However:
* Signing off didn't work: nothing happens. (LIkely a Gnome issue)
* Switching to the VT, it's cloned on both screens.
* Restarting gdm3 with the HDMI cable still plugged in don't change anything: Integrated Screen is off while External Screen is on, when in X.
* But restarting gdm3 with the HDMI cable off make the Integrated Screen work again.