You seem to know a bit about these issues and was wondering if you could help me?
Using a Radeon R9 280X in High Sierra with only one monitor working. The second one is getting a signal but displaying only a black screen. Both worked flawlessly in previous versions of OSX. I've been working at this for months with no joy and followed the steps above too. When I replace the aforementioned kexts with the Sierra versions my one working monitor defaults to what could be said is a VGA display with weird colours.
I agree with your logic that it's a kext issue and therefore by replacing the kexts that worked in Sierra they should also work in High Sierra but that isn't the case for me. It has me wondering if the 3000 kexts are the correct versions for HS? Any further input?
**EDIT** I am convinced it's the DVI that's broken under High Sierra. Dual monitors still works OK in Ubuntu and Windows 10 so it isn't a hardware issue. Definitely software related. System Prefs picks up both monitors (See image) https://imgur.com/a/qylUp
**EDIT2** I was right DVI was not set up correctly. I had to edit a kext file adding frame buffer settings. All sorted now