Then a reboot and ... bingo ! The artefacts were gone, everything back
in order.
So Sven, you must be right as I can see some mesa packages removed or
downgraded in my last move, hope that bug will be resolved soon.
I know it's a bit late, but Piotr asked me for an output, here it is :
$ inxi -G
Graphics:
Device-1: NVIDIA G96C [GeForce 9400 GT] driver: nouveau v: kernel
Display: x11 server: X.Org v: 1.20.11 with: Xwayland driver: X:
loaded: modesetting gpu: nouveau resolution: 1920x1080~60Hz
OpenGL: renderer: NV96 v: 3.3 Mesa 20.3.5
And for good mesure :
$ uname -a && cat /etc/debian_version
Linux birdynam 5.7.0-3-amd64 #1 SMP Debian 5.7.17-1 (2020-08-23) x86_64
GNU/Linux
bookworm/sid
Thanks again to Piotr and Sven and all the subscribers of this great list
Rudu