Is there to make the login screen appear on the external display in 18.04?

This is a known and reported bug with gdm3. The current workaround appears to be:

1. Go into Settings > Devices > Displays and configure your monitors the way you want for your login screen (in your case, internal laptop display disabled). Click the "Save" button when done.
2. Copy your user's monitors.xml file into the home folder for gdm user.

To copy the monitors.xml file, open a terminal and perform the following:

sudo cp ~/.config/monitors.xml ~gdm/.config/monitors.xml
sudo chown gdm:gdm ~gdm/.config/monitors.xml

Then, reboot and see if your changes persist. If this doesn't work, try going through the whole process again. It took me two times in order to get it to stick. I was also able to reboot with my external display disconnected and it switched back to my internal laptop display. I rebooted again and reconnected it and it switches back to my external monitor.

Hope this helps!

This Solved my Isuue:

Go into Settings > Devices > Displays and configure your monitors the way you want for your login screen (in your case, internal laptop display disabled). Click the "Save" button when done.

Copy your user's monitors.xml file into the home folder for gdm user.

To copy the monitors.xml file, open a terminal and perform the following:

sudo cp ~/.config/monitors.xml ~gdm/.config/monitors.xml
sudo chown gdm:gdm ~gdm/.config/monitors.xml

And in the /etc/gdm3/custom.conf uncomment WaylandEnable=false

I too have an external monitor. This worked for me:

• Set your display mode as desired using Settings > Devices > Displays

• Open a terminal window by pressing CtrlAltT and then type:

  sudo cp ~/.config/monitors.xml /var/lib/gdm3/.config

• Hit Enter

• Reboot the computer