A BBC navigation bar component broke depending on the external monitor

Why are websites getting mouse position in screen coordinates in the first place?

I've searched for reasons and couldn't find much. The fact that a website can know where a browser window is located (window.screenX/window.screenY) and that clicks position can be reported in that coordinate system sounds insane for a desktop. TOR Browser seems to spoof screenX and screenY to avoid fingerprinting.

Has anyone seen good use-cases for that feature? I'm thinking about dual window applications that interacts with each other (I think I saw a demo of something like this a while ago on HN but I wasn't able to find it again), or sites where behavior depends on their location on the virtual screen.