code: function overDragThreshold(pos2, mouseDownEvent) {
    var pos1 = eventToPosition(mouseDownEvent);
    var dx = Math.abs(pos1.x - pos2.x);
    var dy = Math.abs(pos1.y - pos2.y);
    return Math.sqrt(dx * dx + dy * dy) > DRAG_THRESHOLD;
  }
docs: px, radius