I tried the solution posted in Ubuntu 15.10 random lightdm crash but when I got to the step of reinstalling lightdm* a couple errors came up (I didn't write down what they were). Now I'm stuck in GNOME with the following issues:
Removing/purging lightdm* doesn't seem to be working. Several packages say "is not installed, so not removed". Another package has unmet dependencies (plasma-workspace) but it is not going to be installed.
If I try to dpkg-reconfigure gdm and set it to lightdm, when I reboot the computer tries to start in "low graphics" mode and basically fails to start lightdm. Would love some help here...
For the record, I'm running an HP Spectre x360.
john@john-desktop:~$ sudo apt-get remove lightdm*
Reading package lists... Done
Building dependency tree
Reading state information... Done
Note, selecting 'liblightdm-gobject-0-0' for regex 'lightdm*'
Note, selecting 'ubuntustudio-lightdm-theme' for regex 'lightdm*'
Note, selecting 'lightdm-gtk-greeter-config' for regex 'lightdm*'
Note, selecting 'razorqt-lightdm-greeter-config' for regex 'lightdm*'
Note, selecting 'liblightdm-qt5-3-0' for regex 'lightdm*'
Note, selecting 'liblightdm-qt-3-0' for regex 'lightdm*'
Note, selecting 'mythbuntu-lightdm-theme' for regex 'lightdm*'
Note, selecting 'lightdm-gtk-greeter' for regex 'lightdm*'
Note, selecting 'gir1.2-lightdm-1' for regex 'lightdm*'
Note, selecting 'liblightdm-qt5-3-dev' for regex 'lightdm*'
Note, selecting 'lightdm-remote-session-uccsconfigure' for regex 'lightdm*'
Note, selecting 'liblightdm-qt-3-dev' for regex 'lightdm*'
Note, selecting 'lightdm-kde-greeter' for regex 'lightdm*'
Note, selecting 'lightdm-gtk-greeter-settings' for regex 'lightdm*'
Note, selecting 'ubuntu-mate-lightdm-theme' for regex 'lightdm*'
Note, selecting 'razorqt-lightdm-greeter' for regex 'lightdm*'
Note, selecting 'liblightdm-gobject-1-dev' for regex 'lightdm*'
Note, selecting 'lightdm-gtk' for regex 'lightdm*'
Note, selecting 'liblightdm-qt-1-0' for regex 'lightdm*'
Note, selecting 'lightdm-webkit-greeter' for regex 'lightdm*'
Note, selecting 'liblightdm-qt-dev' for regex 'lightdm*'
Note, selecting 'lightdm-remote-session-freerdp' for regex 'lightdm*'
Note, selecting 'liblightdm-gobject-1-doc' for regex 'lightdm*'
Note, selecting 'lightdm-greeter' for regex 'lightdm*'
Note, selecting 'lightdm' for regex 'lightdm*'
Note, selecting 'liblightdm-qt-0-0' for regex 'lightdm*'
Note, selecting 'liblightdm-gobject-1-0' for regex 'lightdm*'
Note, selecting 'liblightdm-qt-1-dev' for regex 'lightdm*'
Package 'liblightdm-qt-1-0' is not installed, so not removed
Package 'liblightdm-qt-1-dev' is not installed, so not removed
Package 'liblightdm-qt-3-dev' is not installed, so not removed
Package 'liblightdm-gobject-0-0' is not installed, so not removed
Package 'liblightdm-qt-0-0' is not installed, so not removed
Package 'lightdm-remote-session-freerdp' is not installed, so not removed
Package 'lightdm-remote-session-uccsconfigure' is not installed, so not removed
Package 'lightdm-gtk' is not installed, so not removed
Note, selecting 'qlubuntu-default-session' instead of 'razorqt-lightdm-greeter-config'
You might want to run 'apt-get -f install' to correct these:
The following packages have unmet dependencies:
sddm-theme-breeze : Depends: plasma-workspace but it is not going to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).
EDIT
Thanks for the help. I encountered some errors at the 'sudo dpkg --configure -a' step, and decided not to continue until I get some feedback on them. Here are the errors:
john@john-desktop:~$ sudo dpkg --configure -a
dpkg: dependency problems prevent configuration of sddm-theme-breeze:
sddm-theme-breeze depends on plasma-workspace; however:
Package plasma-workspace is not installed.
dpkg: error processing package sddm-theme-breeze (--configure):
dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of sddm:
sddm depends on sddm-theme-breeze | sddm-theme; however:
Package sddm-theme-breeze is not configured yet.
