*{margin:0;padding:0;}
 body{
 margin:0 auto;
   max-width:10rem;
 }
 header{
 width:10rem;
 height:1rem;
 background:red;
 position:fixed;
 top:0;
 left:auto;
 }
 section{
 height:100%;
 overflow: auto;
 padding:1rem 0;
 }
 .bananers{
 width:100%;
 height:3rem;
 text-align: center;
 line-height:3rem;
 background: aqua;
 }
 .mains{
 width:100%;
 height:1rem;
 background:red;
 display: flex;
 }
 .mains>div{
 width:100%;
 height:100%;
 border:1px solid #dddddd;
 display: flex;
 align-items: center;
 justify-content: center;
 }
 .contents{
 width:100%;
 }
 .contents>div{
 height:50px;
 line-height:50px;
 padding-left:10%;
 border-bottom:1px solid red;
 }
 footer{
 width:10rem;
 height:1rem;
 background: #0086B3;
 position:fixed;
 left:auto;
 bottom:0;
 }
  .fixed-top {
    position: fixed;
    width: 100%;
    top:1rem;
    left:auto;
  }
  .sticky {
    position: -webkit-sticky;/*滚过初始位置时自动吸顶*/
    position: sticky;/*吸顶时的定位*/
    top:1rem;
    left:auto;
    z-index: 999;/*z-index比下方所有层级要高*/
  }

<header>头部</header>
<section>
  <div class="bananers">内容</div>
  <div class="mains">
  <div>导航1</div>
  <div>导航2</div>
  <div>导航3</div>
  </div>
  <div class="contents"></div>
</section>
<footer>页脚</footer>

for(var i=0;i<20;i++){
 $(".contents").append(`<div>${i+1}</div>`)
 }
 var headers=$("header")[0].getBoundingClientRect().height;
 var mains=$(".mains").offset().top;
 var heights=mains-headers;
 $(".bananers").html(mains+"----"+headers);
 var tops = document.querySelector('.mains');
 function fixed(num) {
    var nys= navigator.userAgent;
    var isAndroid = nys.indexOf('Android') > -1 || nys.indexOf('Adr') > -1; 
    var isIOS = !!nys.match(/\(i[^;]+;( U;)? CPU.+Mac OS X/); 
    if(isAndroid) {
      document.body.onscroll = function(e) {
        var scrollT = document.body.scrollTop;
        if (scrollT > num) {
          $(tops).addClass('fixed-top');
        }else {
          $(tops).removeClass('fixed-top');
        }
      };
    }else if(isIOS) {
      $(tops).addClass('sticky');
    }
  }
fixed(heights);