I developed it on a mbp 13", just not the retina version. Someone reported the bug that it doesn't work when the browser is zoomed. May that be the case for you?
Doesn't work on my Retina MBP either, but it works on a connected regular res monitor.
The problem appears to be that it maps 2x pixels to "normal" 1x pixels as output. So all the measurements are scaled up 200%, with the top left corner as the origin.
The problem appears to be that it maps 2x pixels to "normal" 1x pixels as output. So all the measurements are scaled up 200%, with the top left corner as the origin.