Admin menu broken on Menu Locations screen at <782px media query

[johnbillion]

Seen on an Android Nexus 4 and an iOS7 iPad in portrait mode, both of which trigger the <782px media query.

To reproduce:

• Visit the Appearance -> Menus screen on a device such as the above
• Click the Menu Locations tab
• Try to toggle the main admin menu using the hamburger button in the toolbar

Note that the menu doesn't open.

I cannot reproduce this in a desktop browser resized down to 782px. Possibly something to do with touch devices.

[azaozz]

In 26822:

Fix error in postbox.js attempting to change screen columns on mobile devices orientation changes, fixes #26493.