There are times on my 2015 13" rMBP, while I am surfing the net, where a website I'm wanting to look at will come up with a thin, horizontal black line running through it. Sometimes the line will l slowly fill in, while at other times, I will need to refresh the page. I've had earlier versions of the MBP (non-retina) where I have never seen this happen. Not sure what it is that's happening here. Any ideas?