"Save as" should behave like almost other applications
Currently the feature named "Save as" behaves differently from almost every other application, GNOME or otherwise - it saves a copy with the new name, and the current window continues to edit the file with the original name.
This is very confusing behaviour, and it's very frustrating to get halfway through updating what you think is the new file and then realise that you've been editing the old one instead.
At a minimum, the current feature could be renamed "save a copy", and unbound from the "Ctrl-Shift-S" shortcut, since that means "save as" almost everywhere.