I have done a restart and that didn't change anything. This is not that big a deal for me to go through a reinstall. I'm only trying to report what I see as a problem on my iMac in case others are having the same problem, and also in case Apple engineering is not currently aware of this problem. I will also try to file a bug report as I'm already an Apple Developer.
I'm adding camera-generated screen shots as the normal screen shots do not help in showing the 'artifacts' - which I now can tell are pixels from other windows which are "bleeding" through and into the top-most window, and which I can now tell are hidden app windows which are somehow still bleeding into the desktop image.
I'm including some more screen shots, this time taken with a camera (though hand-held and not corrected for lens distortion). But with literal screen shots, I think you are not actually able to see the extraneous pixel details that you can see with your eyes. The camera was able to capture the pixel artifacts.
The 2 vertical images show the artifacts visible in my Finder/Desktop image, and also the hidden window for Transmission which is doing some file downloads for movies. If you look at the image of the Transmission window, you can clearly match up the bleeding content in the Finder when the app window has been hidden. I've also included an image of bleeding coming into a Photoshop window, this time with the Transmission window in the background and not hidden.
