Unknown display (known also as phantom display) seems like a popular, come and go, bug in Linux. It’s associated with false positive output port (like a display port) and isn’t bound to any particular release (for Ubuntu, there are reports from version 12.x, 13.x and I got it on 16.x, after a non-security lib update). It often comes with range of side effects which aren’t described in “unknown display” forum posts, so when looking for a solution, you may not know that’s the real cause of a problem. Also, don’t let system fool you, turning an unknown display OFF (in display settings) won’t do much.
- locked resolution – System locks itself (all displays) in “unknown display’s” maximum resolution – usually 800×600 or 1024×768,
- missing greeter interface – After boot, you are greeted by empty/blank login screen, wallpaper only, no forms. Lightdm will show up after you navigate with cursor to a “real” display (it jumps between active screens), GDM’s UI stays on the “unknown display” (it’s still possible to login blind). Your interface is fine, it just loads in a non-existent area.
- no screen edges – It’s possible to navigate to “unknown display”, there is no edge between real desktop areas and these coming from phantom screen
If you run into one of this issues and have an “unknown display” present, fixing the display issue will probably make the other one go away. Fortunately, solution is very simple – turn off the fake port. First of all, you need a basic idea how your monitors are connected. Most likely you are using an HDMI, Display Port or DVI, but this issue affects VGA too – take a look at your ports and then go to terminal.
Check status of output graphical ports via xrandr.
sudo xrandr -q
For more friendly and adequate list you can check:
Knowing how your displays are connected and what’s the status according to the system, find the most suspicious, connected port. Foremost, it’s a port you are not using, and it may have weirdly low available resolutions. Once you have your suspect, it’s time to disable it via grub config.
Sudo nano /etc/default/grub
Add the port in format of “video=
Save the file with Ctrl+X, confirm with an Y and accept the current naming. Update grub, so it has new settings, with disabled display:
and reboot the system:
Phantom display should be gone with any side effects. If unknown display is still present in the settings, you picked a wrong port and need to try again.