On Juno, Intel Haswell notebook with external monitor, trying to turn off a monitor doesn't actually work. When I use
Settings -> Displays -> any_monitor -> Use This Display -> OFF
the monitor is still on.
xrandr: eDP-1 connected (built-in) DP-1 connected (external monitor)
Running xrandr --output any_output --off
Tried booting into stock 18.10 and the Settings/Displays app was able to set Single Display