Not sure if this is a bug or a wanted behaviour.

If I click the button "Mark Forums Read", all of the main forums are marked read and this is what it should do and does. But If I enter a sub forum, all of the unread topics are still marked as unread. Maybe they should also be marked as read?

It was never designed to mark the threads in a forum as read. This forum does not function like other systems.

Yeah, I was wondering about that and figured it out by myself that this forum is different as coded from scratch by you (if I'm correct) and I really can't emphasis enough how your coding kept TM2 alive. Could you maybe explain me the reason why the function of the button was designed this way and not the other way? Just curious about your philosophy.

You're too kind. You really should thank TGYoshi for keeping it going!

Initially I would have designed it so that threads are marked as read as well, but at the time (couple of years ago) there were performance considerations.

If done right, TGYoshi would be able to accomplish this now. But there could still be performance considerations. I'll let TGYoshi weigh in on this.

It's fixed.