Steps to reproduce :
- Visit the homepage of the forum
- Open the active topics page
- Click a new topic in a forum (if the topic is the only new active topic in this forum)
- Read the last message
- Go back to the homepage : the forum is still flagged as "new"
- Click the forum : there is no topics marked as "new"
- Go back to the homepage : the forum is now marked as read
When visiting a topic, update the forum flag if this subject is the only unread topic for this forum