nixos/pantheon, pantheon.elementary-greeter: Default to pantheon-wayland session

See elementary/session-settings issue 91.

nixos/doc/manual/release-notes/rl-2511.section.md

···
 
 - mate-wayland-session 1.28.4 is now using the default wayfire decorator instead of firedecor, thus `services.xserver.desktopManager.mate.enableWaylandSession` is no longer shipping firedecor. If you are experiencing broken window decorations after upgrade, backup and remove `~/.config/mate/wayfire.ini` and re-login.
 
+- Due to [deprecation of gnome-session X11 support](https://blogs.gnome.org/alatiera/2025/06/08/the-x11-session-removal/), `services.desktopManager.pantheon` now defaults to pantheon-wayland session. The X11 session will be removed before gnome-session 49 lands.
+
 - `services.gitea` supports sending notifications with sendmail again. To do this, activate the parameter `services.gitea.mailerUseSendmail` and configure SMTP server.
 
 - Revamp of the ACME certificate acquisication and renewal process to help scale systems with lots (100+) of certificates.

nixos/modules/services/desktop-managers/pantheon.nix

···
 
       # Without this, elementary LightDM greeter will pre-select non-existent `default` session
       # https://github.com/elementary/greeter/issues/368
-      services.displayManager.defaultSession = mkDefault "pantheon";
+      services.displayManager.defaultSession = mkDefault "pantheon-wayland";
+      programs.dconf.profiles.user.databases = [
+        {
+          settings."io/elementary/greeter" = {
+            last-session-type = "pantheon-wayland";
+          };
+        }
+      ];
 
       environment.extraInit = ''
         ${concatMapStrings (p: ''

nixos/tests/pantheon.nix

···
       services.displayManager = {
         autoLogin.enable = true;
         autoLogin.user = nodes.machine.users.users.alice.name;
-        defaultSession = "pantheon-wayland";
       };
 
       # We ship pantheon.appcenter by default when this is enabled.

pkgs/desktops/pantheon/desktop/elementary-greeter/default.nix

···
   gnome-settings-daemon,
   mutter,
   elementary-icon-theme,
+  elementary-settings-daemon,
   wingpanel-with-indicators,
   elementary-gtk-theme,
   nixos-artwork,
···
 
 stdenv.mkDerivation rec {
   pname = "elementary-greeter";
-  version = "8.0.1";
+  # To allow overriding last-session-type.
+  # nixpkgs-update: no auto update
+  version = "8.0.1-unstable-2025-09-14";
 
   src = fetchFromGitHub {
     owner = "elementary";
     repo = "greeter";
-    rev = version;
-    sha256 = "sha256-T/tI8WRVbTLdolDYa98M2Vm26p0xhGiai74lXAlpQ8k=";
+    rev = "2461ad1be4a4d0e541879abe869cf8911f505215";
+    hash = "sha256-rDlaL2KCm0tz73cwHLgNAD7Ddbn1QFJVa+Syh5eTfWo=";
   };
 
   patches = [
···
   buildInputs = [
     accountsservice
     elementary-icon-theme
+    elementary-settings-daemon
     gala # for io.elementary.desktop.background
     gnome-desktop
     gnome-settings-daemon