Allow tall two-column/curtain headers to be triple-clicked in Firefox

Summary:
Fixes T10905. In Firefox, triple clicking the new headers doesn't select the entire line, so you can't easily copy/paste an entire task title or revision name. It works fine in Safari/Chrome.

This seems to fix that without breaking anything.

Test Plan:

• Viewed headers in Safari, Firefox, Chrome.
• Triple-clicked headers in Safari, Firefox, Chrome.
• Viewed tablet/device layouts in Safari, Firefox, Chrome.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10905

Differential Revision: https://secure.phabricator.com/D15823