I've had this exact problem start popping up on my PSD's and am trying to find a solution. I don't think it's hardware related as I've had it happen on my laptop as well which is completely independent from my desktop (no sharing files etc). It seems very random, I'll just be painting, and then a colored rectangle just pops onto my canvas. Sometimes if I zoom out one level it will dissapear, but it still seems to be there otherwise. I've tried messing around with the openGL settings and turning off hardware acceleration, but haven't found anything that works. This only started happening recently, so I wonder if it's some kind of bug with a recent update?
