Got this too, it just appeared out of nowhere a couple of weeks ago but I have no idea what caused it. The device wasn't dropped or anything like that, the only thing I can think of that may be relevant is that I used it outdoors in relatively low (~ -15C) temperatures a few times (but I can't find any operating temperature specs anywhere).

My spot is thankfully mostly in the window titlebar area so it only gets annoying in full screen mode.