It's very difficult to explain this problem without a picture. See attached
I have 2 Dell monitors. One is on Display port for AMD Firepro W4100, the other is on HDMI for an Intel graphics card. To me it seems this problem occurs when I open the window on the Dell monitor that run the DisplayPort. If I re-size the window it will be fine, it's only in some window sizes this problem occurs. It's very annyoing. I have the most recent display drivers installed. Can I change any settings?