Your own posts are considered unread.
Open a thread, read through it, leave the thread to the sub-forum page and the thread is no longer bold (marked as read).
Go back into the thread, post a response, go back to the sub-forum page and the thread is now marked as not read (bold and star) and no other posts were made to the thread except the one you just created.
Seems to have started fairly recently although I couldn't pin down a specific date. I don't think much more than a week, but probably less.