prevent virtual display from becoming the main display

[ldenoue]

I have an issue with a virtual display becoming the main display.
At the beginning, I have one built-in display and one external display, mirrored.

I then launch FluffyDisplay and create a virtual display.
All is good.

Now I unplug my external monitor.
Problem: macOS makes the FluffyDisplay its main display.

Is there a way to prevent that and make the built-in display the main display instead?