我为带有左侧导航栏的网站设置了jsfiddle。导航栏上始终有一个用于扩展/折叠的按钮。我试图对其进行更改，以便仅在导航栏折叠时才会显示该按钮，例如，当我将屏幕尺寸更改为太薄而左侧导航栏消失时；是我唯一希望显示“展开侧栏”按钮的时间
https://jsfiddle.net/martinradio/a0k2c5rv/
/*
DEMO STYLE
*/
@import "https://fonts.googleapis.com/css?family=Poppins:300,400,500,600,700";
body {
font-family: 'Poppins', sans-serif;
background: #fafafa;
}
p {
font-family: 'Poppins', sans-serif;
font-size: 1.1em;
font-weight: 300;
line-height: 1.7em;
color: #999;
}
a,
a:hover,
a:focus {
color: inherit;
text-decoration: none;
transition: all 0.3s;
}
.navbar {
padding: 15px 10px;
background: #fff;
border: none;
border-radius: 0;
margin-bottom: 40px;
box-shadow: 1px 1px 3px rgba(0, 0, 0, 0.1);
}
.navbar-btn {
box-shadow: none;
outline: none !important;
border: none;
}
.line {
width: 100%;
height: 1px;
border-bottom: 1px dashed #ddd;
margin: 40px 0;
}
/* ---------------------------------------------------
SIDEBAR STYLE
----------------------------------------------------- */
.wrapper {
display: flex;
width: 100%;
align-items: stretch;
}
#sidebar {
min-width: 250px;
max-width: 250px;
background: #7386D5;
color: #fff;
transition: all 0.3s;
}
#sidebar.active {
margin-left: -250px;
}
#sidebar .sidebar-header {
padding: 20px;
/* background: #6d7fcc; */
}
#sidebar ul.components {
/* padding: 20px 0; */
/* border-bottom: 1px solid #47748b; */
}
#sidebar ul p {
color: #fff;
padding: 10px;
}
#sidebar ul li a {
padding: 10px;
font-size: 1.1em;
display: block;
}
#sidebar ul li a:hover {
color: #7386D5;
background: #fff;
}
#sidebar ul li.active>a,
a[aria-expanded="true"] {
color: #fff;
background: #6d7fcc;
}
a[data-toggle="collapse"] {
position: relative;
}
.dropdown-toggle::after {
display: block;
position: absolute;
top: 50%;
right: 20px;
transform: translateY(-50%);
}
ul ul a {
font-size: 0.9em !important;
padding-left: 30px !important;
background: #6d7fcc;
}
ul.CTAs {
padding: 20px;
}
ul.CTAs a {
text-align: center;
font-size: 0.9em !important;
display: block;
border-radius: 5px;
margin-bottom: 5px;
}
a.download {
background: #fff;
color: #7386D5;
}
a.article,
a.article:hover {
background: #6d7fcc !important;
color: #fff !important;
}
/* ---------------------------------------------------
CONTENT STYLE
----------------------------------------------------- */
#content {
width: 100%;
padding: 20px;
min-height: 100vh;
transition: all 0.3s;
}
/* ---------------------------------------------------
MEDIAQUERIES
----------------------------------------------------- */
@media (max-width: 768px) {
#sidebar {
margin-left: -250px;
}
#sidebar.active {
margin-left: 0;
}
#sidebarCollapse span {
display: none;
}
}<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<title>martinbarker.me</title>
<!-- Bootstrap CSS CDN -->
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" integrity="sha384-9gVQ4dYFwwWSjIDZnLEWnxCjeSWFphJiwGPXr1jddIhOegiu1FwO5qRGvFXOdJZ4" crossorigin="anonymous">
<!-- Our Custom CSS -->
<link rel="stylesheet" href="style.css">
<!-- Font Awesome JS -->
<script defer src="https://use.fontawesome.com/releases/v5.0.13/js/solid.js" integrity="sha384-tzzSw1/Vo+0N5UhStP3bvwWPq+uvzCMfrN1fEFe+xBmv1C/AtVX5K0uZtmcHitFZ" crossorigin="anonymous"></script>
<script defer src="https://use.fontawesome.com/releases/v5.0.13/js/fontawesome.js" integrity="sha384-6OIrr52G08NpOFSZdxxz1xdNSndlD4vdcf/q2myIUVO0VsqaGHJsB0RaBE01VTOY" crossorigin="anonymous"></script>
</head>
<body>
<div class="wrapper">
<!-- Sidebar -->
<nav id="sidebar">
<div class="sidebar-header">
<h3>Title</h3>
</div>
<ul class="list-unstyled components">
<li class="active">
<a href="#homeSubmenu" data-toggle="collapse" aria-expanded="false" class="dropdown-toggle">Home</a>
<ul class="collapse list-unstyled" id="homeSubmenu">
<li>
<a href="#">Home 1</a>
</li>
<li>
<a href="#">Home 2</a>
</li>
<li>
<a href="#">Home 3</a>
</li>
</ul>
</li>
<li>
<a href="#">About</a>
</li>
<li>
<a href="#pageSubmenu" data-toggle="collapse" aria-expanded="false" class="dropdown-toggle">Pages</a>
<ul class="collapse list-unstyled" id="pageSubmenu">
<li>
<a href="#">Page 1</a>
</li>
<li>
<a href="#">Page 2</a>
</li>
<li>
<a href="#">Page 3</a>
</li>
</ul>
</li>
<li>
<a href="#">Portfolio</a>
</li>
<li>
<a href="#">Contact</a>
</li>
</ul>
</nav>
<!-- Page Content -->
<div id="content">
<nav class="navbar navbar-expand-lg navbar-light bg-light">
<div class="container-fluid">
<button type="button" id="sidebarCollapse" class="btn btn-info">
<i class="fas fa-align-left"></i>
<span>Toggle Sidebar</span>
<!-- </button>
<button class="btn btn-dark d-inline-block d-lg-none ml-auto" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<i class="fas fa-align-justify"></i>
</button>
-->
<!--
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="nav navbar-nav ml-auto">
<li class="nav-item active">
<a class="nav-link" href="#">Page</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Page</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Page</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Page</a>
</li>
</ul>
</div>
-->
</div>
</nav>
<h2>Collapsible Sidebar Using Bootstrap 4</h2>
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</div>
</div>
<!-- jQuery CDN - Slim version (=without AJAX) -->
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<!-- Popper.JS -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js" integrity="sha384-cs/chFZiN24E4KMATLdqdvsezGxaGsi4hLGOzlXwp5UZB1LY//20VyM2taTB4QvJ" crossorigin="anonymous"></script>
<!-- Bootstrap JS -->
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js" integrity="sha384-uefMccjFJAIv6A+rW+L4AHf99KvxDjWSu1z9VI8SKNVmz4sk7buKt/6v9KI65qnm" crossorigin="anonymous"></script>
<script type="text/javascript">
$(document).ready(function () {
$('#sidebarCollapse').on('click', function () {
$('#sidebar').toggleClass('active');
});
});
</script>
</body>
</html>