This forum support infinite scroll, it wonderful, but sometimes it's annoying. Open long duscussion (with 300-400 answers) is painful, I open read only last few answers and if scroll to top (look at topic theme or breadcrumbs) , I cant do it, because forum load previous pages. It's very annoying for me.
It's possible add button "load more"? I see it this way: when user open topic first time, he see only one page (20 posts for example) and if topic has more posts, buttons "load more" (at top if exist earlier posts and bottom if exists later posts). If user press once "load more" - means we activate infinite scroll as it works now.
Pagination with infinite scroll (better solution than "load more")
Best infinite scroll i meet: https://vk.com/topic-200_2806586
Above discussion placed pagination and when you scroll, work infinite scroll. But scroll work only in direct side, not when you scroll back. For example if user open first page he can read whole discussion with infinite scroll. If he scroll back, its return to first page (where he start reading)
If he open 5th page he can also read to the end. But if he scroll back, in this case he return to 5th page (where he start reading).
So if user open last page (most common case for forum, read last unread post), infinite scroll doesn't work at all. If unread posts several(2 pages, for example), user open last but one page and infinite scroll works only work with 2 last pages.
Hope you understand idea. You can play with VK demo:
Topic from first page
last but one page
My js skill not enough to implement this, so I can only offer idea.