How to get the mouse position relative to the window viewport in javascript?
Try using event.clientY that should always return the correct value regardless of scrolling
I was in similar situation, I required cursor's coordinates w.r.t the Viewport (since my page was scrollable).
I tried other answers here, they didn't seem to work once the screen was scrolled (they worked well with non-scrollable pages).
Upon reading a few documentation page of https://developer.mozilla.org/en-US/docs/Web/API/MouseEventI found that while working with scrollable page, if you need X and Y coordinates w.r.t the current Viewport (i.e. even if scrolled), it would be better to useevent.pageX instead.
var pageX = MouseEvent.pageX;
https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/pageX