Flickering horizontal line when I click on Reading List link Something strange going on with my iMac; it's hard to describe. 

When I click on any link on my Reading List sidebar, for a split second there is a flickering line which goes across the screen. When I say line, what I mean is, a faint copy of the link below flickers for a split second horizontally across part of the screen. 

It only happens on Safari (11.1). No other app, so it must be a problem with Safari only. I have macOS 10.13.4 installed. I have tried resetting Safari, resetting SMC and NVRAM and even deleting the plist file from my Library folder and repairing my disk in Disk Utility......

A short video showing my problem can be viewed here, about 2 seconds into the video......

Youtube

Anybody have this and found a solution?
It's more of an annoyance (bug) in Safari than an actual problem because it doesn't cause anything to fail.  It's also definitely not something that can be fixed by resetting NVRAM, the SMC or performing First Aid on your disk(s).

What you are seeing is a dialog box that pops up; usually when you are hovering over a link.  I extracted the frame from your YouTube video clip to see what was being shown and it appears to be the link below the one you have selected.



As you have already said, the problem only manifests in Safari.  Your best course of action would be to provide feedback to Apple so that they can address the issue/bug.  Provide them with a link to this question/answer so they get the full picture (no pun intended). 

Bottom line:  It's nothing more than a box with a document title/link in it and it only appears for less than a split second.  Annoying?  Perhaps.  Problematic?  Not so much.