Describe the bug

The default behaviour for cage is to "Extend the display across all connected outputs". The greeter therefore spans across all monitors. In multihead setups, this often results in the login prompt being split between two or more monitors. For obvious reasons, that isn't desirable.

Steps To Reproduce

Steps to reproduce the behavior:

1. programs.regreet.enable = true
2. services.greetd.enable = true
3. Have multiple monitors
4. On greetd start, observe that the greeter spans across all monitors

Expected behavior

The greeter appears on one monitor or appears separately on each monitor.

Additional context

cage -m last should restrict cage to the last connected device but that results in an otherwise blank screen with the following error:

Failed to create /var/empty/.cache for shader cache (Operation not permitted)---disabling.

Notify maintainers

@fufexan @Peter015

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 6.1.23, NixOS, 23.05 (Stoat), 23.05pre-git`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.13.3`
 - channels(lord-valen): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`