The issue with FCPX for me was fixed with a direct update (17A344b). However, I wanted to convert to APFS like last time, and downloaded the full installer and to reinstall directly. The full installer now automatically converted the start up drive to APFS, but the issue with FCPX reappeared. After restoring from the previous carbon copy clone and then updating directly from Mac App Store, everything is back to normal. (Of course I'm back to Mac OS Extended journaled) I'm confused. Is there something about APFS and FCPX on my specific Mac? It's just a hard drive format.
Update 8-20-17: I bought another SSD and did an experiment. I first did a clone backup of my start up drive beta 17A344b which is Mac OS Extended format.
I downloaded the standalone installer from the Apple Store linked from the beta.apple.com site.
I performed the full install and it reinstalled and converted the current start up drive to APFS.
Rebooted.
As expected, FCPX has the dark video window issue: start up drive is APFS.
Rebooted from the cloned backup of 17A344b.. and then used latest version of Carbon Copy Cloner to restore previous clone to the current APFS startup drive.
Rebooted. Final Cut Pro X is now correct... and my start up drive is APFS format.
It leads me to suspect there is either a issue with the standalone Hgh sierra Beta installer and my Mac Pro 5,1 with AMD 7950 or my current install has some kind of legacy issue or garbage.
Sent feedback to Apple just in case.
Might have to consider a format and clean install in the future.
Note: Carbon Copy Cloner as of this writing does not support creating bootable APFS clones of your startup drive. The copy needs to be on OS Extended format drive.