How did he make it work? I didn't think this would be possible.
Yes, extremely impressed with the tool.
EDIT: Looked at the code. He takes screenshot of the active tab when extension is clicked. And then parses the PNG in measureDistances function when mouse moves.
For a mouse at position X, Y, it tries to find the farthest Top, Bottom, Right, Left points with same color/threshold. This is why it is able to see through 'transparent' Google logo.
I am very impressed.