When a dropdown menu would render in a way that hides it offscreen, try a different alignment

Summary:
Depends on D20382. Ref T13272. When something near the edge of the screen has a dropdown menu, we currently may render the menu offscreen.

Instead, keep the menu onscreen.

(This is happening because I'm adding dropdown menus to tab query panels.)

Test Plan:
Before:

After:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20383