HDMI screen not recognised after upgrade Gnome 3.20

[Thomas SIMON]

Hello all,

I'm not sure if this is the right mailing list or if I should ask for
help somewhere else (I'm not sure it's a bug so far...). Basically, the
problem is simple, after upgrade of Ubuntu GNOME 16.04 with staging PPA,
my external screen was working (plugged in with HDMI), even after the
first restart. I went away and then switched off entirely my laptop,
when I came back, no way to get the external screen recognised by the
system (doesn't show on "Display" of system settings and doesn't show
with randr command). 

Then, well, I used ppa-purge, but that didn't change anything and tried
to plug in the screen with a VGA cable (and USB adaptor) and that still
wasn't recognised. Finally, I tested the screen with another laptop
(running Win 10) and that got the screen to work, so I'm pretty sure
it's an issue related to the upgrade to Gnome 3.20. 

Would anybody be able to advise please?

Thanks a lot in advance,

Thomas