I experience very clear retention/ghosting on my 18 month old iMac 5K display. When I talk to Apple Care they refer to the article "Avoiding image persistence on Apple LCD displays" and say that retention/ghosting should be expected. This means that I should activate the screensaver every 10 minutes to avoid retention/ghosting. It is alarming that Apple allows this to be normal. It is a top of the range display, and I would never have bought it if I knew that I could expect this, and that Apple would say that it is normal. I think that we should make this known to as many as possible, so THEY know what I didn't know - That Apple displays are expected to show retention and ghosting after only a few minutes. The picture below shows the ghosting from a browser window that has been open for less than 5 minutes.