bootstrap modal 子元素定位为什么是基于模态框？

Question

webkit浏览器下bootstrap modal 模态框里的子元素定位为 position:fixed 时，为什么是基于模态框而不是屏幕视窗的位置？
jsfiddle 示例

<style type="text/css">
.test {
  position: fixed;
  top: 0px;
}
</style>
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#myModal">
  Launch demo modal
</button>

<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">Modal title</h4>
      </div>
      <div class="modal-body">
        <div class="test">test</div>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

Answer 1

transform: translate(0,0);引起的，你谷歌搜一下“transform fixed”有挺多解答的~

这不是bug，这是规范的一部分，规范链接
下面是标准答案： https://www.zhihu.com/question/25552699