Welcome to FT!

It sounds like you're talking about business (or "Signature") class rather than premium economy.

If you want to talk most of the flight, the two middle seats are still your best bet - the partition isn't that high. But many would prefer two windows.

Note that there are many threads addressing this topic. A quick Google search will often turn up answers more quickly than a new thread, e.g. Divider Between Business Class Seats D and G on AC 777s

P.S. It's 787-9, or 789, not 787/900.