我试着让导航菜单重叠每个部分。 但是我尝试使用Z索引，但是它似乎在这种情况下不起作用。

我正在尝试许多解决方案，比如为所有元素添加位置，但仍然不起作用。 这是我的HTML代码

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<div class="wrap-content">
  <header class="header">
    <div class="header__center">
      <nav class="header__center--nav">
        <ul>
          <li class="main-menu"><a class="main-menu__a" href=""><span class="flaticon-right-arrow main-menu__a"></span>
                Elements</a>
            <div class="wrapper-menu">
              <div class="row">
                <div class="col-xl-3 col-lg-3">
                  <h3>Classic</h3>
                  <div class="menu-1">
                    <ul>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>About us</a></li>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>About me</a></li>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>Contact us</a></li>
                    </ul>
                  </div>
                </div>
                <div class="col-xl-3 col-lg-3">
                  <h3>Pretentation</h3>
                  <div class="menu-1">
                    <ul>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>About us</a></li>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>About me</a></li>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>Contact us</a></li>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>Contact</a></li>
                    </ul>
                  </div>
                </div>
                <div class="col-xl-3 col-lg-3">
                  <h3>Infographic</h3>
                  <div class="menu-1">
                    <ul>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>About us</a></li>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>About me</a></li>

                    </ul>
                  </div>
                </div>
                <div class="col-xl-3 col-lg-3">
                  <h3>Typography</h3>
                  <div class="menu-1">
                    <ul>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>About us</a></li>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>About me</a></li>
                      <li> <a href="#"><span class="flaticon-right-arrow"></span>Contact us</a></li>
                    </ul>
                  </div>
                </div>
              </div>
            </div>

          </li>
        </ul>
      </nav>
    </div>
  </header>
  <section class="section-1">

    <div class="container">
      <div class="row">
        <div class="col-6">
          <div class="col-6__inner">
            <h1>Incredible app
              <span class="col__inner--trade">features</span></h1>
            <span></span>
          </div>

          <div class="col-6__title">
            <h3>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Illo sdam dolorem debitis quod.</h3>
          </div>

        </div>
        <div class="col-6">
          <div class="col-6__phone">
            <div class="img-phone-1">
              <img src="./img/phone-img/phone-2.png" alt="">
            </div>

          </div>
        </div>
      </div>
    </div>
  </section>
</div>

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下面是我设置nav-menu的z索引为9999后的结果。 在此处输入图像说明