other files in https://secure.phabricator.com/D9871 it works all fine, except resources/sql/autopatches/20140710.workerpriority.sql. Although there is no horizontal scroll bar showing up, somehow it is still preventing scrolling up/down if you start trying this scroll mode there.

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.

Oh well the original FB post was talking about autoscrolling. Anyway I cannot really repro the regular mouse wheel scrolling scenario.

Then maybe its a system font thing. What font is the default for you on GTK2?

I'm just using a regular mouse wheel on FF 29 / Gnome.

@dancol let me know if you can't do a screencast, not much else actionable here right now.