Comment 3 for bug 1615286

I did several further tests, but unfortunately I won't be able to continue (I have to use my computer, so I'll need to install a new distro). I hope this information will nevertheless be useful for Linux Mint developers.

- After installing lightdm (apt install lightdm) and switching to it as default dm, I tried rebooting but there were some error messages on boot: "Failed to start Light Display Manager".

/var/log/lightdm/lightdm.log conained mentions to "Failed to find session configuration default". Googling about it led me to some AskUbuntu answers suggesting changes in /etc/lightdm/lightdm.conf. Such file did not exist, and only an empty /etc/lightdm/lightdm.conf.d existed. I tried "apt install lightdm-gtk-greeter", and after that I rebooted. I was then able to actually see something resembling LightDM's login screen, though it still gave an X error after trying to login. Apparently it was related to an already existing X session.

Then I tried once again installing gdm (apt install gdm) and setting "gdm3" as default desktop manager. After rebooting, it got in some sort of loop (screen refreshing several times) but never actually got to the login screen. I then uninstalled gdm, forced a reinstall of mdm, and booted again. I was back to the mdm login screen, but after trying to login, I only had a black screen.

Having completely botched my Linux Mint, I decided to try a fresh Xubuntu 16.04.1 install. I still had the display off issue, so I tried again installing gdm, and this time it worked better. Still had an error message but otherwise I could use Xfce fine.

I then noticed that even on Xubuntu's live CD, the display problem was present. So I installed a Debian 8 XFCE live CD this time, and now I can finally turn off my monitor and turn it back again, without issue. I'll probably have other issues, but at least there exists one working configuration for XFCE where I can turn back on my monitor.

SUMMARY OF MY FINDINGS:

- The display off/on issue is NOT related to Linux Mint or mdm at all, since it also happens in Xubuntu;

- However, there are some serious stability issues related to installing other desktop managers in Linux Mint, probably due to mdm, since they were not present in Xubuntu 16.04. For instance, I don't know why it is possible to install lightdm and configure it without having any greeters, which seems to be somewhat useless since LightDM always fails in that case (and then mdm seems to take over and still give a graphical desktop). But the most troubling issue is that installing gdm leads to some reboot loops and a worse configuration than doing it on a Xubuntu.

- The display bug must be Ubuntu-specific, or Debian somehow managed to fix it since. Because I don't have more time to test other things, I'm moving to Debian for now. However, since the issue is visible even on a live CD (Xubuntu or Linux Mint), I could quickly test other versions to see if it is present, and gather debug logs, if that would be useful.