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; https://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.