What about eraser?

Great article! Thanks! I've made the "fixed-width" version, by setting the canvas size to copy it's parent wrapper's size.

  ctx.canvas.width = canvas.offsetWidth;
  ctx.canvas.height = canvas.offsetHeight;

codepen.io/frendhisaido_1470415284/pen/dyXypKB

But I'm still not sure why if the canvas is inside any parent that has a relative position, the mouse draw precision will be off by a few pixels.