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:

enter image description here

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.

  • What model of iMac is this? Have you run Apple Diagnostics to check for hardware issues? - support.apple.com/en-us/HT202731
    – drivec
    Jun 1, 2018 at 18:14
  • It is a 21" iMac18,1 (2017). I ran a diagnostic (booting with 'd' depressed) and the diagnostic reported all well with the hardware.
    – zeeple
    Jun 5, 2018 at 17:23


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