Login screen (gdm3) is purple blank after switching userA -> userB -> userA again

Login screen (gdm3) is purple blank after switching userA -> userB -> userA again.

Sometimes need to switch 5 times.. but definitely after around 5 times gdm3 breaks (it shows just a blank purple screen with the cursor).

## Workaround

Ctrl+Alt+F3 (or F4.. F5...) until I get to the user I need to (and unlock the screensaver there).
OR: switch to the console and run "exec sudo systemctl restart gdm3". ("exec" is not necessary but highly desirable from the security point of view, so there won't be a logged in session).

## Workaround 2

To not use gdm3. There are alternatives such as sddm or ligthdm:

sudo apt install lightdm

Ctrl+Alt+F5 (drop to a console)
sudo systemctl stop gdm3
exec sudo systemctl start display-manager

## Details

arno@z390-linux:~$ dpkg -l gdm3
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-==============-=======================-============-=================================
ii gdm3 3.36.3-0ubuntu0.20.04.4 amd64 GNOME Display Manager

arno@z390-linux:~$ cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-5.11.0-25-generic root=UUID=a39515ec-3bed-493b-8286-3904a4aac9de ro quiet splash vt.handoff=7

arno@z390-linux:~$ nvidia-smi
Wed Aug 11 16:42:07 2021
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 460.91.03 Driver Version: 460.91.03 CUDA Version: 11.2 |
|-------------------------------+----------------------+----------------------+
| GPU Name Persistence-M| Bus-Id Disp.A | Volatile Uncorr. ECC |
| Fan Temp Perf Pwr:Usage/Cap| Memory-Usage | GPU-Util Compute M. |
| | | MIG M. |
|===============================+======================+======================|
| 0 GeForce RTX 207... Off | 00000000:01:00.0 On | N/A |
| 0% 37C P8 17W / 215W | 473MiB / 7979MiB | 1% Default |
| | | N/A |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Processes: |
| GPU GI CI PID Type Process name GPU Memory |
| ID ID Usage |
|=============================================================================|
| 0 N/A N/A 2211 G /usr/lib/xorg/Xorg 35MiB |
| 0 N/A N/A 10496 G /usr/lib/xorg/Xorg 106MiB |
| 0 N/A N/A 10626 G /usr/bin/gnome-shell 157MiB |
| 0 N/A N/A 16828 G /usr/lib/xorg/Xorg 35MiB |
| 0 N/A N/A 38063 G ...AAAAAAAAA= --shared-files 80MiB |
+-----------------------------------------------------------------------------+

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gdm3 3.36.3-0ubuntu0.20.04.4
ProcVersionSignature: Ubuntu 5.11.0-25.27~20.04.1-generic 5.11.22
Uname: Linux 5.11.0-25-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Wed Aug 11 16:37:35 2021
InstallationDate: Installed on 2020-09-26 (318 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)