It’s a bit bugged: in mega long threads, say if i’m at post 666 and click the button, it only takes me about 20 posts up (to post 640), not to post number one.
test here: What happens when a topic has over 1000 replies? - #576 by Neil - general - Theme Creator
Later edit: i don’t know how to add the link to the theme preview and the particular topic at the same time, so you will have to search for the long thread by hand.