function ClientCoordinates(clientX, clientY) {
            this.clientX = clientX;
            this.clientY = clientY;
        }
Coordinates within the application's client area (i.e. origin is document's scroll position).

For example, clicking in the top-left corner of the client area will
always result in a mouse event with a client.x value of 0, regardless
of whether the page is scrolled horizontally.