Fix flickering hovercards

Summary:
Refs T1048, T2902 - This should fix flickering hovercards. Probably does not fix displaced hovercards, though could (because of the flickering doing something bad to rendering).

I'm bad with JS D:

Test Plan: Tried out plenty of reload & hover combos. No flickering anymore. Never again.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1048

Differential Revision: https://secure.phabricator.com/D5607