Just installed Mavericks on my wife's computer (2012 non-retina MBP). She has an external Dell display connected over mDP to DVI. When it's connected it gets this crazy artifact thingy. Anyone seen this? It does it on both displays equally, but goes away when the exernal display is disconnected. EDIT: The is happening anytime the machine is using the discreet graphics chip. I tried a clean install and it even did it during the installation.