Bug 514110

SUMMARY

When locking the screen, all my screens will go black and stay black, showing only the mouse cursor, but not offering any way to unlock the screen using a password. Ctrl-Alt-Fn to switch to another VT still works. I have found no way to get my complete session back in those circumstances.

STEPS TO REPRODUCE
1. Press the keyboard shortcut for screenlocking (Meta+L in my case) OR walk away from the computer for long enough for the automatic screen locking to kick in

OBSERVED RESULT

All connected screens will show a completely black screen, apart from the mouse cursor, which can still be moved around. There is a process "kscreenlocker_greet" running and consuming 100% CPU.

EXPECTED RESULT

The screen should show password entry input widget which allows me to get my running session back.

SOFTWARE/OS VERSIONS
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: 
KDE Plasma Version: 6.5.4
KDE Frameworks Version: 6.20.0
Qt Version: 6.10.1

ADDITIONAL INFORMATION

This is on a Wayland session.

There have recently been major updates to KDE Plasma itself, KDE Frameworks and Qt in short succession on my Gentoo system. This bug is a regression from one of those updates, but I don't know which one.

While switching through VTs, I noticed another graphical program running on VT 2 showing a black screen and a differently styled mouse cursor. This is not present during normal operations. (It's probably something left over by sddm.)

When the screenlocker is in this state, the screen can not be unlocked using loginctl. The session can also not be terminated using loginctl. kscreenlocker_greet can also not be killed in this state with an ordinary SIGTERM, only with a SIGKILL. Killing the process in this way will get rid of the kscreenlocker_greet process (it won't come back, either), but the session will still be inaccessible.

The only way I found to get a part of my session back in this case is to kill kwin_wayland. (I was really just trying to get rid of the running, now inaccessible session so I could log in again.) In that case, I get back a mostly empty screen with a working krunner accessible through Alt-F2, and one of the windows from my last session. (A konsole window). The rest of those windows (I think most if not all of them were Firefox) were gone, as was plasmashell.

Note that the nature of this bug makes it very likely to cause data loss.

The bug is not 100% reproducible. It seems to affect mainly the first session after system boot. After having killed a session as described above and then logged out and in again, screen locking seems to work properly again.

My workaround is to disable screen locking. Note that this workaround is completely unacceptable in a lot of settings.