Is there a forum with tree-like replies and manual marking as read?

Question

CMS

We need a non-public forum with a tree-like structure of responses, in which one could manually mark read / unread messages.
Usually forums only support automatic display of the status of posts - by the date the topic was last opened.
I am ready to consider both services with such functionality and products. If there is no ready-made solution, what can be easily modified to such functionality?