Out of the blue I started getting these weird borders around my windows. It either happens after coming back from full-screen applications, or after system resume. A reboot fixes this problem, but only temporarily. I have not updated my graphics drivers (not manually, automatically maybe) or had any problems since installing last year.
My video card is a NVIDIA Quadro K1000M, and I've been using NVIDIA binary driver version 375.39 since launch. I have not yet tried tinkering with the drivers, seeing if it would fix the problem or not.