Click on the page to set the three test layers. The red areas denote the actual coordinates returned by the property pairs, while the tell us where theory expects them to be.
Zoom factor: .
This test interprets the coordinates as page coordinates (i.e. pageX/Y). I calculate the expected positions as follows:
clientX/Y = pageX/Y - window.pageX/YOffset
screenX/Y = clientX/Y * devicePixelRatio * zoom,
zoom = documentElement.clientWidth / window.innerWidth