Login screen appears on only one monitor and it's not the one I want

Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug 1723025, so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Feel free to continue to report any other bugs you may find.

I don't think this is a dupe of Bug #1723025, which is discussing behavior when a screen is disabled such as a laptop being closed while an external monitor is connected. In my case, the laptop screen is not disabled (the laptop is not designed to operate with the lid closed). The laptop screen simply cannot be seen when using the external monitor and keyboard due to its physical positioning.

Ideally, for this bug to be fixed the screens would be mirrored and the login screen displayed on all screens. Fallback would be to implement previous release behavior, where the login controls followed the mouse pointer and would appear on whatever screen the mouse pointer is moved to.

It's a different project that implemented "login controls followed the mouse".

I know many people would like to see the same in gdm3. It just hasn't been done yet.

Please log a request for the feature here:
  https://bugzilla.gnome.org/enter_bug.cgi?product=gdm
and then let us know its URL.

https://bugzilla.gnome.org/show_bug.cgi?id=794971

Upstream issue shuffled to to https://gitlab.gnome.org/GNOME/gdm/issues/372

Reworded the bug title because I keep finding duplicates of this issue and it's hard to direct people here while the bug title only describes one possible solution instead of describing the problem. Other people have requested other solutions like honoring their account's own primary monitor preference as a global setting.

In general it's good bug hygiene to describe the problem only.

That said, I personally agree that mirroring the login screen on all monitors would be best.

I switched to lightdm but the problem still exists: if I attach an external monitor to my laptop, no login screen appears, nevertheless, I can enter my credentials blind, and than everything's fine.
If I boot with nomodeset than at least the login is ok. I've got a dell laptop with integrated intel video.

I have exactly the same issue. This is really annoying bug.

I agree mirroring login screen to all displays is the ideal default behavior.

WORKAROUND: replace gdm3 with slick-greeter + lightdm-settings + gnome-screensaver + numlockx(optionally)

Greeter will auto-appear on whichever screen your mouse is on or moves to, dynamically.

See: https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1820423

Are there some solution for it?

In addition, for me, install lightdm solve the problem. Now, I can see login screen on both monitors :)

This still happens on Ubuntu 20.04 (Focal), too. In my case, I have one external monitor which is mirrored. But the login screen appears only on the laptop's screen but not on the external monitor's display.

same issue here, on Ubuntu 20.04(Focal)

Hello!

I am experimenting the described behaviour in Ubuntu 20.04.1

Moved to gnome-shell. Any graphical aspect of the login screen is implemented in gnome-shell, not gdm.

WORKAROUND/FIX:

1. Log in and configure your multi-monitor desktop the way you want.

2. sudo cp ~/.config/monitors.xml ~gdm/.config/

3. Edit ~gdm/.config/monitors.xml to make sure the monitor you want as primary is set as <primary>yes</primary>

4. Log out.

We could add a button to gnome-control-center that does the above. Label the button "Apply on login screen".

The problem with the workaround/fix is that that fix can and will get overwritten by newer versions of GDM3. That just happened to me and I had a hard time figuring out how to fix it until I came across this again...

Another problem with the workaround/fix is that it does NOT work when wayland is enabled for the login screen.

You have to uncomment WaylandEnable=false in /etc/gdm3/custom.conf and then systemctl restart gdm.