Visit http://en.m.wikipedia.beta.wmflabs.org/wiki/Main_Page whilst anon in stable without any query parameters. The menu is invisible.
One of the browser tests relating to the main menu is failing as a result:
e.g. https://integration.wikimedia.org/ci/job/browsertests-MobileFrontend-SmokeTests-linux-chrome-sauce/117/testReport/junit/(root)/Menus%20open%20correct%20page%20for%20anonymous%20users/Check_links_in_menu/
It seems under some circumstances the menu is not rendering at all.
I'm seeing this on mediawiki.org @phuedx so we should fix this before next deploy...