Revert use of "user-select: all" to modify tab selection behavior
However, this has a side effect: the sequence is now selected with a single click. I didn't read the docuementation on the CSS property thoroughly and missed this in testing, since I was focused on drag-selection behavior.
This behavior is enough of a net negative that I think we're in a worse state overall; revert it.
Test Plan: Straight revert.
Differential Revision: https://secure.phabricator.com/D21429