This is already present in many (if not all?) editors already. In vim or emacs you can open a new buffer/window/pane (whatever terminology is used) on the same file and put different views of the same file (with live changes) side by side. I often do this when I need to keep an eye on the header/import section of a file and also work at the bottom of the file at the same time.

I'm pretty sure a lot of other editors/IDEs I've used in the past also support this. It's nothing new.