clientX/Y: The location in relation to the visible portion of the document. If you click point A, then scroll the page, then click again without having moved the mouse, a subsequent click will return the same value because your X/Y position within the visible area has not changed. This has no relation to the component.
pageX/Y: The location in relation to the entire document (page). If you click point A, then scroll the page, then click again without having moved the mouse, a subsequent click will return a different value, because the Y position is no longer the same as the first click.
screenX/Y The location in relation to the actual physical monitor screen. Scrolling will have no effect on these values. They do no correlate to the session/page - only to the user’s monitor.