After logging in on, say, hu.wikipedia.org:
- the login itself works
- second-level domain cookies work - I am instantly logged in on en.wikipedia.org
- edge login doesn't work (not necessarily, see comments) - I am not instantly logged in an any of the sister projects
- autologin works - if I wait a second or two, I get logged in on any project.
Tested in Chrome 65, 83 and 84 on Ubuntu (on mobile and desktop; also with and without "keep me logged in" - neither seems to make much difference). samesite-sandbox.glitch.me suggests I am not opted into the new SameSite treatment yet.
I think the start of this coincided with the SameSite patch rollout.
See also T258121: Logging in to a wiki sometimes fails with 'sessionfailure' error (coinciding with SameSite rollout) which might be a different incarnation of the same cookie handling problem.