You need 2 users, both of who have unread forum post tracking turned on. Make a forum. Start a discussion as user 1. Log in as user 2 and verify that user 1's forum thread is shown an continain 1 unread post. Click on the thread and verify that the unread forum post is visually highlighted. Reply to user 1's post. Log in as user 1, and verify that the forum thread is shown an continain 1 unread post. Click on the thread and verify that the unread forum post is visually highlighted.