R1:6288d8a7d4b5

Support scrollbar snapback on Windows

Summary: See D11472. I eyeballed the "140" number by screenshotting / measuring in Paint.

Test Plan: Made the snapback thing return `true` and got snapback on OSX.

Reviewers: chad

Reviewed By: chad

Subscribers: avivey, epriestley

Differential Revision: https://secure.phabricator.com/D11485