Bug#956121: Blank screen on 2nd display using wayland

[Pierre Cheynier]

Package: mutter
Version: 3.34.4-1
Severity: important

Dear Maintainer,

When upgrading mutter from 3.34.3-1 to 3.34.4-1 (testing), my second
screen (eDP / 3440x1440) is completely white, the second one show
glitches and freezes during a few tens sec. probably while trying to
sync the other one before aborting.
Downgrading to 3.34.3-1 + restart gdm3 fixes the issue.

HW setup: i915

$ lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core
processor Graphics Controller (rev 09)
01:00.0 VGA compatible controller: NVIDIA Corporation GK107GLM [Quadro
K2000M] (rev a1)

Not able to see something specific neither here:
https://gitlab.gnome.org/GNOME/mutter/-/compare/3.34.4...3.34.5#0cc1139e3347f573ae1feee5b73dbc8a8a21fcfa
nor there:
https://salsa.debian.org/gnome-team/mutter/-/commits/upstream/3.34.x
(and BTW most 3.34.5 patches have been backported).

Anything relevant that would help troubleshooting the issue?

-- System Information:
Debian Release: bullseye/sid
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-debug'), (500,
'testing'), (500, 'stable'), (2, 'unstable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 5.4.0-4-amd64 (SMP w/8 CPU cores)
Kernel taint flags: TAINT_WARN
Locale: LANG=fr_FR.utf8, LC_CTYPE=fr_FR.utf8 (charmap=UTF-8),
LANGUAGE=fr_FR.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages mutter depends on:
ii  gnome-settings-daemon-common  3.36.0-1
ii  gsettings-desktop-schemas     3.36.0-1
ii  libc6                         2.30-4
ii  libgles2                      1.3.1-1
ii  libglib2.0-0                  2.64.1-1
ii  libmutter-5-0                 3.34.3-1
ii  libwayland-server0            1.18.0-1
ii  libx11-6                      2:1.6.9-2
ii  libxcomposite1                1:0.4.4-2
ii  mutter-common                 3.34.3-1
ii  zenity                        3.32.0-5

mutter recommends no packages.

Versions of packages mutter suggests:
ii  gnome-control-center  1:3.36.1-1
ii  xdg-user-dirs         0.17-2

-- no debconf information