When completing a compatibility SSO login, if the login was already consumed the error page is confusing

[sandhose]

When a compatibility SSO login is being completed, MAS redirects to the client redirect_uri. If that URI is a native callback, it can happen that the browser window stays open, and sometimes even reload.

When that happens, the SSO login gets "consumed" twice, which currently gives this feedback:

Two things to fix:

• make the feedback more explicit (like "the login already completed" or something)
• make the redirect happen ~client side? Like display an HTML page with an <meta http-equiv="refresh" content="0;{redirect_uri}"> or something, and a link on the page to open the callback

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#1865