While I can understand the author's need for screenX and screenY, the question remains. Why would screenX return the real screenX position instead of the positi

While I can understand the author's need for screenX and screenY, the question remains. Why would screenX return the real screenX position instead of the position within the renderer (I don't think that exists?) or the rendered page (layerX and layerY)? The author's need would be met the same with the renderer position and window positions wouldn't be leaked to all visited websites.