I can’t say I’ve ever encountered the need for a dynamic “back button” while using Discourse, and I consume 3-4 Discourse forums daily. You don’t have an x-to-y post-position in Twitter. In Discourse you do, and it’s hard not to pay some attention to it.
Say I’m reading a book. On page 34 it says “remember that thing we discussed on page 12?” or “see glossary on page 202”; I won’t have a hard time juggling those two different positions.
Discourse even has read/unread callouts, so if you have to retrace your steps a bit just “follow the blue dots until you’re back where you started”.
All of the above can be further reinforced with a custom scrollbar, which seems like a very likely addition at this point (though not necessarily as mocked up in my post).