If I use GDM and Gnome, the default gsettings make guesses about how to scale the screen. I’m not happy with the results. This is easy to configure for my normal user with gsettings or dconf-editor, but in order to make those same changes for the login screen, I need to set them for the GDM user.
If I do something like sudo -u gdm dconf-editor I can make the changes, but they do not seem to take hold or persist through a reboot. I see the services.xserver.desktopManager.gnome3.extraGSettingsOverrides option, but am not sure if that’s something that might help me solve this problem.
As you found out, extraGSettingsOverrides are not passed to a display manager consistently.
As for changing the keys with database editor, GDM does not have a writable home directory, nor does it use user-db in its profile so it has nowhere to persist those changes.
What we want to do instead is to generate a database programmatically with a NixOS module:
I made some progress, but this didn’t end up totally resolving my problem and I’m not clear whether it’s a bug or not. I wrote a module similar to the one in the link which sets the window scaling to 1 (no scaling) and font scaling to 1.5x. The font scaling takes hold, but the window scaling seems to be stuck at auto-detect.
Since one change is working, I know that I’ve gotten things in the right place. I’ve checked my spelling about 6 times in the configuration file, so I don’t think it’s that.