Twenty Thirteen: Expanded menu disappears on small screens

[johnbillion]

Steps to reproduce:

1. View the Twenty Thirteen theme (demo site: ​http://twentythirteendemo.wordpress.com/) using a device with a screen narrower than 643 pixels (such as a mobile phone in portrait orientation). You should be served the narrowest layout of Twenty Thirteen, which includes a dropdown navigation menu in place of the horizontal one.
2. Open the menu.
3. Scroll down in order to access one of the menu item's that are off the bottom of the screen.

Observe that the menu abruptly disappears once you scroll past the header image, making it impossible to access menu items further down the menu.

I tested this using an iPhone on landscape and portrait and whilst it does go below and whilst it appears you can scroll it's very jumpy and doesn't really let you click.

I worked a few things using CSS but it seems due to the jumping we need a JavaScript solution here. I can confirm though it's only an issue in landscape.

In 23533:

Twenty Thirteen: disable fixed navbar on small screens. Props obenland, fixes #23647.

Also closes #23526, #23535, and #23566

If the resolution is "fixed", milestone should stay.