I just installed Nixos and Xmonad on top of X11 (in VirtualBox).
I can't find a command to trigger the shutdown from within the GUI. When I quit Xmonad I'm facing the graphical login, without any button.
This is what I did so far:
ALT + SHIFT + Q to quit Xmonad CTRL + ALT + F1 to open a console login as root $ halt
Then I power off the VM when it's ready. I think you agree that it is kind of weird way to shut down a system :-)
EDIT: just found that the default DM for Nixos is SLiM which has been abandoned and I don't know how to configure it. I'm going to install a different, lightweight DM and try to configure it.