HERE'S WHAT WORKED FOR ME:
My problems with the display going spastic started when I did the point-update that was released at the same time as the 15.2 release. So, It wasn't Sequoia that did it. It actually began with the last point-update of Sonoma.
COMPLETELY by accident. I couldn't get anything to work to stabilize my display after the upgrade, so I had gotten out my old Studio Display with the mini-DisplayPort plug on it and was searching for my C to MiniDisplayPort adapter. During that process I stumbled on a short C to DisplayPort (the big, rectangular plug with the one corner angled) cable. My LG has a C, 2x HDMI, and one DisplayPort input.
Up until now I've been using C to C with no success, 3 separate breakout hubs (HDMI, Network, USB, SD Cards, etc) with HDMI to HDMI. Two of those gave me the flashing issue and one didn't work at all. I had done everything suggested (delete /Library preferences, different cables, different breakouts, unplug everything, change ColorSync profiles, lowered resolution to 1080i and 760x480, unplugged everything except power and display cable, zap P-RAM and Low-Level RAM, 15 jumping jacks, and turned around while patting my head.
POSSIBLY SIGNIFICANT: On the two breakouts that worked, there was a not-insignificant amount of static both when the image was properly sized and when the image was incorrectly sized (and bleeding off one side and onto the other side of the display). I've seen this type of static before with clock mismatches on my old video scaling products. They are pixel-sized dots that flash randomly and are evenly distributed across the screen.
At any rate, using this cable plugged directly into one of my C ports to the display's DisplayPort and I'll be ****** if the picture wasn't perfect. Whoo hoo - put the old Studio Display back in the closet.
Mind you, I have no idea where this cable came from. It's been in my collection for at least a couple of years. So, I looked it up and here's the info I could find on it:
USB-C to 4K DisplayPort™ by J5Connect
Configuaration:
- iMac Retina 4K, 21.5", 2019 (3 GHz, 6-core Intel i5 with Radeon Pro 560X 4 GB w/16 GB 2667 MHz DDR4)
- 2 USB C + 3x USB3
- External display: LG 34BQ77QB wide-screen display native 3440 x 1440
Good luck everybody (that's what it took for me)
And here's to Apple waking up to the downward spiral they've been in for the last 10 years.
