NixOS configuration broke, brightness keys don't work, no idea what I changed

The reason I’m asking about See diff between generation configurations. Or see previous generation's configuration.nix is because my brightness keys stopped working, and it is now difficult to figure out what exactly I changed to break it.

TLDR: there’s no way to see what my previous configuration was, and I unfortunately did not version control my configuration.nix.

The only thing I can try to do is rollback, but that’s not helpful because I won’t know what the actual problem is. Overall it is difficult to actually know what a problem is, as rolling back doesn’t really help you with that.

Not sure what your question is here, but it may not be the configuration.nix changes that did anything. Did you update your channel in the meantime ? Did you reboot in the meantime ? To know for sure, you could rollback and see if the problem disappears. If not, it is not linked to that system change.

As far as I know, I didn’t. I haven’t explicitly ran with the --update flag. I did also reboot a few times. I decided to move forward and try tweaking configuration.nix, so I added several packages, and the problem seems to have fixed itself after some changes, though I don’t recall changing anything major except setting xkbOptions and adding some packages.