如何在Bootstrap 3中消除.dropdown与.dropdown-menu之间的边框？

我来给你拆解一下怎么实现这种下拉菜单和导航项无缝衔接、没有分隔边框的效果，核心就是通过调整边框、背景和过渡动画，让hover状态下的导航项和下拉菜单完美融合：

关键CSS样式调整要点 #

• 先清除导航项默认的底部边框，避免和下拉菜单的边框冲突：

.navbar-default .navbar-nav>li>a {
  border-bottom: 0;
}

• 给下拉导航项设置初始透明边框，hover时切换为实色边框并同步背景色：
  这样做是为了让hover时导航项的左右边框和下拉菜单的左右边框对齐，同时把底部边框设为0，消除和下拉菜单上边框的分隔线

.dropdown {
  border-left: 1px solid transparent;
  border-right: 1px solid transparent;
}
.dropdown:hover {
  background: white;
  border-left: 1px solid #ccc;
  border-right: 1px solid #ccc;
  border-bottom: 0;
}

• 改造下拉菜单的样式，让它和导航项衔接：
  去掉默认的阴影、圆角，设置和导航项一致的上边框，再添加平滑展开的过渡动画，初始状态隐藏，hover时展开

.dropdown .dropdown-menu {
  border-top: 1px solid #ccc;
  z-index: 1;
  transition: all 0.5s;
  max-height: 0;
  display: block;
  overflow: hidden;
  opacity: 0;
  box-shadow: none;
  border-radius: 0;
}
.dropdown:hover .dropdown-menu {
  max-height: 400px;
  opacity: 1;
}

完整代码示例 #

HTML代码

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
<div class="container">
  <div class="row">
    <nav class="navbar navbar-default">
      <div class="container-fluid">
        <div class="navbar-header">
          <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
            <span class="sr-only">Toggle navigation</span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
          </button>
          <a class="navbar-brand" href="#">Brand</a>
        </div>
        <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
          <ul class="nav navbar-nav">
            <li class="active"><a href="#">Link <span class="sr-only">(current)</span></a></li>
            <li><a href="#">Link</a></li>
            <li class="dropdown">
              <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
              <ul class="dropdown-menu">
                <li><a href="#">Action</a></li>
                <li><a href="#">Another action</a></li>
                <li><a href="#">Something else here</a></li>
                <li role="separator" class="divider"></li>
                <li><a href="#">Separated link</a></li>
                <li role="separator" class="divider"></li>
                <li><a href="#">One more separated link</a></li>
              </ul>
            </li>
            <li class="dropdown">
              <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
              <ul class="dropdown-menu">
                <li><a href="#">Action</a></li>
                <li><a href="#">Another action</a></li>
                <li><a href="#">Something else here</a></li>
                <li role="separator" class="divider"></li>
                <li><a href="#">Separated link</a></li>
                <li role="separator" class="divider"></li>
                <li><a href="#">One more separated link</a></li>
              </ul>
            </li>
          </ul>
          <ul class="nav navbar-nav navbar-right">
            <li><a href="#">Link</a></li>
            <li class="dropdown">
              <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
              <ul class="dropdown-menu">
                <li><a href="#">Action</a></li>
                <li><a href="#">Another action</a></li>
                <li><a href="#">Something else here</a></li>
                <li role="separator" class="divider"></li>
                <li><a href="#">Separated link</a></li>
              </ul>
            </li>
          </ul>
        </div>
      </div>
    </nav>
  </div>
</div>

CSS代码

.navbar-default .navbar-nav>li>a { 
  border-bottom: 0; 
} 
.dropdown { 
  border-left: 1px solid transparent; 
  border-right: 1px solid transparent; 
} 
.dropdown:hover { 
  background: white; 
  border-left: 1px solid #ccc; 
  border-right: 1px solid #ccc; 
  border-bottom: 0; 
} 
.dropdown .dropdown-menu { 
  border-top: 1px solid #ccc; 
  z-index: 1; 
  transition: all 0.5s; 
  max-height: 0; 
  display: block; 
  overflow: hidden; 
  opacity: 0; 
  box-shadow: none; 
  border-radius: 0; 
} 
.dropdown:hover .dropdown-menu { 
  max-height: 400px; 
  opacity: 1; 
}

内容的提问来源于stack exchange，提问作者Mona Coder