Use requestAnimationFrame to prevent hint pop-over from flickering

Before:

![before](https://f.cloud.github.com/assets/55838/1834527/73186870-73de-11e3-9ab4-294c32c05f66.gif)

After:

![after](https://f.cloud.github.com/assets/55838/1834529/7aebee00-73de-11e3-996b-8f2451a0de63.gif)

Fixes #1852.

http://caniuse.com/requestanimationframe