HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>pending + selected</title>
  <style>
    #snap-box {
      width: 220px; height: 160px; overflow-y: scroll;
      border: 2px solid #333; scroll-snap-type: y mandatory;
    }
    #snap-box section {
      height: 160px; scroll-snap-align: start;
      display: flex; align-items: center; justify-content: center;
      border-bottom: 1px solid #ccc; font: 16px sans-serif;
    }
    #snap-box section.pending { outline: 3px dashed #06c; background: #e8f3ff; }
    #snap-box section.selected { outline: 3px solid #0a7; background: #e8fff4; }
  </style>
</head>
<body>
  <div id="snap-box">
    <section>Card 1</section>
    <section>Card 2</section>
    <section>Card 3</section>
    <section>Card 4</section>
  </div>
  <p>Dashed = pending · Solid green = selected</p>
  <script>
    var box = document.getElementById("snap-box");
    box.addEventListener("scrollsnapchanging", function (event) {
      box.querySelectorAll(".pending").forEach(function (el) {
        el.classList.remove("pending");
      });
      if (event.snapTargetBlock) {
        event.snapTargetBlock.classList.add("pending");
      }
    });
    box.addEventListener("scrollsnapchange", function (event) {
      box.querySelectorAll(".pending, .selected").forEach(function (el) {
        el.classList.remove("pending", "selected");
      });
      if (event.snapTargetBlock) {
        event.snapTargetBlock.classList.add("selected");
      }
    });
  </script>
</body>
</html>

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Element scrollsnapchanging — pending + selected (Try It)