Apple left the display market back in 2016 with the discontinuation of the outdated Thunderbolt Display. Apple did, however, say they were working on a new screen alongside the new Mac Pro. Apple's Phil Schiller said in 2017:
With regards to the Mac Pro, we are in the process of what we call "completely rethinking the Mac Pro." We’re working on it. We have a team working hard on it right now, and we want to architect it so that we can keep it fresh with regular improvements, and we’re committed to making it our highest-end, high-throughput desktop system, designed for our demanding pro customers.Kuo describes the new monitor as a "6k3k" display.
As part of doing a new Mac Pro — it is, by definition, a modular system — we will be doing a pro display as well. Now you won’t see any of those products this year; we’re in the process of that. We think it’s really important to create something great for our pro customers who want a Mac Pro modular system, and that’ll take longer than this year to do.