GTR implemented in the past a notebook which allowed to have several PO files opened in tabs. This feature disappeared at any moment in the past, but source code remains, so we have two options:
Recover the feature tho show tabs in GUI. This could complicate currently fixed features like search and replace (at least in the past this was a failure point). If it breaks nothing and is easy to add to GUI it would be ok for me.
Leave the GUI as is and do a code cleanup to remove gtk's notebook references, to simplify it
Of course, this is not an urgent issue so it can wait or simply bury it and forget about it ;-)