Noticed this on our own install after upgrading recently, and perhaps only in firefox (haven't tested much in Chrome), but the div with a class of differential-changeset gets a vertical scrollbar in certain situations (haven't determined what triggers it exactly), which can interrupt scrolling when you're trying to scroll down a long diff (as the div takes over the scrolling, but doesn't actually have any scrolling to do, outside of a few pixels).

It happens to me on this diff here on secure.phabricator.com:

https://secure.phabricator.com/D8999

Here's a screenshot of the dual scrollbars:

{F151214}

If I add overflow-y: hidden to the div (.differential-changeset CSS block) the scrollbar goes away and all seems ok, but not sure if hiding the vertical overflow on that div is acceptable or not.