水平导航栏的下拉菜单中出现双边框

<nav>
  <ul>
    <li><a href="#">菜单1</a></li>
    <li><a href="#">菜单2</a>
      <ul>
        <li><a href="#">子菜单1</a></li>
        <li><a href="#">子菜单2</a></li>
      </ul>
    </li>
    <li><a href="#">菜单3</a></li>
  </ul>
</nav>

nav ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
}

nav ul li {
  display: inline-block;
  position: relative;
}

nav ul li a {
  display: block;
  padding: 10px;
  text-decoration: none;
  color: #000;
}

nav ul li ul {
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  width: 200px;
  padding: 10px;
  background-color: #fff;
  border: 1px solid #ccc;
  border-top: none;
}

nav ul li:hover ul {
  display: block;
}

nav ul li ul:before {
  content: "";
  position: absolute;
  top: -1px;
  left: -1px;
  width: 100%;
  height: 100%;
  border: 1px solid #ccc;
  border-bottom: none;
}