In my browser (Firefox), the top-level navigation collapses into a hamburger menu when the window is reduced to about 930 pixels wide. We should keep it as a real nav menu instead of turning it into a hamburger menu until the window is resized to typical mobile sizes.
(Even then, in general collapsing navigation into a hamburger menu is a design error and should be re-thought. See http://jamesarcher.me/hamburger-menu)