[RedTyger]

Should be really very easy to do, I may well get around to it myself.

But it's not marking the thread as unread, it's marking every post in the thread as unread. There's no such thing as an unread thread per se unless it's a thread with only one post. You can't mark individual posts as unread I don't think, you'd run into a world of usability trouble.

Edit: I suppose what you could do is just mark the first post of a thread as unread and use that as an unread thread marker, that might work better.

And it would still be marked as read by the "Mark all as read" default feature.