My other forum also recognises when you have looked at a page with New posts on and doesn't continue to flag that page as New later on, but, if there are also New posts on the following page(s), then the New flag remains.
Clicking on the alert for New posts on each thread takes you the the first new (unseen) post, rather than the most recent.
It also recognises that a post that you have just posted is not a New post from your point of view and doesn't flag up a New post on that thread, unless there happens to also be one that you haven't seen..
Handy