abc-nix
lightdm worked for me after upgrading the core live system, installing seatd, elogind, libpam-elogind, dbus-x11, lightdm, xserver-xorg-then and icewm, then installing to disk (didn’t work on the live session, so maybe this is your issue), and after the reboot lightdm worked (I did install the “recommended packages” with lighdm though, so maybe that is the issue).
Lightdm already has a runit service folder if you are really using runit. Please check in /usr/share/runit/sv and in /etc/sv (if not present in the last folder copy it over from the first folder).
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