Jquery如何定位:固定了一个元素,直到页面的某个div结束

    $(window).on("scroll", function() {
      var scrollTop = $(this).scrollTop();

      var containerOffset = $(".content").offset().top;
      var containerHeight = $(".content").height();
      var f1 = parseInt(containerHeight);
      var f2 = parseInt(containerOffset);
      var sum = f1 + f2;

        if (scrollTop > sum - 240) {
          $(".item")
            .css("position", "relative")
            .css("top", f1 + "px")
        }  else {
          $(".item")
            .css("position", "fixed")
            .css("top", "20px");
        }

    });

.content {
  height: 1000px;
  border: 1px solid red;
  background: grey;
}

.item {
  position: fixed;
  top: 20px;
  height: 40px;
  background: #fff;
  width: 100%;
}

.another {
  background: orange; 
  height: 400px;
}

.more {
  background: darkblue;
  height: 400px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="container">
  
  <div class="content">
    <div class="item">
      scroll only until end of grey CONTENT DIV
    </div>
  </div>
  <div class="another">
    <h1>Another div</h1>
  </div>
  <div class="more">
    <h1>More content</h1>
  </div>

</div>