[Guide] Debian Testing (Bullseye) with GNOME is broken: this is how it is fixed

Today users of Debian Testing (Bullseye) with GNOME will have found that the latest update has broken the desktop environment, preventing access to even GDM, so the user cannot access even the most elementary part of the graphical part of the system.

Now that I am using Debian Testing, I have eaten this error and at first, I tried to change GDM for LightDM first and SDDM later, but despite letting me start the graphical session the desktop did not show up. The origin is apparently that the Debian Testing maintainers have not measured the dependencies well, causing a general breakdown of the GNOME installations and showing the following image every time you try to start the system.

After seeing the thread of the failure in the Debian Reddit, I took the solution proposed at the beginning and decided to make it more detailed so that only the broken part was repaired without the rest of the system being affected since for this you need to enable the Unstable repository of the distribution.

Luckily the only thing that is broken is GNOME, so the virtual consoles can be accessed. To do this, press simultaneously, for example, the key combination Ctrl + alt + F3. With the virtual console in front, the user has to log in with his password and then install a text editor that works in the console. In my case, I have used Nano, but you can use Vim or whatever the user wants.

sudo apt install nano

Once the console text editor has been installed, the Debian Unstable repository must be enabled. In case you have chosen a different editor than Nano, in the commands you have to put the one chosen by the user instead.

sudo nano /etc/apt/sources.list.d/unstable.list

Once the file is opened, enter the following text, save the changes and exit the text editor:

deb http://ftp.debian.org/debian/ unstable main
deb-src http://ftp.debian.org/debian/ unstable main

Now it's time to refresh the repositories to load the Debian branch packages Unstable:

sudo apt update

With the packages of the Unstable branch loaded, GNOME Shell is repaired by installing the corresponding package indicating the source branch:

sudo apt -t unstable install gnome-shell

Debian's Unstable branch can be dangerous and it is not that you want the entire system to be updated by that branch, but rather the purpose is to repair a broken GNOME Shell. To do this, the file created to enable the Unstable repository is deleted:

sudo rm /etc/apt/sources.list.d/unstable.list

The repositories are refreshed again to remove the packages from Unstable and have only those from Testing:

sudo apt update

Finally, the system is restarted:

sudo shutdown -r now

After performing these steps Debian Testing Bullseye would have to work correctly again, now with version 3.36 of both GNOME Shell and Mutter.