Reboot with an external display attached. You should see the same login dialog on both monitors, although when tested it showed up only on the built-in monitor. Regardless, disconnect the external monitor. The remaining, internal monitor now displays a black background with just the mouse cursor remaining. Re-connecting the external monitor adds it and extends the workspace onto it such that the mouse cursor can travel onto it, but both displays are empty and black.

Conversely, attaching an external monitor if the system has been booted without one does nothing.