Simple Responsive Menu is a drop-in replacement for the standard Joomla vertical menu.
On a wide enough screen, this page has a vertical menu on the right hand side. Below 800 pixels wide, the menu changes to a select list, fixed at the top left of the window. Try it.
Simple Responsive Menu lets you configure the screen width where the switch takes place, and the position of the select list.
|Simple Responsive Menu User Guide
|Simple Responsive Menu Module