Javascript 如果文档中存在绝对位置的 div，如何 Select 背景文本

var mousePosition;
var offset = [0, 0];
var div;
var isDown = false;

div = document.createElement("div");
div.style.position = "absolute";

div.style.left = "100px";
div.style.top = "100px";
div.style.width = "100px";
div.style.height = "100px";

div.style.background = "red";
div.style.color = "blue";

document.body.appendChild(div);

div.addEventListener('mousedown', function(event) {
  if (event.target !== div) { /* <---- */
    return;
  }
  isDown = true;
  offset = [
    div.offsetLeft - event.clientX,
    div.offsetTop - event.clientY
  ];
}, true);

document.addEventListener('mouseup', function() {
  isDown = false;
}, true);

document.addEventListener('mousemove', function(event) {
  if (isDown) {
    event.preventDefault(); /* <---- */
    mousePosition = {
        x : event.clientX,
        y : event.clientY
    };
    div.style.left = (mousePosition.x + offset[0]) + 'px';
    div.style.top  = (mousePosition.y + offset[1]) + 'px';
  }
}, true);

body{
  height: 1000px;
  width: 1000px;
}

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