I always assumed the behaviour and categorization was in their backend. I think its weird that the view (HTML vs javascript) is not decoupled from the model?
While that is possible it would make no sense. Why spend time refactoring code to improve a legacy UI for old browsers almost nobody is using? Legacy UI doesn't know anything about categorization because it was added later.
Actually Gmail's approach is sane. They just keep old UI working as it used to many years ago although the backend could have changed completely.
what exactly are they supposed to do in the backend?
they probably have an api in which you can limit the results by category. As the old interface doesn't have any limiter implemented, they're getting all the results back.
you can also disable categories in the newer interface and the effect would be the same (everything in the inbox)
