Probably fix scratchpad race condition

Summary: Probably fixes https://trello.com/c/bwkvMurk/1196-sporadic-clicking-causes-a-scratchpad-svg-to-block-useful-clicks. Not 100% sure.

Test Plan: In local mode, clicked to show the scratchpad.

Reviewers: eater, marcia

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D6848