Try logging into a non-GNOME Xorg session on Workspaces if you can. Then use the 'xrandr' command to set a mode to turn on one of the unused virtual monitors. If that causes the DCV client to go blank then we have proven it's not mutter's fault.
Try logging into a non-GNOME Xorg session on Workspaces if you can. Then use the 'xrandr' command to set a mode to turn on one of the unused virtual monitors. If that causes the DCV client to go blank then we have proven it's not mutter's fault.