I always seen it like this: trolling: silly arguments, maybe attacking a person in a thread about their viewpoint (ad hominem) during a thread. While harassment is insulting a person, everywhere regardless of the thread going on, as well as attacks while the person is not participating in a/the discussion.

Yeah, I get that. But what happens is that threads on one site become trolling topics on other sites. So harassment becomes part of the trolling game.