List of steps to reproduce (step by step, including full links if applicable):
What happens?:
The last step of central login happens on the desktop domain (https://commons.wikimedia.beta.wmflabs.org/wiki/Special:CentralLogin/complete?token=[redacted]), which fails with "No active login attempt is in progress for your session." (centralauth-error-nologinattempt) - probably due to the cookies not being available. The local login on the mobile domain works (but since the user ends up the desktop domain where they are not logged in, this isn't obvious).