Triggered when a viewport is under 600 pixels wide. This functionality depends on JS being enabled, and will naturally fall back to a normal, expanded menu for a client without JS.
Props iandstewart for the original JS file and concept.
git-svn-id: http://svn.automattic.com/wordpress/trunk@20007 1a063a9b-81f0-0310-95a4-ce76da25c4cd
lancewillett