I googled this issue to see if I was the only one, I can’t believe this problem has been around for half a decade and it still remains a mystery.
I also have some weird memory (Mandela effect?) of the half circle pulling out some type of menu on my Galaxy S7 Edge, now it just sits on the top right hand corner above the time. It would be nice if someone can answer this mystery. I don’t even use the phone anymore but it still bothers me that I have no idea what it is.
It is also indeed a perfect half circle, doesn’t look like any dead pixels I’ve ever seen before.