Dear Maintainer,

   * What led up to the situation?

This is on a laptop msi CX61 2PC 8 GB RAM (no RAM problem), with an Optimus
nvidia graphic card that is unused, no packages at all about it ; I only use
the intel one. I have another screen (1920x1080) plugged on the VGA output that
right-extends it's tiny display.

As the Xfce4 fast user switch package disappeared (?), I use the lock screen
trick to open a 2nd session for another user, flipping between both by locking
again and re-login from one to another.

As soon as the screen's locked, the power button goes orange (nvidia card) then
(~10-12s) comes back to blue (intel) and then, both screens are off.
I tried everything, but the only possible outcome is to CTRL-ALT-F1, which
works and revive the laptop screen, then ALT-F7 to come back to Xfce, wait
5-6s, then the padlock appears on both screens, ~12s and the login screen of
lightdm comes to live on both screens.

   * What exactly did you do (or not do) that was effective (or
     ineffective)?

This is the only way I found to get the screens back to live after a lock.
The problem is, it is a long procedure to switch from one user to the other.