I would presume that when settings the value of org.gnome.settings-daemon.plugins.power lid-close-suspend-with-external-monitor to false the laptop will not sleep when an external display is plugged in. However this doesn’t seem to work in 18.04. The laptop still sleeps. The value True, will always sleep the laptop. Can someone else test this? Setting to False should not sleep the laptop 
Did some testing. xubuntu (lightdm) ubuntu (gdm) work with this. Mate not 
I presume it’s all to do with the display manager and not display server.
Mate seems to use /org/mate/power-manager/ and not org.gnome.settings-daemon.plugins.
@Wimpy Is there a way we can use power lid-close-suspend-with-external-monitor in /org/mate/power-manager/?