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CSS

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JavaScript

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HTML 

              
                <h1>Scroll Snap Event Visualizer</h1>
<main>
  <div>
    <p>Snap Change</p>
    <output id="snapchange" aria-live="polite"></output>
  </div>
  <section id="scroller" tabindex="0">
    <div>1</div>
    <div>2</div>
    <div>3</div>
    <div>4</div>
    <div>5</div>
    <div>6</div>
    <div>7</div>
    <div>8</div>
    <div>9</div>
    <div>10</div>
  </section>
  <div>
    <p>Snap Changing</p>
    <output id="snapchanging" aria-live="polite"></output>
  </div>
</main>

<footer>
  <label for="snapalign">Align to:</label>
  <select id="snapalign">
    <option>center</option>
    <option>start</option>
    <option>end</option>
  </select>
</footer>
!

CSS 

              
                @import "https://unpkg.com/open-props" layer(support.design-system);
@import "https://unpkg.com/open-props/normalize.min.css" layer(support.demo);

::view-transition-group(.message) {
  animation-duration: .5s;
  animation-timing-function: var(--ease-3);
}

/* allow interrupting the view transition with mouse */
::view-transition {
  pointer-events: none;
}

@layer support.demo {
  html {
    view-transition-name: none;
  }
  
  body {
    display: grid;
    place-items: center;
    place-content: center;
    padding: var(--size-5);
    gap: var(--size-5);
  }
  
  main {
    display: grid;
    gap: var(--size-5);
    grid-template-columns: max-content 1fr max-content;
    grid-template-rows: min(250px, 30vh);
    
    @media (width <= 1000px) {
      grid-template-rows: 20ch 30vh 20ch;
      grid-template-columns: calc(90vh - 40ch);
    }
    
    > div {
      display: grid;
      justify-content: center;
      grid-template-rows: auto 1fr;
      
      > p {
        border: 1px solid var(--surface-3);
        padding: var(--size-2) var(--size-3);
      }
    }
  }
  
  section {
    display: grid;
    grid-auto-flow: column;
    inline-size: 100%;
    max-inline-size: min(1280px, 100vw);
    justify-self: center;
    
    overflow-x: auto;
    overscroll-behavior-x: contain;
    scroll-snap-type: x mandatory;
    
    background: light-dark(white, var(--gray-10));
    padding: var(--size-3);
    scroll-padding-inline: var(--size-3);
    gap: var(--size-3);
    border: 1px solid var(--surface-3);
    
    &::before, &::after {
      content: "";
      display: block;
      inline-size: 50vw;
    }
    
    > * {
      inline-size: 200px;
      scroll-snap-align: var(--_snap-align, center);
      border: 1px solid var(--surface-2);
      
      display: grid;
      place-items: center;
      font-size: var(--font-size-6);
      color: var(--surface-4);
      padding: var(--size-2);
      
      &.changing {
        background: var(--surface-2);
        color: var(--text-1);
      }
      
      &.change {
        background: var(--indigo-5);
        color: white;
      }
    }
  }
  
  output {
    border: 1px solid var(--surface-2);
    border-block-start: none;
    display: grid;
    align-content: start;
    gap: var(--size-1);
    padding: var(--size-2);
    scroll-padding: var(--size-3);
    
    overflow-y: auto;
    overscroll-behavior-y: contain;
    scroll-snap-type: y mandatory;
    scroll-behavior: smooth;
    
    &:not(:focus-within, :hover) > :last-child {
      scroll-snap-align: end;
    }
    
    > * {
      transition: opacity .75s var(--ease-spring-2), transform .75s var(--ease-spring-2);
      background: var(--surface-3);
      border-radius: var(--radius-2);
      padding-inline: var(--size-2);
      padding-block: var(--size-1);
      font-size: var(--font-size-0);
      
      @starting-style {
        opacity: 0;
        transform: translateX(-100%);
      }
    }
  }
  
  #snapchange > :last-child {
    color: white;
    background: var(--indigo-5);
  }
  
  footer {
    display: flex;
    place-items: center;
    gap: 1ch;
    
    > select {
      background: var(--surface-2);
      padding-inline-end: var(--size-2);
    }
  }
  
  h1 {
    font-size: max(2.5vw, 2rem);
  }
}
!

JS 

              
                const state = {
  changed: null,
  changing: null,
  queue: [],
  count: 0,
}

scroller.addEventListener('scrollsnapchange', event => {
  // create the log node
  const message = document.createElement('div')
  message.innerText = 'Changed to ' + event.snapTargetInline.innerText
  message.style.viewTransitionName = '--message-' + state.count++
  message.style.viewTransitionClass = 'message'
  snapchange.appendChild(message)
  
  // manage snap selected state
  state.changed?.classList.remove('change')
  event.snapTargetInline.classList.add('change')
  state.changed = event.snapTargetInline
  
  setTimeout(() => {
    state.queue.push({
      parent: snapchange,
      child: message,
    })
  }, 3000)
})

scroller.addEventListener('scrollsnapchanging', event => {
  // create the log node
  const message = document.createElement('div')
  message.innerText = 'Changing to ' + event.snapTargetInline.innerText
  message.style.viewTransitionName = '--message-' + state.count++
  message.style.viewTransitionClass = 'message'
  snapchanging.appendChild(message)
  
  // manage snapping state
  state.changed?.classList.remove('change')
  state.changing?.classList.remove('changing')
  event.snapTargetInline.classList.add('changing')
  state.changing = event.snapTargetInline
  
  setTimeout(() => {
    state.queue.push({
      parent: snapchanging,
      child: message,
    })
  }, 3000)
})

setInterval(() => {
  if (!state.queue.length) return
  
  document?.startViewTransition 
    ? document.startViewTransition(() => {
        let node = state.queue.shift()
        node.parent.removeChild(node.child)
      })
    : snapchanging.removeChild(state.queue.shift())
}, 1000)

snapalign.oninput = event => {
  document.body.style.setProperty('--_snap-align', event.target.selectedOptions[0].innerText)
}
!
999px

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