This makes the menu great for iPhones and other mobile phones where display space is a premium. The clever work is done in the jQuery code. It’s implemented as lazy code; the select field is only created when the window width is less than or equal to the defined minimum width. So it works on your desktop browser too!
A jQuery responsive menu for WordPress. When the window size reduces to a defined width (default 640) then the selected navigation menu is converted to a simple select menu.