Operating system and graphics card
Broken: 2.78, 2.78a, 2.78b
Short description of error
While trying to key green screen if my viewer node is on the second monitor it always picks up black color. I have to sample the color within the same monitor to make it work.
Exact steps for others to reproduce the error
You need two monitors to test it.
Open two windows, one NODE EDITOR, the other UV/IMAGE EDITOR.
Drag one of them to the other monitor
Then try to sample color from viewer node.
You will see that it always bring black color.