Ever since upgrading to macOS Sierra, previously solid background colors in Atom and on certain websites in Chrome are now not uniform. It almost looks like I highlighted stuff with my cursor or something.
It only happens when I plug my mac into an external monitor. If I am working on just my macbook pro screen, everything is normal. When I plug my HDMI into an external monitor, the non-uniform backgrounds start again — including on my macbook pro screen, if mirrored. The HDMI and monitor were checked with other laptops of mine, and the problem never came up. I also tried a different HDMI with the mac, and same problem.
Certain apps like Safari, do not display this problem at all. I've included a screenshot for example: