PSA: there's a problem with the `gdm` package (gnome display/login manager) on MNT Pocket/Reform, or on Debian unstable for that matter, please don't upgrade the package right now https://community.mnt.re/t/attention-when-upgrading-only-black-screen-with-cursor-instead-of-gdm/4150
Attention when upgrading: only black screen with cursor instead of gdm

Hi, when upgrading to the latest packages from Debian unstable and rebooting, you will only see a black screen with a cursor on the upper left corner instead of gdm coming up with the login prompt. I was unable to track down the problem so far. Help is welcome. But you should probably hold off on upgrading your system until this is fixed. Update 1: Pressing ctrl+alt+F5 (or F2, F3, F4, …) gets you to a textual login prompt. You can start sway from there or continue working on the text termina...

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@mntmn Too late :-/
@duck if you still have shell access to the machine, try `apt update` and `apt install libmozjs-140-0` again, it should be fixed now!
@mntmn yep, thanks, just an apt update & upgrade solved it. Somehow it didn't know about version 140.6.0-1 as suggested in joschs post.