There's a minor caveat to all of the above. The display new posts only works once you've visited a thread in the new forum/software. When you open a thread that was copied from the old board, for the first time in the new board, it apparently doesn't have the last visited data, so you get post #1.
Thanks. All three work, but none seems as simple as before. I think I'll use the double-click method. Just out of curiosity, is this documented somewhere? It seems unlikely you'd just happen upon it. I can't see why anyone would make such an obviously important function hidden. Why not one of those snazzy "new" icons on the thread title?
When you open a thread FOR THE FIRST TIME by clicking the thread title, you're taken to Post #1 in the thread. If the posts span multiple pages, the system keeps track of how many pages deep you scroll into the thread and assumes you've "seen" every post on each of the pages you viewed.
The next time you open the thread by clicking the thread title, you're taken to the first unread post. If there are no unread posts, you're taken to Post #1.
(There is some discussion/disagreement in the Xenforo community on what post you should see when there are no unread posts.)
Of course, to see the most recent post in a thread, you click on the date in the last column in forum view (or the date shown when in mobile view).