Man, not to be a Negative Nancy, but it just does not work if you resize your browser even slightly quickly.
Hum, what browser and OS are you using ? If the browser supports media-queries, it will automatically adapt, it's the media-query technlogy that's build so. I've tried resizing it very quickly on Chrome, Firefox and even IE9 and everything works smoothly, so I'm really wondering how you got it not to work. Also, no "real" user quickly resizes its browser "even slightly quickly" every two seconds in "real world", most of the time such menus are loaded on mobile or tablets on which you can't resize :)