Hey,
It is cool that I can pop out the preview, but I cannot select another tab instead of a separate window. Am I blind, or has this not been implemented?
Greetigs
I don’t think it has. Why do you want to have it in another tab?
Right now, I can’t really think of a good reason 
Say you only have one screen to work with but want full screen for both the code and render. In this case you are right, creating a new window works just fine. But what if your task bar is already full of windows and adding yet another duplicate icon down there will not help the situation? With this added pressure a tab works better.
Anyways, it seems like a personal preference thing.
That’s true, but I can work better with tab management than with two open windows that I have to toggle through all my programs. It would be nice to be able to go directly from preview to code in one window with one click, or merge the pop-out preview with the open windows.
But anyway. If this is not possible, my question is answered.
Thank you for your answers.