title: What is the proper way to get the mouse position in a React Component?
body: <p>I have a component that renders a canvas element.  I have an <code>onMouseMove</code> handler, but when I move my mouse towards the top of the element <code>event.clientY</code> prints 86.  Why not 0?  Is there an synthetic event property that will give me the point relative to the element's coordinate system and not the windows?</p>