Arch Linux

Steps to reproduce
------------------
Use KDE Plasma, SDDM, NVIDIA proprietary drivers.
Suspend the computer.
Wake it up and unlock.
Lock the screen again.

Problem
-------
(Whenever locking the screen in any way), Instead of showing the lock screen normally, the screen just freezes and becomes unresponsive, other than showing the mouse cursor movement. You can't unlock the screen, other than going to TTY and `sudo loginctl unlock-sessions`.
The problem persists from after the first suspend and until a reboot (`systemctl restart sddm` was also observed to help).

Package versions
----------------
kscreenlocker 5.19.4-1
nvidia 450.57-6
sddm 0.18.1-2
qt5-base 5.15.0-4
xorg-server 1.20.8-3

Hardware
--------
NVIDIA GeForce GTX960

Upstream bug
------------
https://bugs.kde.org/show_bug.cgi?id=347772#c51
- exactly comment 51 reflects my symptoms the best; the rest may or may not be related, but one thing in common is that the workaround of "QSG_RENDER_LOOP=basic" helps in the same way:
https://bugs.kde.org/show_bug.cgi?id=347772#c22

Workaround
----------
Running `QSG_RENDER_LOOP=basic /usr/lib/kscreenlocker_greet` works fine, unlike when done without the environment variable. I applied the workaround permanently this way:

$ sudo mv /usr/lib/kscreenlocker_greet{,.bin}
$ printf '#!/bin/sh\nQSG_RENDER_LOOP=basic exec /usr/lib/kscreenlocker_greet.bin "$@"\n' |
sudo tee /usr/lib/kscreenlocker_greet
$ sudo chmod +x /usr/lib/kscreenlocker_greet