What problem does it solve?
As a disadvantage: it becomes difficult to understand the boot sequence and so less hackable

It's simply the canonical way to do things. I'm not sure how it makes the boot sequence difficult, in the end the display manager will have a session file.

@volth The main benefit is reducing the NixOS modules in favour of upstream solutions, just like we do with tmpfiles.d, systemd services and so on.

@volth i use Pantheon for some time now :)

From IRC with @jtojnar

worldofpeace https://github.com/NixOS/nixpkgs/pull/78421 should probably be mentioned in release notes as it disallows using Xfce as a DE with a different WM

worldofpeace: We use it as such in the example `defaultSession = "xfce+icewm";` in the release highlights 🤣️

Jan Tojnar: oh no, that's actually no good. I know a lot of people in NixOS use xfce with a different wm and they totally wouldn't be happy with that

Ideally the Xfce session would use sessionPackages and users can still construct + wm sessions.
@jtojnar can we even do that? If not, I guess we can revert this on master and release.

We could provide both upstream session and our old one but dm=xfce with wm=none would clash with it.

We could provide option for switching between the two but that would be ugly.

Finally, we could add new option desktopManager.mixNmatchableDesktops and have:

services.xserver.desktopManager.session = map (desktop: {
      name = "mm-${desktop}";
      bgSupport = true;
      start = ''
        ${pkgs.dex}/bin/dex "${desktop}" &
        waitPID=$!
      '';
    }) cfg.desktopManager.mixNmatchableDesktops;

and then add desktopManager.mixNmatchableDesktops = "xfce"; to this module.

We could provide both upstream session and our old one but dm=xfce with wm=none would clash with it.

We could provide option for switching between the two but that would be ugly.

Finally, we could add new option desktopManager.mixNmatchableDesktops and have:

services.xserver.desktopManager.session = map (desktop: {
      name = "mm-${desktop}";
      bgSupport = true;
      start = ''
        ${pkgs.dex}/bin/dex "${desktop}" &
        waitPID=$!
      '';
    }) cfg.desktopManager.mixNmatchableDesktops;

and then add desktopManager.mixNmatchableDesktops = "xfce"; to this module.

It seems we have more features to implement 😄
I'm going to revert this one for now.