GDM monitor configuration

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Hey oberstal,

I ran into this myself after getting frustrated with GDM always trying to display on the TV I have plugged in (which is usually off) rather than my main monitor. Digging around in gdm.nix I noticed there is already a pulseaudio config being symlinked into /run/gdm using systemd.tmpfiles.rules and figured that pattern would work for monitors.xml as well.

Here’s what I have in my configuration.nix and it seems to be working well:

  systemd.tmpfiles.rules = [
    "L+ /run/gdm/.config/monitors.xml - - - - ${pkgs.writeText "gdm-monitors.xml" ''
      <!-- this should all be copied from your ~/.config/monitors.xml -->
      <monitors version="2">
        <configuration>
            <!-- REDACTED -->
        </configuration>
      </monitors>
    ''}"
  ];

Here is where the tmpfiles are set up in gdm.nix, if you’re curious: https://github.com/NixOS/nixpkgs/blob/cd63096d6d887d689543a0b97743d28995bc9bc3/nixos/modules/services/x11/display-managers/gdm.nix#L152-L160

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