My 2011 iMac has developed a strange pattern that displays over every application window. It looks like a chessboard grid of yellow squares. If I scroll the window, the pattern moves as well and rebooting doesn’t fix it. It only affects the application windows themselves – the desktop background is fine. Upgrading to Yosemite didn’t help. Mike Doulston Try these two tests. Take a screenshot with ç+ß+3 and set that as your desktop background. Then restart with ß held down, to start up in Safe mode. If the pattern appears on the desktop but doesn’t show on the app windows while you are in Safe mode, it’s a graphics drivers fault. Reinstalling OS X using ç+R to restart in Recovery mode may help, otherwise report it to Apple. But I don’t think you’ll need to cross that bridge. More likely, the pattern will still be there in Safe mode. That indicates a hardware fault. If you can see the pattern on the desktop, it’s a graphics card fault. If the desktop looks fine but the app windows are still messed up, it’s an LCD display fault. Repairing either isn’t cheap and you may want to cut your losses and get a new one or a working secondhand model.