Piotr, Sven, Thank you for your help, see below.
I've been already trying some downgrades, of :
Downgrade: xserver-xorg-video-nouveau:amd64 (1:1.0.17-2, 1:1.0.17-1),
xserver-xorg-input-evdev:amd64 (1:2.10.6-2+b1, 1:2.10.6-2),
xserver-xorg-core:amd64 (2:21.1.4-1, 2:1.20.11-1+deb11u1)
Purge: xserver-xephyr:amd64 (2:21.1.4-1)
Downgrade: xserver-xorg:amd64 (1:7.7+23, 1:7.7+22), xserver-common:amd64
(2:21.1.4-1, 2:1.20.11-1+deb11u1)
... the stable versions of those packages.
But nothing changed.
Then, Piotr drew my attention to this libdrm-nouveau2 :
Downgrade: libdrm-nouveau2:amd64 (2.4.112-3, 2.4.104-1),
libdrm-nouveau2:i386 (2.4.112-3, 2.4.104-1)
... still no luck.
Looking at its dependencies, I noticed a libdrm2 and libdrm-common
packages. Their downgrade had some consequences :
Install: libllvm11:amd64 (1:11.1.0-6+b2, automatic),
kwin-wayland-backend-x11:amd64 (4:5.25.4-2, automatic),
libdrm-intel1:amd64 (2.4.104-1, automatic)
Downgrade: libglx-mesa0:amd64 (22.2.0~rc2-1, 20.3.5-1), libgbm1:amd64
(22.2.0~rc2-1, 20.3.5-1), libgl1-mesa-dri:amd64 (22.2.0~rc2-1,
20.3.5-1), libdrm-common:amd64 (2.4.112-3, 2.4.104-1), xwayland:amd64
(2:22.1.3-1, 2:1.20.11-1+deb11u1), libglapi-mesa:amd64 (22.2.0~rc2-1,
20.3.5-1), libdrm-amdgpu1:amd64 (2.4.112-3, 2.4.104-1),
libdrm-radeon1:amd64 (2.4.112-3, 2.4.104-1), libdrm-radeon1:i386
(2.4.112-3, 2.4.104-1), libdrm2:amd64 (2.4.112-3, 2.4.104-1),
libdrm2:i386 (2.4.112-3, 2.4.104-1), libegl-mesa0:amd64 (22.2.0~rc2-1,
20.3.5-1), libdrm-intel1:i386 (2.4.112-3, 2.4.104-1)
Remove: libxcb-present0:i386 (1.15-1), libglx-mesa0:i386 (22.2.0~rc2-1),
libglvnd0:i386 (1.4.0-1), libxshmfence1:i386 (1.3-1), libdecor-0-0:i386
(0.1.0-3), libfltk1.1:i386 (1.1.10-29), libwayland-cursor0:i386
(1.21.0-1), libxcb-dri2-0:i386 (1.15-1), libxcb-dri3-0:i386 (1.15-1),
libgbm1:i386 (22.2.0~rc2-1), libwayland-server0:i386 (1.21.0-1),
libglx0:i386 (1.4.0-1), libdrm-nouveau2:i386 (2.4.104-1), libllvm14:i386
(1:14.0.6-2), kwin-wayland-backend-drm:amd64 (4:5.25.4-2), libz3-4:i386
(4.8.12-1+b1), libxcvt0:amd64 (0.1.2-1), libgl1-mesa-dri:i386
(22.2.0~rc2-1), libxkbcommon0:i386 (1.4.1-1), freeglut3:i386 (2.8.1-6),
libxcb-glx0:i386 (1.15-1), libwayland-egl1:i386 (1.21.0-1),
libglapi-mesa:i386 (22.2.0~rc2-1), libsdl2-2.0-0:i386 (2.0.22+dfsg-6),
libdrm-amdgpu1:i386 (2.4.112-3), libgl1:i386 (1.4.0-1),
libwayland-client0:i386 (1.21.0-1), libxcb-sync1:i386 (1.15-1),
libatomic1:i386 (12.1.0-8), libxcb-xfixes0:i386 (1.15-1)
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