doc/dev/plugins/problems/scss/_comparison-sliders.scss

// Image comparison slider
// Credit: http://codepen.io/solipsistacp/pen/Gpmaq
.comparison-slider {
  height: 50vh;
  overflow: hidden;
  position: relative;
  width: 100%;
  -webkit-overflow-scrolling: touch;

  .comparison-before,
  .comparison-after {
    height: 100%;
    left: 0;
    margin: 0;
    overflow: hidden;
    position: absolute;
    top: 0;

    img {
      height: 100%;
      object-fit: cover;
      object-position: left center;
      position: absolute;
      width: 100%;
    }
  }

  .comparison-before {
    width: 100%;
    z-index: 1;

    .comparison-label {
      right: $unit-4;
    }
  }

  .comparison-after {
    max-width: 100%;
    min-width: 0;
    z-index: 2;

    &::before {
      background: transparent;
      content: "";
      cursor: default;
      height: 100%;
      left: 0;
      position: absolute;
      right: $unit-4;
      top: 0;
      z-index: $zindex-0;
    }

    &::after {
      background: currentColor;
      border-radius: 50%;
      box-shadow: 0 -5px, 0 5px;
      color: $light-color;
      content: "";
      height: 3px;
      position: absolute;
      right: $unit-2;
      top: 50%;
      transform: translate(50%, -50%);
      width: 3px;
    }

    .comparison-label {
      left: $unit-4;
    }
  }

  .comparison-resizer {
    animation: first-run 1.5s 1 ease-in-out;
    cursor: ew-resize;
    height: $unit-4;
    left: 0;
    max-width: 100%;
    min-width: $unit-4;
    opacity: 0;
    outline: none;
    position: relative;
    resize: horizontal;
    top: 50%;
    transform: translateY(-50%) scaleY(30);
    width: 0;
  }

  .comparison-label {
    background: rgba($dark-color, .5);
    bottom: $unit-4;
    color: $light-color;
    padding: $unit-1 $unit-2;
    position: absolute;
    user-select: none;
  }
}

@keyframes first-run {
  0% {
    width: 0;
  }
  25% {
    width: $unit-12;
  }
  50% {
    width: $unit-4;
  }
  75% {
    width: $unit-6;
  }
  100% {
    width: 0;
  }
}