chrome ToT r624798, platform 11574, eve, SingleProcessMash enabled

* Open a page that is > 1 screen tall (e.g. about:version, xkcd.com)
* Trackpad fling scroll to the bottom of the page
* Fling scroll down a few more times

Sometimes (1 in 5?) the page "bounces up", scrolling up by a few lines. It can be tricky to repro.

It happens both with default display resolution (2x on eve) and custom resolution (zoomed out a bit).

It never happens with touch scrolling.

It never happens when SingleProcessMash is off.

Suggestions for other repro steps to narrow this down?