I have a recently rebuilt recently purchased (read: really new) iMac that is running High Sierra, Chrome, and a few other very standard apps. Nothing unusual at all. A very basic set up. The user who uses this machine for their daily driver took a video of something that happens almost everyday when he is preparing for class. Here is a screen shot of it:
Also, he had activity monitor already open and the most taxing thing that was going on was Chrome using like 6.2% of the CPU. What is causing this visual artifact to appear, and then stay on the screen?
Update: The user is still having this problem, now with the second iMac of the exact same configuration. I had performed a hardware diagnostic on the previous iMac where this was happening with the results being all thumbs up. This is still a mystery.