severity 973474 grave thanks Upgrading the bug severity since it requires killing one's session to recover from, taking all unsaved work with it.
I installed Debian testing today in a VM and have the same problem in the GNOME+X11 session. When I type the wrong password in the lock screen, there is a spinner and after a few seconds an error message. When I type the correct password, it *immediately* goes back to asking for the password. I can CTRL+ALT+F2 to log in at a text console to debug this, but I'm not sure how to go about this. "DISPLAY=:0 xlsclients" does not show a screensaver process running. Is the locking done by gnome-shell itself these days? I do not see anything related in syslog. Attaching a screenshot of what was logged while I switched to the locked session, attempted a login, and switched back to text console, in case someone wants to take a look.
