Fix flickering tooltips in Chrome when the tip container overlaps the triggering element
Fixes T8440. See that task for discussion.
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
Differential Revision: https://secure.phabricator.com/D19808