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

Question

Today I answered another question when a modal window had buttons with a click in FF and buttons without clicks in Chrome.

Although I found out that chrome set the z-index of the modal parent to 0, and FF set it to auto.

In the demo below, the modalwindow is placed inside the .fixeddiv. To fix the problem, I installed z-indexof .fixeddiv more than .modal-backdrop(say, 1041). Then I wondered why I cannot set the z-index .fixeddiv as auto, since FF sets it to auto, but it still does not work in Chrome. Can someone explain what is 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 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>

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Answer 1

z-index stacking. position:fixed , .modal-backdrop, .

body , :

$('.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 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>

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