Package sddm-theme is not installed.
Package sddm-theme-breeze which provides sddm-theme is not configured yet.
dpkg: error processing package sddm (--configure):
dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of qlubuntu-default-session:
qlubuntu-default-session depends on sddm; however:
Package sddm is not configured yet.
dpkg: error processing package qlubuntu-default-session (--configure):
dependency problems - leaving unconfigured
Errors were encountered while processing:
sddm-theme-breeze
sddm
qlubuntu-default-session
john@john-desktop:~$
EDIT
I was able to purge the offending packages, although in the reverse order you listed them. Everything else was working fine until the echo commands:
john@john-desktop:~$ echo '[SeatDefaults]' | sudo tee /etc/lightdm/lightdm.conf.d/50-unity-greeter.conf
tee: /etc/lightdm/lightdm.conf.d/50-unity-greeter.conf: No such file or directory
[SeatDefaults]
john@john-desktop:~$ echo 'greeter-session=unity-greeter' | sudo tee -a /etc/lightdm/lightdm.conf.d/50-unity-greeter.conf
tee: /etc/lightdm/lightdm.conf.d/50-unity-greeter.conf: No such file or directory
greeter-session=unity-greeter
I have no lightdm.conf.d directory. I do have a lightdm-gtk-greeter.conf file and a lightdm-gtk-greeter.conf.d directory which has 01_ubuntu.conf, 30_lubuntu.conf, and 99_ubuntu-mate.conf. Should I redirect the echo commands (or just write those lines) in one of these other files? I don't know if any are equivalent to 50-unity-greeter.conf.
john@john-desktop:/etc/lightdm$ ls
lightdm.conf lightdm-webkit-greeter.conf
lightdm-gtk-greeter.conf mythbuntu-lightdm-gtk-greeter.conf
lightdm-gtk-greeter.conf.d users.conf
For a better description of what happens when I try to start after switching to lightdm, it first pops up a dialogue saying that it could not properly detect my graphics and input devices, so I will have to configure those myself. When I close out of that, it then says that it will try to start in low graphics mode. I ultimately have to ctrl+alt+F2 to a new terminal and reconfigure back to gdm to get out of it.
EDIT 02-15-2016
Still looking for help here. In addition to the above, I've tried manually creating lightdm.conf.d and 50-unity-greeter.conf and then continuing with the reconfigure back to lightdm, but still no dice. When I try to start in lightdm, I still get the 'could not detect your display, graphics, and input devices' error and then asking if it should start in low-graphics mode. When I ctrl+alt+F1 to the terminal, the following is shown ...
[ OK ] Started ACPI event daemon.
[ somenumber] [drm:intel_pipe_config_compare [i915]] *ERROR* mismatch in base.adjusted_mode.crtc_clock (expected 138780, found 92519)
[ somenumber] [drm:intel_pipe_config_compare [i915]] *ERROR* mismatch in base.adjusted_mode.crtc_clock (expected 138780, found 92519)
[ somenumber] [drm:intel_pipe_config_compare [i915]] *ERROR* mismatch in base.adjusted_mode.crtc_clock (expected 138780, found 92519)
EDIT 02-16-2016
I followed all the instructions in EDIT 2 (in the answer section) and no errors were thrown. The Intel packages installed flawlessly with little green checkmarks for each step in their GUI. I had the right directory structure for lightdm.conf.d and the echo commands went through. Nevertheless, when I did the sudo dpkg-reconfigure lightdm command and rebooted, it once again tried to start in low-graphics mode. I appreciate all the help so far and, believe me, I wish I could mark this as resolved - but alas, I need more help. What are the next steps?
EDIT 02-19-2016
So I finally gave up and just reinstalled Ubuntu 15.10. First thing I did after the new install was to upgrade the Intel graphics drivers, as suggested below. Hopefully I will not see the crash-to-login behavior as I did before. If I do, I'll come back to this post and will try again, without using the wildcard after the lightdm package name.
I don't think I should necessarily mark this as resolved, but if one of the mods wants to mark it as inactive, that's fine. There's good info here, but I wasn't ultimately able to get down to the root of the problem I was having. Thanks, mchid, for all your help.