On my Ubuntu "retina" notebook (ThinkPad) it did not work at first, because I set my default zoom level in Chrome to 175%. Once I scaled back to 100% the extensions works great. I guess it doesn't really make since to do pixel measurements when the pixels arent 1:1, so I'm not sure this should be fixed or just documented.

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.

Doesn't work on my mbp 13" either... Also only grabs some random measurements

I have the exact same problem. Hopefully it gets resolved, this looks really helpful!