I am not sure where you even want us to add these views.

The main editor view of course. Right behind the text like in Terminal.app’s pre-installed Homebrew theme.

Allan is right, it’s tricky to choose where to put those. I thought of having transparency only in the File Browser, like a sidebar app, but you had another idea. It’s not a simple unanimous design decision.

There are no “default system tabs construction”.

I thought that NSTabView would be used by Apple apps to make those tabs, but I was clearly wrong.

Well, I guess we’ll have to wait a bit more on this, which is perfectly fine. :-)