black screen showing mouse pointer only.

[Germano Massullo]

In past ~3 days some plasma related updates have broken something: on
many machines with different FOSS VGA drivers I got a black screen
showing mouse pointer only. Usually after SDDM login, but sometimes even
while kscreenlock is active. The only way to escape from the situation
is to reboot the system.
Has anybody experienced a similar problem?