I have a 21.5" iMac with a dual screen set up. My second screen is just a regular VGA>thunderbolt adapter hookup. When I play blizzard games, I like to flip back and forth to a web page on my secondary screen. Prior to el-captain, all animations and graphics on my primary screen would continue to play as if it were my primary screen, despite the control being on my secondary screen. After installing el-captain, blizzard games will freeze the screen when I'm on my off screen. The games still run, and when I flip back I can see that processes have continued, but the screen remains frozen from a graphics standpoint until I flip back. Thoughts?