Hi! I've been getting these weird black boxes from some time, since I installed El Capitán. It has never happened before, and it occurs randomly, without any "specific" way to cause them. I think it's a graphic issue, but I don't know if it's a known bug, or if it affects people other than me. They are persistent, and stay at desktop level (I can see other windows on top, but the boxes persist behind it). Force quitting Finder won't fix it. If I switch to another Space, those boxes aren't shown, but after going back to the previous one, they're still there. If I choose another desktop background it goes away, but it eventually occurs again. Why might that be? What could be causing it? Thanks.