汉堡包中的Bootstrap 4下拉菜单显示在“后面”

Question

问题：

如果我们打开汉堡包，然后激活下拉菜单，我们会发现其内容显示不正确。

执行以下操作：

下拉菜单将z-index设置为默认值1000，它似乎不起作用。 怎么修呢？ https://jsfiddle.net/zx3skern/ 下面是我的代码，上面是我的小提琴

     <nav id="mainnavbar" style="z-index: 0;-moz-box-shadow:0 5px 5px rgba(182, 182, 182, 0.75);
    -webkit-box-shadow: 0 5px 5px rgba(182, 182, 182, 0.75);
    box-shadow: 0 5px 5px rgba(182, 182, 182, 0.75); z-index:1; background:white;" class="navbar navbar-expand-lg navbar-light bg-light fixed-top">
        <img id="scrollToTop" src="photos/navground.png" style=" cursor:pointer;">
        <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
        </button>
        <div class="collapse navbar-collapse" id="navbarSupportedContent">
            <ul class="navbar-nav mr-auto">
                <li class="nav-item active">
                </li>
                <li style="height:60px;" class="nav-item spannavd dropdown">
                    <span class=" dropdown-toggle" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                      Strefa Zawodnika
                    </span>
                    <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                        <a class="dropdown-item" href="#">Action</a>
                        <a class="dropdown-item" href="#">Another action</a>
                        <div class="dropdown-divider"></div>
                        <a class="dropdown-item" href="#">Something else here</a>
                    </div>
                </li>
                <li style="height:60px;" class="nav-item spannavd dropdown">
                    <span class=" dropdown-toggle" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                      Wspólpraca
                    </span>
                    <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                        <a class="dropdown-item" href="#">Action</a>
                        <a class="dropdown-item" href="#">Another action</a>
                        <div class="dropdown-divider"></div>
                        <a class="dropdown-item" href="#">Something else here</a>
                    </div>
                </li>
                <li style="height:60px;" class="nav-item spannavd dropdown">
                    <span class="dropdown-toggle" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                      Konferencja
                    </span>
                    <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                        <a class="dropdown-item" href="#">Action</a>
                        <a class="dropdown-item" href="#">Another action</a>
                        <div class="dropdown-divider"></div>
                        <a class="dropdown-item" href="#">Something else here</a>
                    </div>
                </li>
                <li class="spannav">Cennik</li>
                <li onclick="window.open('https://www.facebook.com/pg/WartaChallenge/photos/?ref=page_internal', '_blank');" class="spannav">Galeria</span>
                    <li class="spannav">Kontakt</li>
            </ul>
        </div>
    </nav>