Thanks, una and @DocJohn. So I took it back to ten posts, but now if I click on the little icon after the name of the last person who posted so I can go to the last post, I end up halfway through the thread. Like on a 78-page thread, I end up on page 39.

ETA: likewise if I make or edit the last post in a thread, after hitting post or save I’m taken back to the halfway point of the thread.