Implement more correct white-icon menu behavior

Summary: I didn't get this quite right.

Test Plan:

• Clicked to open, saw white, then closed by:
  • Clicking document outside menu;
  • clicking menu icon again;
  • clicking a different menu icon.
• In all three cases, got correct close + un-white behavior.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10967