HDMI only works after login, DP works since kernel boots. How to make HDMI behave like DP?

Support Graphics & Display
21

Finally a shed of light! So, I’m experimenting with SDDM’s Xsetup script this way:

xrandr --listmonitors >> /tmp/monlog.txt
xrandr --output HDMI-1-0 --auto 2>&1 >> /tmp/monlog.txt # enable external monitor
xrandr --output eDP-1 --off 2>&1 >> /tmp/monlog.txt # disable internal monitor
xrandr --listmonitors >> /tmp/monlog.txt

Guess what’s the content of /tmp/monlog.txt

Monitors: 1
 0: +*eDP 2560/344x1440/193+0+0  eDP
Monitors: 2
 0: +*eDP 2560/344x1440/193+0+0  eDP
 1: +HDMI-1-0 2560/597x1440/336+0+0  HDMI-1-0

both turning on the HDMI connected monitor and turning off the internal display (this one actually fails silently, no idea who’s to blame, again no issue when the external monitor is connected through DisplayPort) outputs nothing, but --listmonitors only list my HDMI connected monitor AFTER it’s enabled. Damn weirdo…

This should be enough for a workaround. Only bluetooth device reconnect on that login screen issue now, but that’s for another post (if I can’t find the solution on my own).