Having exactly same issues on High Sierra. Especially on dark grey backgrounds - see attached photo below
All those lighter elements are artifacts that are like ghosting of things that were displayed earlier - like fb page or icons of the background visible on the right side.
You should actually see a smooth grey background - below is a screenshot of the same thing:
- artifacts above are visible only on my iMac screen
- they are not visible on the 2nd monitor
- had to take a photo just like LuxViz as they are not visible on screenshots
- when they start to appear - it seems that my iMac is slowing down - especially when I jump from one desktop to another - it's just jerky animation
- I assume that all that suff have something to do with Metal 2 introduced by Apple in High Sierra - as you can read on the internet: "Metal 2 includes driver optimizations, and Apple claims that it brings up to a 10 times better draw call throughput. More debugging tools are now available, and the Mac Window Server has been migrated to Metal as well to enhance performance of the system-wide animations."
- 2nd monitor issues:
I do not have artifacts but have strange grey rectangles appearing on dark backgrounds. And I can take screenshots of those "rectangles". Sometimes they are flickering. See below screenshots of Spotify app on 2nd montior and on iMac:
iMac DISPLAY (THIS IS HOW IT SHOULD LOOK)
On top of that I am experiencing "locking" issues - I have to restart my iMac as only my mouse cursor is responsive and nothing else, including not responsive keyboard and screens are not refreshed at all. But this is a known issue to Apple and you can find more info about this elsewhere.
I hope Apple will solve this issue soon - seems that 10.13.1 update is even worse.
My iMac specs:
iMac 27" Late 2012
3.4 GHZ Intel Core i7
16 GB RAM
NVIDIA GeForce GTX 680MX 2GB