nixos/xserver: `xserver.nix` doesn't enable any window manager by default

[steelman]

Nixpkgs version

• Stable (25.11)

Describe the bug

In a minimal default configuration lightdm does not start X11 session. Apparently no window/desktop manager is enabled along with lightdm in xserver.nix. When I set services.xserver.windowManager.awesome.enable to true, rebuilt the system and restarted display-manager.service, I was finally able to log into an X11 session.

Steps to reproduce

1. Boot the minimal ISO image to install NixOS
2. run nixos-generate-config
3. uncomment services.xserver.enable = true;
4. run nixos-install and reboot
5. try logging in as a normal user

Expected behaviour

Lightdm starts an X11 session with some default window/desktop manager.

Screenshots

No response

Relevant log output

/var/log/lightdm/lightdm.log.old:[+1115.30s] DEBUG: Seat seat0: Failed to find session configuration default
/var/log/lightdm/lightdm.log.old:[+1115.30s] DEBUG: Seat seat0: Can't find session 'default'


lightdm tries to open `/nix/store/i9qxi7827jwbs3793p4i39nrmv44lylb-desktops/share/xsessions/default.desktop` but fails because the directory is empty.

Additional context

No response

System metadata

• system: "x86_64-linux"
• host os: Linux 6.12.76, NixOS, 25.11 (Xantusia), 25.11.7702.48652e9d5aea
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.31.2
• nixpkgs: /nix/store/dkhiwsn2mxi293907klnpzdwa6m2xb0i-nixos-25.11.7702.48652e9d5aea/nixos

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