Are you trying to say you want to convert this to a feature request, you never actually said that clearly? As I said above (added emphasis):
Up to you to raise a __separate__ feature request
The history of this issue and having been closed as invalid in the past is not appropriate for a feature request.
But _within_ Geany you can certainly have your feature request refer to this for background.
You should add to the FR how you suggest it should be addressed, especially how the selected tab title and arrows stuff is to be handled, etc. And as I said, adding a PR would be worthwhile, since so far nobody has cared enough to make the changes to the default GTK notebook behaviour for both sidebar and message window notebooks.
On your GTK request, I would also add, check the documentation, and check if there has been a previous similar issue before and what the answer was.