Fix flickering tooltips in Chrome when the tip container overlaps the triggering element

Summary:
Fixes T8440. See that task for discussion.

Ref T13216. See PHI976.

Test Plan:
In Chrome, hovered a timestamp and moved the mouse up to the "overlap" area (see T8440). Before: flickered like crazy. After: no flickering.

(I couldn't reproduce the original issue in modern Firefox or Safari.)

Reviewers: amckinley, avivey

Reviewed By: avivey

Maniphest Tasks: T8440, T13216

Differential Revision: https://secure.phabricator.com/D19808