Bug#769960: gdm3: upgrade from 3.14.0 to 3.14.1 results in blank screen

[Richard B. Kreckel]

Suddenly, a few hours ago, the same symptoms started on a machine that
had been running Debian jessie fine for many months. Also, 'systemctl'
reports that gdm runs as expected, but there's no screen content. Also,
'startx' works and launches a Gnome session. 'journalctl'
reports nothing unusual (the 'Failed to give slave programs access to
the display. Trying to proceed' message seems to be normal.) I don't
see anything about a suspicious llvmpipe. Graphics card is an NVidia
GeForce GT 430 with the nvidia driver.

This is triple strange. I have no idea what caused this. I didn't change
anything except for today's upgrade of cups-related packages, but a
similar upgrade on another system did not cause a similar problem. (The
upgrade to jessie 8.1 has happened three days ago and did definitely not
cause this.)

I attached to gdm3 and Xorg processes using gdb and compared the bt with
a working system, but was unable to spot any difference.

Ideas how to debug this, anybody?