The MouseEvent.clientY read-only property returns the vertical coordinate within the application's client area at which the event occurred (as opposed to the coordinates within the page). For example, clicking in the top-left corner of the client area will always result in a mouse event with a clientY value of 0, regardless of whether the page is scrolled vertically.

API CSSOM View DOM Events MouseEvent Property Read-only Reference Référence

The MouseEvent.screenX read-only property provides the horizontal coordinate of the mouse pointer in global (screen) coordinates.

API CSSOM View DOM Events MouseEvent Property Read-only Reference Référence