Hide window endpoints for visualizing derivatives in Windows 7/Firefox

Summary: Fix https://s3.amazonaws.com/uploads.hipchat.com/6574/98377/oqxx622xiefgu2q/upload.png (Windows 7/Firefox)

Test Plan: Tested locally; plan to test on Windows 7/Firefox

Reviewers: eater

Reviewed By: eater

CC: alpert, michaelchang

Differential Revision: http://phabricator.khanacademy.org/D1063