clickableRegion.js

  var down = false;

  canvas.addEventListener("mousedown", function (evt) { //event listener
    down = onButton(getMousePos(evt));
  });

  //handle the document mouseup to reset the "down" variable every time.
  document.addEventListener("mouseup", function (evt) {
    if (onButton(getMousePos(evt)) && down) {
      //we got a click!
    }
  });