Comment 29 for bug 1245474

I've found something that may be useful both for diagnosis and as a workaround.

If you define a "display-setu-script" in the lightdm configuration, like this:

[SeatDefaults]
display-setup-script=/path/to/display-setup.sh

and put this command in the "display-setup.sh":

xset dpms 120 125 130

this does NOT fix the problem. But, if you write instead:

(
  sleep 10
  xset dpms 120 125 130
) &

it DOES help, i.e. login screen goes dark after two minutes.
Apparently, it implies that "something" that is started as a part of the greeter "session", i.e. the greeter itself, or one of the indicators, resets the DPMS settings.

In the meanwhile, my display-setup script may be used as a workaround by those who need it.