Bonjour
Since I don't have the correct solution to your problem, I'll suggest the one I used when lightdm was buggy:
I installed sddm
sudo pacman -Sy xfce4 xfce4-goodies
sudo pacman -Sy sddm-openrc
sudo rc-update add sddm default
Auto login :
sudo usermod -a -G autologin user
sudo mkdir /etc/sddm.conf.d
sudo touch /etc/sddm.conf.d/autologin.conf
sudo nano /etc/sddm.conf.d/autologin.conf
or maybe
sudo mousepad /etc/sddm.conf.d/autologin.conf
## Add this lines :
[Autologin]
User=tintin
Session=xfce
sudo pacman -Rsu lightdm