My laptop comes with

lspci | grep VGA

00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09) 
01:00.0 VGA compatible controller: NVIDIA Corporation GK107GLM [Quadro K2000M] (rev a1)

The laptop is able to detect DisplayPort monitors without any issue. Now I tried connecting an HDMI monitor, but it is not detected. The monitor works fine when I boot to Windows.

After googling a bit, I found this Nvidia / Wayland / Ubuntu 18.04 : Unable to find display on any available system
Where it is mentioned 'Wayland does not work with nvidia'. I had the 390 drivers installed tried switching to Nouveau (reboot always) and still the same.

Then I found this External monitor not detected on Ubuntu 18.04 and followed it up with this. So I ran sudo dpkg-reconfigure lightdm, as gdm was not installed.

The problem wasn't solved after rebooting, although I could see a change, as the login screen now had a lower resolution. I think I was still using nouveau at this time. I then changed to nvidia 390 and this time the login screen appears in the HDMI display. After logging in, the HDMI display turns to black, the laptop display turns on and the HDMI remains undetected. If I lock the screen, the HDMI display turns on again, showing the login screen to enter the password. Also now the login screen is always sent to the HDMI monitor, even if it is not connected. So I cannot login without the monitor attached... The HDMI monitor is at tty7. tty1-6 are displayed on the laptop's screen, text mode.

Then I tried disabling Wayland by following the instructions here, but this also didn't change anything. If I change the option on the login screen from Ubuntu (Wayland) (white circle) to Ubuntu default (with the ubuntu logo), then the computer uses the HDMI screen only for login and desktop enviroment. If I logout and switch back to the option Ubuntu (Wayland), I get back the previous behaviour.

During all this time, nvidia-settings

ERROR: Unable to find display on any available system

And I could not find a fix for this online. Although it may help, it's not necessary, as the system is able to use the HDMI monitor just fine to display the login/use selection screen.

During all this time, the 'Displays' window only showed my laptop's built-in display.

This is what I get from xrandr, on my laptop screen, even after reconfiguring to lightdm

Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
XWAYLAND0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 340mm x 190mm
    1920x1080     59.96*+

This is what I get from xrandr, on the HDMI screen:

Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 16384 x 16384
DP-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 480mm x 270mm
    1920x1080     60.00*+  59.94    50.00
    1680x1050     59.95
    1600x900      60.00
    1440x900      59.89
    1400x1050     59.98
    1280x1024     75.02    60.02
    1280x800      59.81
    1280x720      60.00    59.94    50.00
    1152x864      75.00
    1024x768      75.03    60.00
    800x600       75.00    60.32
    720x576       50.00
    720x480       59.94
    640x480       75.00    59.94    59.93
DP-1 disconnected (normal left inverted right x axis y axis) 
DP-2 disconnected (normal left inverted right x axis y axis) 
DP-3 disconnected (normal left inverted right x axis y axis) 
DP-4 disconnected (normal left inverted right x axis y axis)
DP-5 disconnected (normal left inverted right x axis y axis)
  • I'm facing the same problem with Linux Mint. Since Mint comes with Lightdm as the default display manager I've installed gdm3. But this showed to be not enough and then I've uninstalled all the NVIDIA drivers. So, using the default xserver drivers and gdm3 solved the problem, but I might say in an unsatisfactory way because I use linux mainly for multimedia work and can't use the full potential of my hardware without the Nvidia drivers. Many people has this problem too and looks like there is no specific reason for this to happen and it's sad. – Fabio Silva Apr 9 '19 at 0:26

I fixed the problem by upgrading to 18.10.

Using a VGA cable I was able to dual monitor but even that was was problematic, the system wouldn't allow any other resolution than 1024x768. Dual-booting to Windows proved there was no problem with the monitor, cables, etc.

I wish the LTS versions would be a little bit more stable, even for less mainstream stuff like this.

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