Last night, I put my mac screen down, without shutting the computer. Look, I know, it's bad and I shouldn't do it, it's easy. I fully take responsibility for doing it. This morning I opened it and it had a weird blue stripe across the screen. The photo is from later, but it looked exactly like this.
Barely noticeable, but annoying. I tested that it showed up regardless of what was up on screen. I tried rebooting the laptop, just in case it would help. And it did. It completely went away by rebooting.
And then I had to move rooms with the laptop (a good 4+ hours since the issue first fixed itself) and I closed the screen because I would be on the laptop again in less than two minutes. The stripe reappeared. I tried rebooting again, but the stripe is still here.
There was nothing between the screen and the keyboard that could have been caught in the closure.
I've never experienced any issues. The laptop is a couple years old, but is fully functioning, and when compared to my wife's even newer mac, a bit faster. I don't know what to do, it's just noticeable enough that I can't try to forget about it.