Ghosting of screen images
I have this problem with a 2015 iMac with 21.5" retina display, but it only happens with certain streaming videos, nothing else.
So 90% of the time I have not problem, but if I stream a video that's embedded in a web page, occasionally it will instantly start retaining/ghosting the image.
It then takes a while to dissipate, most effectively by filling the screen with a white page. Turning the computer off does not fix the problem.
It does not happen when I edit video with Adobe Premiere, or use Photoshop, or any other program. Just certain streaming videos, and not all of them! Very frustrating.