Ah, okay.
So, the workaround for others:
- Ctrl-Alt-F2 to a terminal.
- Login.
- sudo nano /etc/lightdm/lightdm.conf
- Comment out the line with greeter-session=lightdm-webkit2-greeter
- Create a new line with greeter-session=lightdm-gtk-greeter
- Save your file.
- Run pacman -S lightdm-gtk-greeter
Those steps at least got me running again. I’m listing them here to help someone else who might stumble into this.