[refresh] navigation is buggy in landscape mode on mobile

[alexgibson]

Description

When trying to use the new navigation on a mobile device in landscape mode, it is hard to operate because it uses fixed positioning when there is very little vertical space.

Steps to reproduce

1. On a mobile device, open https://www-dev.allizom.org/en-US/ and rotate the phone to landscape.
2. Tap "Products" to open the product navigation items.
3. Notice how "Thunderbird" is below the bottom of the visual view port (at least it is on my device).
4. Try to scroll to Thunderbird.
5. Try to close the navigation menu (notice you can't, there's no close button!).

Expected result

1. You should be able to scroll and see all visible items in the navigation menu.
2. You should also be able to close the menu via a button.

Actual result

You become trapped in the navigation and can't leave. The only option is to choose a visible link, or refresh the page and try again.

Environment

iPhone