@CaseySJ Ok, so I've tried a few different Monitors/ports, all the same result. I also tried enabling CSM, same result. I do have a 580 but it's in another running machine right now so before I mutilate that workstation did you have any other suggestions? One other piece of info is that the system doesn't seem stable when booted, I can move the mouse but once I click on the mystery window the cursor disappears.
Was this a fresh installation and Migration Assistant was not used?
Correct, fresh installation. After the successful installation and reboot (waiting about 12 minutes for the OS to configure itself) it loaded into the aforementioned screen.
I have not tested my RX 5700 XT on that system, but my RX 580 works absolutely brilliantly. I'm thinking it might be good to install the RX 580 now so we can at least determine whether the problem is due to the 5700 XT. In other words, if the RX 580 works, then we can conclude that the 5700 XT is at fault. But if the RX 580 also fails, then we know that we need to look elsewhere for a solution.
Ok, popped in the 580 and got black screens on both monitors, on a single monitor, on DP and HDMI.
when I did the upgrade (using system prefs--software update) from 10.15.5 to 10.15.7 the other day, I noticed that strange screen popped up at the end of the last part of the update, and then it went away and the desktop appeared. you said you're doing a clean install, and therefore I assume you downloaded the Catalina installer and created a USB install media thumb drive. if so, I would try re-downloading Catalina, then re-making the installer thumb drive, before you tear apart your other machine.
@CaseySJ Actually I still had CSM on. I turned CSM off and got the same exact screen as before with the 5700. So no change with the 580.
Okay, some suggestions:
