As maintainer of TLP this has nagged me too. I trust you saw we support excerpts and thumbnail previews?
The one major feature that is missing is presenting the post feed immediately below the topic preview without having to change route, “in context” as you say. I’ve not experimented with that idea yet because there have been so many other features to implement and improve, but it’s something I’d like to get to.
I appreciate it’s one of the reasons some people are put off when considering moving from Facebook Groups to Discourse - the immediacy of the former (the other major one being consolidated notifications from all their feeds - though this is somewhat mitigated by Discourse Hub but there isnt a Discourse site for every Facebook Group). However there are still countless reasons for doing so in any case.