如何在折叠时为响应式导航栏添加平滑过渡？

<nav class="navbar">
  <div class="navbar-toggle" onclick="toggleNavbar()">
    <span class="icon-bar"></span>
    <span class="icon-bar"></span>
    <span class="icon-bar"></span>
  </div>
  <ul class="navbar-links">
    <li><a href="#">Home</a></li>
    <li><a href="#">About</a></li>
    <li><a href="#">Services</a></li>
    <li><a href="#">Contact</a></li>
  </ul>
</nav>

.navbar {
  background-color: #333;
  color: #fff;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 60px;
  transition: height 0.3s ease, opacity 0.3s ease;
}

.navbar.collapsed {
  height: 40px;
  opacity: 0.8;
}

.navbar-links {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  justify-content: flex-end;
  align-items: center;
  height: 100%;
}

.navbar-links li {
  margin: 0 10px;
}

.navbar-links li a {
  color: #fff;
  text-decoration: none;
}

.navbar-toggle {
  display: none;
  cursor: pointer;
}

.icon-bar {
  display: block;
  width: 20px;
  height: 2px;
  background-color: #fff;
  margin-bottom: 4px;
}

@media (max-width: 768px) {
  .navbar-links {
    display: none;
  }
  
  .navbar-toggle {
    display: block;
  }
  
  .navbar.collapsed {
    opacity: 1;
  }
}

function toggleNavbar() {
  var navbar = document.querySelector('.navbar');
  navbar.classList.toggle('collapsed');
}