Login screen goes to broken internal display, working external display shows black (unless xorg.conf set up)

[Jani Uusitalo]

This is a laptop with a broken internal display ("LVDS1" in xrandr output), and therefore connected to a working external LCD ("VGA1").

After upgrading from Precise to Trusty, boot ends with a black screen (on the external display). The greeter sound effect is played. Typing my password (I'm the sole user) does log me in, and brings up the desktop on the display.

So the login functionality is there, just the visuals are missing. And they do flash by (corrupted) when the desktop is loading; if I set automatic login for myself, I get a desktop with the login screen as the background image (I'll attach a screenshot).

There are existing, similar reports but all with subtle differences from this (such as Bug #1252748, where the user only gets black screen half the time) or too vague for me to claim to be the same issue with certainty (such as Bug #1242213). A lightdm restart (such as in Bug #969489) doesn't fix this (i.e. black screen still after lightdm restart).

I'm not sure whether this is a lightdm issue or a unity-greeter one, so my reporting this against the latter is just a guess.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity-greeter 14.04.1-0ubuntu1
ProcVersionSignature: Ubuntu 3.12.0-4.10-generic 3.12.1
Uname: Linux 3.12.0-4-generic x86_64
ApportVersion: 2.12.7-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Nov 23 20:53:43 2013
InstallationDate: Installed on 2012-05-25 (547 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
SourcePackage: unity-greeter
UpgradeStatus: Upgraded to trusty on 2013-11-17 (6 days ago)

[Jani Uusitalo (uusijani)]

Further details: mouse cursor is briefly visible just prior to the black (login) screen. GDM does show up on the display, but in too big a resolution, so that I only get some of the grey background visible on screen and still have to log in blindly.

It occurred to me that with lightdm, the symptom appears as I would expect if it failed to do monitor detection properly and thinks it's displaying the login screen on the (broken) internal display only. Well, apart from the funky "login screen desktop background" phenomenon (which could be a separate issue though).

(Just to be clear: this same combination of broken internal + working external display did display the login screen correctly on the external one in Precise.)

[Jani Uusitalo (uusijani)]

I've made some progress. By setting up an xorg.conf with a modeline for the external display, I got graphics on the (lightdm) login screen to show up on it. But the login input fields weren't on this screen (it was just the background), so this was further evidence that there's monitor misdetection going on (greeter/lightdm happily thinks it's displaying the input fields on the broken internal LCD).

So I further tweaked xorg.conf to force X to ignore the internal display, and now I've got a working login screen back on the external display. I'll attach my xorg.conf below; it is a satisfactory workaround.

[Jani Uusitalo (uusijani)]

With this workaround in place, desktop wallpaper (on the working display) cannot be set using the 'Appearance' configuration tool, and with automatic login the 'login screen as background' effect (the image attached to #1) is there too. This could be Bug #1159430, and as mentioned there, I can work around it using Compiz' Wallpaper plugin (i.e. setting a wallpaper in Compiz sets it on the external display).

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Bastian Gebhardt (buzz-t)]

i have the same issue on a desktop with only one monitor.

Side note: When i boot in recovery mode and choose "resume normal boot" without doing anything else in the recovery console, i get the normal lightdm screen and i'm able to login normally.