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HTML

              
                  <div class="SideScroller">
    <div class="SideScroller__wrap">
      <div class="SideScroller__foreground"></div>
      <div class="SideScroller__mask">
        <div class="SideScroller__line">
          <div class="SideScroller__handle">
            <svg role="img" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path fill="currentColor" d="M377.941 169.941V216H134.059v-46.059c0-21.382-25.851-32.09-40.971-16.971L7.029 239.029c-9.373 9.373-9.373 24.568 0 33.941l86.059 86.059c15.119 15.119 40.971 4.411 40.971-16.971V296h243.882v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.568 0-33.941l-86.059-86.059c-15.119-15.12-40.971-4.412-40.971 16.97z"></path></svg>
          </div>
        </div>
      </div>
    </div>
  </div>

              
            
!

CSS

              
                $c-handle: #FFFFFF
$c-bg: #afb69e

=square($n)
  height: $n
  width: $n

=circle($n)
  +square($n)
  border-radius: 50%

=center
  position: absolute
  top: 50%
  left: 50%
  transform: translate(-50%, -50%)

=cover
  position: absolute
  top: 0
  left: 0
  bottom: 0
  right: 0

body
  background: $c-bg
  margin: 0

// - - - -

.SideScroller
  padding: 1rem
  max-width: 900px
  margin: 0 auto
  height: 15rem

  @media (min-width: 576px)
    height: 20rem

  @media (min-width: 768px)
    height: 30rem

  &__wrap
    height: 100%
    position: relative
    background: url(https://garethweaver.com/codepen/images/shark-1.jpg) no-repeat center center
    background-size: cover

  &__foreground
    +cover
    background: url(https://garethweaver.com/codepen/images/shark-2.jpg) no-repeat center center
    background-size: cover

  &__mask
    height: 100%
    position: relative

  &__line
    height: 100%
    width: 2px
    background: $c-handle
    position: absolute
    top: 0
    right: 0
    bottom: 0

  &__handle
    +circle(2rem)
    +center
    background: $c-handle
    position: absolute
    padding: 10%

    svg
      +square(1.5rem)
      +center

              
            
!

JS

              
                class SideScroller {
  constructor(selector) {
    this.elmClassName = selector.split('.').pop()
    this.wrap = document.getElementsByClassName(`${this.elmClassName}__wrap`)[0]
    this.handle = document.getElementsByClassName(`${this.elmClassName}__handle`)[0]
    this.mask = document.getElementsByClassName(`${this.elmClassName}__mask`)[0]
    this.fg = document.getElementsByClassName(`${this.elmClassName}__foreground`)[0]
    this.pos = 50
    this.create()
  }

  create() {
    this.handle.addEventListener('mousedown', this.drag)
    this.handle.addEventListener('touchstart', this.drag)
    window.addEventListener('resize', this.resizeHandler)
    this.setPosition()
  }

  drag = e => {
    e.preventDefault()
    document.addEventListener('mousemove', this.dragHandler)
    document.addEventListener('touchmove', this.dragHandler)
    document.addEventListener('mouseup', this.stopDragHandler)
    document.addEventListener('touchend', this.stopDragHandler)
  }

  resizeHandler = e => {
    this.setPosition()
  }

  dragHandler = e => {
    let percent = this.getPercent(e, this.wrap)
    if (percent < 0) {
      percent = 0
    } else if (percent > 100) {
      percent = 100
    }
    this.setPosition(percent)
  }

  stopDragHandler = e => {
    document.removeEventListener('mousemove', this.dragHandler)
    document.removeEventListener('mouseup', this.stopDragHandler)
  }

  getPercent(evt, elm) {
    let ref = evt.touches
      ? evt.touches[0].pageX
      : evt.pageX
    let rect = elm.getBoundingClientRect()
    return ((ref - rect.left) / rect.width) * 100
  }

  setPosition(percent = this.pos) {
    this.pos = percent
    this.mask.style.width = percent + '%'
    const clip = `inset(0 0 0 ${this.wrap.offsetWidth * percent / 100}px)`
    this.fg.style.clipPath = clip
    this.fg.style.webkitClipPath = clip
  }
}

new SideScroller('.SideScroller')

              
            
!
999px

Console