Hallo - i found that discussion somehow interesting and funny. However, when I correct understand the different points and reduce the polemic part: what you want is, that both screens works independently. like, if you have the mouse in one screen and wipe, then only that screen should change the selected desk. The other remains the same.
I am not sure, is there an example where it works that way ? As far as I can remember, I do not know that functionality in Windows or Linux (but I am willing to get educated).
To be honest, I had not really a point where I missed that and I am regularly in customer meetings and use the Projector for the customer view and the laptop screen for my view.
I can imagine, it would be a nice feature to separate the screens and then to have customer views sorted over different desks and the primarly screen is showing my view, also sorted as I want.
However, I am using the Maximize and Full Screen and did not have a real issue. But other people might see it different. With the foresaid limitations, the second screen is an extension of the primarly, I was always able to manage my stuff in way in front of the customer that it was fully satisfying.
@bjohnson: not sure what you meant with "reverting to mirror..." but what I can see is, that my Mac is remembering the second screen and my arrangements (like the different wallpaper) and only for new second devices it is in a mirror mode. I experienced that also during a customer meeting. I entered the room, attached their projector and found an arrangement which I made months ago (i even forgot that I was already in that room)
Ciao