Mark whole threads as read in catch-up mode

**Is your feature request related to a problem? Please describe.**

If I use catch-up to access a post which is part of a thread, only that post is marked as read in the list. Other parts of the thread are shown as unread. This makes the "read" feature much less useful to me.

**Describe the solution you'd like**

I would like all posts in a thread to be marked as read if I open any post in that thread. Either I have read all the posts, or I have decided not to read them -- either way, I have "dealt with" the whole thread and want to mark the whole thing as read.

It would be nice if this also extended to multiple participants in a threaded conversation -- but only to replies which were expanded in the sidebar view of the thread (assuming that you have a way of detecting which replies are visible immediately and which replies I have to click to expand).