In this case, I don’t believe the subject is exhausted on its subject matter. It may be for some participants, but not for others. Some threads are ongoing for far longer than 675 posts.
Defining a thread hijack can be very subjective and is usually up to moderator discretion. If you’re looking for a diagram that tells you exactly when you’re engaging in a hijack, I don’t think that’s possible. But as you pointed out, you kind of know when it’s happening. Right?
I think if someone is going so far afield as to litigate past elections, single out politicians for complaint or drift off into a discussion about student loan debt, these are easy calls re hijacking. We have so many other threads where these specific things are being or have been discussed. Use those threads, or start a new one, please.
From our most recent specific rules in P&E:
Hope this clarifies.