固定定位div不尊重相对定位父级?

//Code to make sub-nav and cart scroll with the page
$(document).ready(function() {
  var stickySidebar = $('.sideSubNav, .cartcontainer').offset().top;

  $(window).scroll(function() {
    if ($(window).scrollTop() > stickySidebar) {
      $('.sideSubNav, .cartcontainer').addClass('affix');
    } else {
      $('.sideSubNav, .cartcontainer').removeClass('affix');
    }
  });
});

.sideSubNav.affix,
.cartcontainer.affix {
  position: fixed;
  top: 0;
}

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<div class="col-sm-9">
  <div class="sideSubNav">
    <ul class="nav">
      <li class="nav-item">
        <a class="nav-link" href="#">Link</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link</a>
      </li>
    </ul>
  </div>
</div>

<div class="col-sm-3">
  <div class="cartcontainer">
    <!--shopping cart-->
  </div>
</div>