Lately I've been witnessing this weird visual glitch on the screen of my MacBook (non-retina 2012, hi-res screen):
I have had to take a picture of it because whenever I try to take a screenshot it goes away right before the screenshot is taken.
I thought this glitch only happened when I had Adobe Illustrator open, which is when I would see it appear more often, but I've now also caught it in situations where the application was not running (the picture above is one of those cases).
I cannot pinpoint the exact cause of this and I cannot tell whether it's a hardware or a software issue, but I can only say it's not one of those flickering effects that appear for a second and then vanish. This is actually very persistent and will only go away when I switch to another application or in some way move the current window.
Does anybody know what this is and what might cause it?
Here are more pictures: