I know this is an ancient question but I have found an answer (at least for my use-case).
I have a automatic process I want to run that uses multiple windows. I don't need to interact with it, but I need to keep an eye on it and stop it if it goes wrong. It takes a couple of hours to run, so I want to be able to use the computer while it's running the process. So what I wanted was a little window that was basically an extension of my desktop.
This is what I found: http://new.monitorsanywhere.com/how-to-evaluate-monitors-anywhere-without-zero-clients/
Sure enough, it creates a virtual monitor in a window, which sure enough I can extend my desktop to it. And happily, it also scales when you resize the window so I just have it in the corner of my screen and now I can use the computer as normal while keeping an eye on the process.
I also found deskpins, which allows me to make the virtual monitor window always on top.
I've seen a few places where people have been asking for this but with answers that involve hardware or old answers that don't work any more.
This works on windows 10, and it's July 2019 as I write this.
Hope this helps someone!