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HTML

              
                <nav aria-label="navigation menu">
  <ul>
    <li><a href="#">Home</a></li>
    <li class="selected">
      <a class="toggle" href="#submenu">Billing (click to expand/collapse)</a>
      <div class="toggle-content" id="submenu">
        <ul>
          <li><a href="#">My Bill</a></li>
          <li><a href="#">Pay History</a></li>
        </ul>
      </div>
    </li>
    <li><a href="#">Employees</a></li>
    <li><a href="#">Claims</a></li>
    <li><a href="#">Docs</a></li>
  </ul>
</nav>
              
            
!

CSS

              
                .toggle-content {
  display: none;
  height: 0;
  overflow: hidden;
  transition: height .3s ease-in-out;

  &.is-visible {
    display: block;
    height: auto;
  }
}














/* general styling */
:root {
  --border-width: 6px;
  --color-main-hover: #f2f6f9;
  --color-sub-hover: #d8ebf9;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Oxygen-Sans, Ubuntu, Cantarell, sans-serif;
  margin: 0;
}

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

  li {
    color: #404040;
    font-size: 1rem;
    letter-spacing: .5px;
    text-transform: uppercase;

    ul {
      padding: 0;  
    }

    li {
      text-transform: initial;
    }

    a {
      color: inherit;
      display: flex;
      padding: 1.25rem 2rem;
      text-decoration: none;

      &:hover {
        background-color: var(--color-main-hover);
      }
    }
  }
}

.selected {
  background-color: var(--color-main-hover);

  a {
    &:hover {
      background-color: var(--color-sub-hover);
    }
  }
}

.toggle {
  padding-left: calc(2rem - var(--border-width));
}

.toggle,
.toggle-content {
  border-left: var(--border-width) solid #015295;
}
              
            
!

JS

              
                // Show an element
const show = function (elem) {

  // Get the natural height of the element
  const getHeight = function () {
    elem.style.display = 'block'; // Make it visible
    const height = elem.scrollHeight + 'px'; // Get it's height
    elem.style.display = ''; //  Hide it again
    return height;
  };

  const height = getHeight(); // Get the natural height
  elem.classList.add('is-visible'); // Make the element visible
  elem.style.height = height; // Update the max-height

  // Once the transition is complete, remove the inline max-height so the content can scale responsively
  window.setTimeout(function () {
    elem.style.height = '';
  }, 350);

};

// Hide an element
const hide = function (elem) {

  // Give the element a height to change from
  elem.style.height = elem.scrollHeight + 'px';

  // Set the height back to 0
  window.setTimeout(function () {
    elem.style.height = '0';
    console.log('window.setTimeout');
  }, 1);

  // When the transition is complete, hide it
  window.setTimeout(function () {
    elem.classList.remove('is-visible');
    console.log('window.setTimeout:remove is-visible');
  }, 300);

};

// Toggle element visibility
const toggle = function (elem, timing) {

  // If the element is visible, hide it
  if (elem.classList.contains('is-visible')) {
    hide(elem);
    return;
    console.log('hide element');
  }

  // Otherwise, show it
  show(elem);
  console.log('show element');

};

// Listen for click events
document.addEventListener('click', function (event) {

  
  // Make sure clicked element is our toggle
  if (!event.target.closest('a').classList.contains('toggle')) {
    return;
  }

  // Prevent default link behavior
  event.preventDefault();

  // Get the content
  console.log(event.target.closest('a').hash);
  const content = document.querySelector(event.target.closest('a').hash);
  if (!content) return;

  // Toggle the content
  toggle(content);

}, false);
              
            
!
999px

Console