My problem is basically this: https://github.com/NixOS/nixpkgs/issues/24563
When I open System Settings as a regular user, it lets me modify the power settings, but the changes have no effect, and I get this error message when I open it as root, the whole power settings page is disabled and can't be modified:
However, in the Startup and Shutdown menu there's nothing related to the power management. Session saving is off, and through Synaptic I updated the powerdevil, powerdevil-dev and powerdevil-data packages with no result. It was unable to update powerdevil-dbg, whatever that is.Power Management configuration module could not be loaded.
The Power Management Service appears not to be running.
This can be solved by starting or scheduling it inside "Startup and Shutdown"
The ksmserverrc file doesn't contain powerdevil entries, so the solution written in the above linked topic doesn't work for me.
It's really frustrating because whenever I have to leave the laptop for a few minutes, I have to start playing a movie to prevent the screen from switching off. Even though the default setting is to lower the brightness after 5 minutes and switch off after 10, the brightness is lowered after 2 minutes and the screen is switched off after 5, which is the default setting for the battery-powered usage (but the laptop is plugged in and fully charged, so really what the hell). I had a full reinstall recently, but this problem was present in the previous installation too. Is this a common bug? Could a separately installed screensaver overwrite these default settings?