feat(gnome): Declaratively set monitor config

conf.d/gnome-monitors.xml

@@ -0,0 +1,71 @@
+<monitors version="2">
+  <configuration>
+    <logicalmonitor>
+      <x>0</x>
+      <y>1440</y>
+      <scale>1</scale>
+      <primary>yes</primary>
+      <monitor>
+        <monitorspec>
+          <connector>DP-2</connector>
+          <vendor>GSM</vendor>
+          <product>LG ULTRAWIDE</product>
+          <serial>207NTYT1A970</serial>
+        </monitorspec>
+        <mode>
+          <width>5120</width>
+          <height>1440</height>
+          <rate>143.979</rate>
+        </mode>
+      </monitor>
+    </logicalmonitor>
+    <logicalmonitor>
+      <x>5120</x>
+      <y>320</y>
+      <scale>1</scale>
+      <transform>
+        <rotation>left</rotation>
+        <flipped>no</flipped>
+      </transform>
+      <monitor>
+        <monitorspec>
+          <connector>HDMI-0</connector>
+          <vendor>VSC</vendor>
+          <product>VX2478 Series</product>
+          <serial>UPN172200089</serial>
+        </monitorspec>
+        <mode>
+          <width>2560</width>
+          <height>1440</height>
+          <rate>59.951</rate>
+        </mode>
+      </monitor>
+    </logicalmonitor>
+    <logicalmonitor>
+      <x>1680</x>
+      <y>0</y>
+      <scale>1</scale>
+      <monitor>
+        <monitorspec>
+          <connector>HDMI-1</connector>
+          <vendor>SPT</vendor>
+          <product>Sceptre C35</product>
+          <serial>0000000000000</serial>
+        </monitorspec>
+        <mode>
+          <width>3440</width>
+          <height>1440</height>
+          <rate>59.999</rate>
+        </mode>
+      </monitor>
+    </logicalmonitor>
+    <disabled>
+      <monitorspec>
+        <connector>DP-1</connector>
+        <vendor>SAM</vendor>
+        <product>SAMSUNG</product>
+        <serial>0x01000e00</serial>
+      </monitorspec>
+    </disabled>
+  </configuration>
+</monitors>

home-manager/modules/gnome/default.nix

@@ -1,32 +1,35 @@
-{ isLinux, ... }:
+{ isLinux, pkgs, ... }:
 if isLinux then {
   imports = [ ./dconf.nix ];
-  # workaround for https://github.com/nix-community/home-manager/issues/3447
-  # autostart 1Password in the background so I can use the SSH agent without manually opening the app first
-  xdg.configFile."autostart/1password.desktop".text = ''
-    [Desktop Entry]
-    Name=1Password
-    Exec=1password --silent
-    Terminal=false
-    Type=Application
-    Icon=1password
-    StartupWMClass=1Password
-    Comment=Password manager and secure wallet
-    MimeType=x-scheme-handler/onepassword;
-    Categories=Office;
-  '';
-  # autostart Signal
-  xdg.configFile."autostart/signal-desktop.desktop".text = ''
-    [Desktop Entry]
-    Name=Signal
-    Exec=signal-desktop --no-sandbox --start-in-tray %U
-    Terminal=false
-    Type=Application
-    Icon=signal-desktop
-    StartupWMClass=Signal
-    Comment=Private messaging from your desktop
-    MimeType=x-scheme-handler/sgnl;x-scheme-handler/signalcaptcha;
-    Categories=Network;InstantMessaging;Chat;
-  '';
+  xdg.configFile = {
+    "monitors.xml".source = ../../../conf.d/gnome-monitors.xml;
+    # workaround for https://github.com/nix-community/home-manager/issues/3447
+    # autostart 1Password in the background so I can use the SSH agent without manually opening the app first
+    "autostart/1password.desktop".text = ''
+      [Desktop Entry]
+      Name=1Password
+      Exec=1password --silent
+      Terminal=false
+      Type=Application
+      Icon=1password
+      StartupWMClass=1Password
+      Comment=Password manager and secure wallet
+      MimeType=x-scheme-handler/onepassword;
+      Categories=Office;
+    '';
+    # autostart Signal
+    "autostart/signal-desktop.desktop".text = ''
+      [Desktop Entry]
+      Name=Signal
+      Exec=signal-desktop --no-sandbox --start-in-tray %U
+      Terminal=false
+      Type=Application
+      Icon=signal-desktop
+      StartupWMClass=Signal
+      Comment=Private messaging from your desktop
+      MimeType=x-scheme-handler/sgnl;x-scheme-handler/signalcaptcha;
+      Categories=Network;InstantMessaging;Chat;
+    '';
+  };
 } else
   { }

nixos-modules/desktop_environment.nix

@@ -1,4 +1,8 @@
-{ pkgs, ... }: {
+{ pkgs, ... }:
+let
+  monitor_config = pkgs.writeText "gdm_monitors.xml"
+    (builtins.readFile ../conf.d/gnome-monitors.xml);
+in {
   nixpkgs.overlays = [
     # GNOME 46: triple-buffering-v4-46
     # See:
@@ -18,6 +22,10 @@
       });
     })
   ];
+  # make GDM use the same monitor config as GNOME;
+  # see: https://discourse.nixos.org/t/set-external-monitor-as-primary-display-before-login/37844/4
+  systemd.tmpfiles.rules =
+    [ "L+ /run/gdm/.config/monitors.xml - - - - ${monitor_config}" ];
   services = {
     displayManager.defaultSession = "gnome-xorg";
     xserver = {