27-inch iMac Late 2012, 3.2 GHz Intel Core i5. I'm currently running the most recent GM Candidate version (15A282a) of OS 10.11 El Capitan.
I added 8 GB memory to the 8 existing GB, a few hours before the problem occurred and everything seemed normal and good.
Just before the trouble:
I was working with the WineSkin Winery and WineBottler apps. The only way I could make them work was to turn off System Integrity Protection by booting to Safe Mode and entering csrutil disable in the terminal. The applications may require access to fonts or other system elements that SIP was preventing. I had some success creating wrappers with both applications but after booting to Safe Mode again, enabling SIP and rebooting there were problems with the Video.
What's happening:
The Desktop icons are ghosting clearly on most screens including the login screen, but not the start-up screen with the Apple logo and progress bar. The icons even show up on the Desktop of another user. They are most vivid in application windows with dark backgrounds. You might also see the user images from the login screen on the Desktop.
Ghosts of recently opened applications remain on the screen for hours after being open for just a few minutes. When moving an icon on the Desktop, a ghost of the icon can remain behind. It's the ghosts and not the icons themselves that show up very vividly in the dark portions of open application windows, although you might not see the ghost icons or application windows on the Desktop itself.
Actual icons appear normal, clear and bright but everything else except for black text on white, has a slight low contrast grayish look. What should be black in windows is gray. There may also sometimes be a horizontal shimmering effect in dark application windows.
Depending on how dark they are, application windows appear to be translucent to almost transparent with the Desktop picture and icon ghosts visible through the application window. White windows are opaque with no ghosts.
There are horizontal ghost extensions of windows and the graphics and text within them, that spread all of the way across the screen. A line of text within a window will be a dim horizontal line extending past the window for the width of the screen.
Some screens like the logout and shutdown screens have a brief horizontal shimmering effect.
I tried to take some screenshots of these things but in the pictures, contrary to what’s on the screen, everything appears completely normal.
What I've done to try to fix it:
I removed the new memory and then put it back when there was no change.
I zapped the NVRAM (restart with Command - P - R).
I ran the Diagnostic (restart wth D). Everything tested normal.
I deleted the "com.apple.windowserver.plist" prefs
I attempted to calibrate the display but it only allows whitepoint to be set and not the Set Up function which it skips over.
I reinstalled the 10.11 beta and the 15A278b update and later the 15A282a 10.11 GM Candidate but the problem remains.
Before reverting to Yosemite, I'm hoping for a guess as to whether this is a hardware problem or something that might be fixed in an upcoming El Capitan release?