SCSS

HTML

<nav id="left-navbar">
  <div id="left-menu">
    <ul id="menu-menu" class="left-bar">

      <li class="menu-item menu-item-has-children">
        <a href="#">Menu</a>
        <ul class="sub-menu">
          <li class="menu-item"><a href="#">Submenu</a></li>
          <li class="menu-item"><a href="#">Submenu</a></li>
        </ul>
      </li>

      <li class="menu-item menu-item-has-children">
        <a href="#">Menu</a>
        <ul class="sub-menu">
          <li class="menu-item"><a href="#">Submenu</a></li>
          <li class="menu-item"><a href="#">Submenu</a></li>
        </ul>
      </li>
      
      <li class="menu-item menu-item-has-children">
        <a href="#">Menu</a>
        <ul class="sub-menu">
          <li class="menu-item"><a href="#">Submenu</a></li>
          <li class="menu-item"><a href="#">Submenu</a></li>
        </ul>
      </li>
      
      <li class="menu-item menu-item-has-children">
        <a href="#">Menu</a>
        <ul class="sub-menu">
          <li class="menu-item"><a href="#">Submenu</a></li>
          <li class="menu-item"><a href="#">Submenu</a></li>
        </ul>
      </li>
      
      <li class="menu-item menu-item-has-children">
        <a href="#">Menu</a>
        <ul class="sub-menu">
          <li class="menu-item"><a href="#">Submenu</a></li>
          <li class="menu-item"><a href="#">Submenu</a></li>
        </ul>
      </li>

      <li class="menu-item">
        <a href="#">Menu</a>
      </li>

      <li class="menu-item">
        <a href="#">Menu</a>
      </li>

      <li class="menu-item">
        <a href="#">Menu</a>
      </li>
      
      <li class="menu-item">
        <a href="#">Menu</a>
      </li>
      
      <li class="menu-item">
        <a href="#">Menu</a>
      </li>
      
      <li class="menu-item">
        <a href="#">Menu</a>
      </li>
      
      <li class="menu-item">
        <a href="#">Menu</a>
      </li>
    </ul>
  </div>
</nav>

CSS

#left-navbar{
    display: flex;
    z-index: 10;
    width: 100%;
    background-color: #379264;
    font-weight: bold;
    box-shadow: 0px 1.5px 2px rgba(0, 0, 0, 0.7);
    text-shadow: 1px 1px 2px black;
}

#left-navbar{
    width: 350px;
    height: 100%;
    flex-direction: row;
    position: fixed;
}

#left-navbar:hover{
    overflow-y: auto;
}

.left-bar li a{
    padding: 0.8vw;
}

#left-menu{
    height: auto;
    width: 100%;
    position: relative;
}

#left-menu .left-bar{
    margin-top: 0;
    height: 100vh;
}

.left-bar{
    display: flex;
    flex-direction: column;
    padding: 0; 
    list-style-type: none;
}

.left-bar li{
    position: relative;
}

.left-bar li a{
    text-decoration: none;
    padding-left: 2vw;
    padding-right: 2vw;
    font-weight: bold;
    display: block;
}

#left-navbar ul li a{
    line-height: 20px;
}

.left-bar li a:hover{
    background-color: #16663d;
    text-shadow: 1px 1px 1px black;
    box-shadow: inset 0px 0px 4px 0.5px rgba(0, 0, 0, 0.5);
}

.left-bar li .sub-menu{
    display: none;
    left: 100%;
    padding: 0;
    list-style-type: none;
    max-width: 15vw;
    box-shadow: 0px 1.5px 3px rgba(0, 0, 0, 0.7);
    border-top-right-radius: 5px;
    border-bottom-right-radius: 5px;
    background-color: #379264;
}

.left-bar .sub-menu li{
    border-bottom: 0.5px solid rgba(0, 0, 0, 0.25);
}

.left-bar .sub-menu li:last-of-type{
    border-bottom: none;
}

.left-bar .sub-menu a, .left-bar > li > a, .left-bar li a:hover{
    color: white;
}

.left-bar > .menu-item-has-children:hover > .sub-menu{
    display: block;
    width: 100%;
}

.left-bar .menu-item-has-children .sub-menu > .menu-item-has-children:hover > .sub-menu{
    display: block;
}

.left-bar .menu-item-has-children .sub-menu > .menu-item-has-children:hover > .sub-menu{
    position: absolute;
    top: 0;
    left: 100%;
    width: 100%;
}

.sub-menu{
    z-index: 100;
}

.left-bar > .menu-item-has-children:hover > .sub-menu{
    position:...