Why there is a difference in the way css is applied to the modal window in Chrome and FF?

问题

I was answering another question today where the modal window had clickable buttons in FF and non-clickable buttons in Chrome.

Though I have figured out the issue that chrome was setting z-index of modal's parent to 0 and FF was setting it to auto.

In the demo below a modal window is placed inside of .fixed div. To fix the issue I had set z-index of .fixed div more than .modal-backdrop (let's say 1041). Then I thought why can't I set the z-index of .fixed div myself as auto as FF sets it to auto, but it still didn't work in Chrome. Can anyone explain what's happening here.

$(function(){
  $('.modal-body > span').html($('.fixed').css('z-index'));
});

.fixed{
      position: fixed;
    }

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" integrity="sha384-1q8mTJOASx8j1Au+a5WDVnPi2lkFfwwEAa8hDDdjZlpLegxhjVME1fgjWPGmkzs7" crossorigin="anonymous">
<div class="fixed">
      <!-- Button trigger modal -->
      <button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
        Launch demo modal
      </button>

      <!-- Modal -->
      <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 testing</h4>
            </div>
            <div class="modal-body">
              z-index: <span></span>
            </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>
    </div>
    <!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <!-- Latest compiled and minified JavaScript -->
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js" integrity="sha384-0mSbJDEHialfmuBBQP6A4Qrprq5OVfW37PRR3j5ELqxss1yVqOtnepnHVP9aJ7xS" crossorigin="anonymous"></script>

回答1:

z-index sets the order of elements within a stacking context. position:fixed on the parent element of the modal contents places it in a different stacking context than the .modal-backdrop, used to hide the rest of the page.

The common solution for this is to place all the instances of modal contents from the page as direct children of the body element, after page loads, using this line:

$('.modal[role="dialog"]').appendTo('body');

$(function(){
  $('.modal-body > span').html($('.fixed').css('z-index'));
});

$('.modal[role="dialog"]').appendTo('body');

.fixed{
    position: fixed;
}

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" integrity="sha384-1q8mTJOASx8j1Au+a5WDVnPi2lkFfwwEAa8hDDdjZlpLegxhjVME1fgjWPGmkzs7" crossorigin="anonymous">
<div class="fixed">
      <!-- Button trigger modal -->
      <button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
        Launch demo modal
      </button>

      <!-- Modal -->
      <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 testing</h4>
            </div>
            <div class="modal-body">
              z-index: <span></span>
            </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>
    </div>
    <!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <!-- Latest compiled and minified JavaScript -->
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js" integrity="sha384-0mSbJDEHialfmuBBQP6A4Qrprq5OVfW37PRR3j5ELqxss1yVqOtnepnHVP9aJ7xS" crossorigin="anonymous"></script>

来源：https://stackoverflow.com/questions/34468117/why-there-is-a-difference-in-the-way-css-is-applied-to-the-modal-window-in-chrom