I just did confirmed that the current in the repos breaks my KDE:
mvanorde@mvanorde-manjaro ~ master ● grep mesa /var/log/pacman.log|tail
[2020-10-06T15:08:18-0400] [ALPM] upgraded libva-mesa-driver (20.1.6-1 -> 20.1.8-1)
[2020-11-04T18:21:10-0500] [ALPM] upgraded mesa (20.1.8-1 -> 20.2.1-1)
[2020-11-04T18:21:42-0500] [ALPM] upgraded lib32-libva-mesa-driver (20.1.8-1 -> 20.2.1-1)
[2020-11-04T18:21:43-0500] [ALPM] upgraded lib32-mesa (20.1.8-1 -> 20.2.1-1)
[2020-11-04T18:21:47-0500] [ALPM] upgraded libva-mesa-driver (20.1.8-1 -> 20.2.1-1)
[2020-11-12T15:54:35-0500] [PACMAN] Running 'pacman -U mesa-20.1.8-1-x86_64.pkg.tar.zst'
[2020-11-12T15:54:42-0500] [ALPM] downgraded mesa (20.2.1-1 -> 20.1.8-1)
[2020-11-12T16:36:29-0500] [ALPM] upgraded mesa (20.1.8-1 -> 20.2.1-1)
[2020-11-12T16:38:36-0500] [PACMAN] Running 'pacman -U mesa-20.1.8-1-x86_64.pkg.tar.zst'
[2020-11-12T16:38:39-0500] [ALPM] downgraded mesa (20.2.1-1 -> 20.1.8-1)
The timestamp 2020-11-12T16:36:29-0500 was from running:
sudo pacman-mirrors -f && sudo pacman -Syyu
Immediately following, I restarted plasmashell and got the same symptoms:
kquitapp5 plasmashell && kstart5 plasmashell
Re-downgraded and restarted plasmashell once more, and everything is stable again.