Do you mean posts within a thread, or actual threads.

If you mean threads, then it should work this way already. As you can see in this screenshot of Scuttlebutt forum, I've read all the threads except the Branscombe one, which is highlighted, and shows there is one new post in it. I've been away from the forum to do other things, and returned again, and the unread thread is still highlighted as shown. When I open the thread, the software will take me straight to the new post.

If you mean unread posts within a thread, then no, the software won't know which of the posts you've scrolled through.