then you dont need to (actually you cant at all - atleast accurately) show it in the EDITED file... just on the COPY - the programmer cant see the server-side copy and has no use of it, he needs to track it visually on the COPY and make changes to ORIGINAL
this discussion is getting more confusing than the real thing would be

If I got you right, you suggest me to have two windows for each file, one for stepping and another for editing? It would confuse a little...

yes it would be confusing. it is upto you to implement it or not
my vision of the tabbar:
index.php* | classes.php | Output | Help | index.php [D]

I said from the beginning, that's not a simple feature...