Disable window shadow on 10.11 / 10.12?
edit 01-11-2019: Works on Mojave.

There is a solution - plugin called winBuddy which is available through MacForge (previously known as mySIMBL).

It allows you to turn off window shadow and/or window borders for every app.

I've just tested it and it seems to work properly, there are some minor glitches here and there but nothing major.

Minor glitches include:


You can't change window options for apps which don't have menu - like Bartender for example
You can't change window options for some apps, as winBuddy does not show up under 'Window' in menu.