Xfce 4.12 it's dumb and stupid. So I need to manually inform the order of monitors and their panel's location

start-displays.sh

#!/bin/bash

# source https://forum.xfce.org/viewtopic.php?pid=46361#p46361
# 
# Check if our external monitor name is in the list connected displays
grep_output=`xrandr | grep "HDMI-1 connected"`

if [ "$grep_output" != "" ]; then
    # The external monitor IS connected. Put it right of there internal and move xfce-panel there

    xrandr --output LVDS-1 --mode 1366x768 --preferred --pos 2560x312 --output HDMI-1 --mode 2560x1080 --primary --pos 0x0
    xfconf-query -v -c xfce4-panel -p /panels/panel-1/output-name -s monitor-1
else
    # The external monitor IS NOT connected.
    xrandr --verbose --output LVDS-1 --mode 1366x768 --pos 0x0
    xfconf-query -v -c xfce4-panel -p /panels/panel-1/output-name -s monitor-0
fi
exit