I have two displays, one built-in to a laptop and another external, and
am not able to get any signal to the external one. The same arrangement
works fine with other distros. I've tried with the display managers for
both XFCE4 and LXQt, as well as xrandr.

For example, I have tried the following, but the external screen stays
black:

$ xrandr --output eDP-1  --auto \
         --output HDMI-1 --auto \
         --same-as eDP-1

$ uname -sr; lsb_release -rd
Linux 4.9.0-4-amd64
Description:    Devuan GNU/Linux 2.0 (ascii)
Release:        2.0

What should I be looking for with xrandr to cause a usable signal?

/Lars