Turning off the display (un55hu6950) and then turning it on will not yield a picture. Unless the xrandr configuration is refreshed by changing the resolution and then changing it back to 4K. Particularly problematic on the GDM login screen, where I can’t just set a shortcut to do this quickly.
I’m looking into ways of setting GDM at a lower resolution than the main GNOME desktop, which I can fix quickly.
I’m running NVIDIA 349.16 on Fedora 22
The following steps will always reproduce it
click the lock screen button, the PC should go to sleep immediately
turn off the display
turn on the display
press any key, the lock screen will show on all other monitors except the 4K one
if you manage to log in without the 4K monitor, even on GNOME, you will only see output from the other non 4K monitors.