Make xrandr config permanent

Install autorandr

pacman -S autorandr

Apply config (for instance, by running screen.cast.external)

screen.cast.external

Save current config

autorandr --save external

Create a file /etc/X11/xinit/xinitrc.d/10-autorandr.sh with the following contents

#!/bin/sh
[ -x /usr/bin/autorandr ] && /usr/bin/autorandr --change

Make it executable

chmod a+x /etc/X11/xinit/xinitrc.d/10-autorandr.sh

Logout/login.