Not sure if anyone else has run into this. I returned my first iMac 5k (4ghz/295x) because of the issue, but the replacement does the same thing. Apple phone support has never heard of the issue. When moving windows around the screen, the drawing is best described as "tearing" like where the contents of the window trail the window by a few milliseconds. Additionally the window contents seem to have corruption in drawing while moving (the window gets broken into blocks which move around). It all corrects itself once the window stops moving. Rebooting usually clears it up but it returns. My first iMac it came back within minutes. The replacement ran about 12 hours before doing it again. The only app running is Chrome with about 15 tabs open or so. This is running the latest OSX with all the patches, etc. Hard to believe it's a hardware issue now (originally it looked like bad VRAM to me). Anyone else see this?