<nav class="navbar">
  <ul class="navbar__menu">
    <li class="navbar__item">
      <a href="#" class="navbar__link"><i data-feather="home"></i><span>Home</span></a>
    </li>
    <li class="navbar__item">
      <a href="#" class="navbar__link"><i data-feather="message-square"></i><span>Messages</span></a>        
    </li>
    <li class="navbar__item">
      <a href="#" class="navbar__link"><i data-feather="users"></i><span>Customers</span></a>        
    </li>
    <li class="navbar__item">
      <a href="#" class="navbar__link"><i data-feather="folder"></i><span>Projects</span></a>        
    </li>
    <li class="navbar__item">
      <a href="#" class="navbar__link"><i data-feather="archive"></i><span>Resources</span></a>        
    </li>
    <li class="navbar__item">
      <a href="#" class="navbar__link"><i data-feather="help-circle"></i><span>Help</span></a>        
    </li>
    <li class="navbar__item">
      <a href="#" class="navbar__link"><i data-feather="settings"></i><span>Settings</span></a>        
    </li>
  </ul>
</nav>

$borderRadius: 10px;
$spacer: 1rem;
$primary: #406ff3;
$text: #6a778e;
$linkHeight: $spacer * 3.5;
$timing: 250ms;
$transition: $timing ease all;

@mixin gooeyEffect($i) {
  @keyframes gooeyEffect-#{$i} {
    0% {
      transform: scale(1, 1);
    }
    50% {
      transform: scale(0.5, 1.5);
    }
    100% {
      transform: scale(1, 1);
    }
  }
}
@import url('https://fonts.googleapis.com/css2?family=Open+Sans:wght@600&display=swap');
body{
  background: #eaeef6;
  font-family: 'Open Sans', sans-serif;
}
.navbar{
  $ref: &;
  position: fixed;
  top: $spacer;
  left: $spacer;
  background: #fff;
  border-radius: $borderRadius;
  padding: $spacer 0;
  box-shadow: 0 0 40px rgba(0,0,0,0.03);
  height: calc(100vh - #{$spacer*4});
  &__link{
    position:relative;
    display: flex;
    align-items: center;
    justify-content: center;
    height: $linkHeight;
    width: $spacer * 5.5;
    color: $text;
    transition: $transition;
    span{
      position: absolute;
      left: 100%;
      transform: translate(-($spacer*3));
      margin-left: 1rem;
      opacity: 0;
      pointer-events: none;
      color: $primary;
      background: #fff;
      padding: $spacer *0.75;
      transition: $transition;
      border-radius: $borderRadius * 1.75;
    }
    &:hover{
      color: #fff;
    }
    .navbar:not(:hover) &:focus,
    &:hover{
      span{
        opacity: 1;
        transform: translate(0);
      }
    }
  }
  &__menu{
    position: relative;
  }
  &__item{
    &:last-child{
      &:before{
        content: '';
        position: absolute;
        opacity: 0;
        z-index: -1;
        top: 0;
        left: $spacer;
        width: $linkHeight;
        height: $linkHeight;
        background: $primary;
        border-radius: $borderRadius * 1.75;
        transition: $timing cubic-bezier(1, 0.2, 0.1, 1.2) all;
        
      }
    }
    
    @for $i from 1 to 12 {
      &:first-child:nth-last-child(#{$i}),
      &:first-child:nth-last-child(#{$i}) ~ li {
        &:hover {
          ~ li:last-child:before {
            opacity: 1;
          }
        }
        &:last-child:hover:before {
          opacity: 1;
        }
        @for $j from 1 to $i {
          &:nth-child(#{$j}):hover, {
            ~ li:last-child:before {
              @include gooeyEffect($j);
              top: (100% / $i) * ($j - 1);
              animation: gooeyEffect-#{$j} $timing 1;
            }
          }
        }
        &:last-child:hover:before, {
          @include gooeyEffect($i);
          top: (100% / $i) * ($i - 1);
          animation: gooeyEffect-#{$i} $timing 1;
        }
      }
    }

  }
  
}

feather.replace()
//https://twitter.com/One_div