Previously, login/signup happened on the same domain so if you were logged in when clicking the login link, you were logged in on the login form as well. (There is no login link on the UI when you are logged in, but there are help pages etc. with links. Both the login and signup page are meaningful while logged in - for switching to another account, and for creating an account for another user, respectively.) With SUL3 this is not the case anymore, and this is causing confusion, e.g.:
T391862: 'change password' link does not allow user to find the password reset page
T391324: Missing "send temporary password through email" option in Wikimedia SUL account creation

We should let the central domain know (probably via a URL parameter) that the user is expected to be logged in; when that is the case, there should be some warning message (and we should probably add an extra login redirection, ie. send them to the login page and set returnto / returntoquery to the login or signup page, and make sure RedirectingLoginHookHandler doesn't redirect them back).

See also:
T392359: Visiting Special:UserLogin to switch accounts doesn't work in SUL3
T389064: Notify WebAuthn users about SUL3 changes (T389064#10960666)