Display problem with ghost window (iMac Retina 5K, 27-inch, Late 2014)

Hello,

I have been experiencing a strange issue for a while now, where a Safari window remains visible behind other programs at all times, even when Safari is not open or running.

It is particularly noticeable when I display an image full screen in Preview, or when I am working on a photo in Photoshop (or Lightroom, Capture One, etc.). I can see 'ghostly' elements of a Safari window (e.g. address bar, Back button, etc.) at the top of the window, behind the image. Sometimes the 'ghost' window can also cause light patches to appear in other areas of the screen as well.

An imprint of the top menu bar was also always visible, until I checked the option to auto-hide it.

I have tried turning off the transparency effects, but it didn't help. The problem is not solved by restarting. It occurs after every reboot, even with no intensive processes or programs running.

Stranger still, I took a screenshot of the problem, and it doesn't show up in the screenshot file! So, I am attaching photos taken with my phone. These show examples of an image in full screen in Preview and a Photoshop window. In all cases, there is no actual Safari window open behind them.

I am also attaching a screenshot of my system specs, in case that helps.

Has anyone else experienced this problem, and does anyone know how to fix it?

Thank you in advance!

Kind regards,

Ben

PS: Both the Safari bar and the Dock icons (which are auto-hidden) are also visible behind the desktop when running Windows via Parallels