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: https://develop.svn.wordpress.org/trunk@20007 602fd350-edb4-49c9-b593-d223f7449a82