This question is answered on Unix StackExchange: https://unix.stackexchange.com/questions/101809/how-can-i-automatically-update-my-monitor-layout-in-xfce/120945#120945 However, since it's not possible to mark this question as a duplicate of a question on another site, I'm just going to copy the answer here.
One way is to create an udev rule, but as I wanted something more portable, I have this bash script. It relies on inotifywait support, does not have some kind of loops and is considered efficient.
external-lcd.sh
#!/bin/sh
# inspired of:
# http://unix.stackexchange.com/questions/4489/a-tool-for-automatically-applying-randr-configuration- when-external-display-is-p
# http://ozlabs.org/~jk/docs/mergefb/
# http://superuser.com/questions/181517/how-to-execute-a-command-whenever-a-file-changes/181543#181543
export MONITOR2=/sys/class/drm/card0-VGA-1/status
while inotifywait -e modify,create,delete,open,close,close_write,access $MONITOR2;
dmode="$(cat $MONITOR2)"
do
if [ "${dmode}" = disconnected ]; then
/usr/bin/xrandr --auto
echo "${dmode}"
elif [ "${dmode}" = connected ];then
/usr/bin/xrandr --output VGA1 --auto --right-of LVDS1
echo "${dmode}"
else /usr/bin/xrandr --auto
echo "${dmode}"
fi
done
Don't forget to make the file executable (chmod +x external-lcd.sh
). Then just start it whenever you launch your DE.
I am using this on archlinux so I think it should work. You can change xrandr parameters or swap it to use arandr configurations.