HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>scrollBy smooth</title>
  <style>
    body { font-family: sans-serif; margin: 1rem; }
    #box {
      width: 220px; height: 140px; overflow: auto;
      border: 2px solid #0f766e; border-radius: 8px;
      background: #f0fdfa;
    }
    .content { width: 500px; height: 500px; padding: 1rem; }
    button { margin-top: 0.75rem; padding: 0.5rem 1rem; cursor: pointer; }
  </style>
</head>
<body>
  <div id="box">
    <div class="content">Nudge me smoothly by top: 100, left: 100.</div>
  </div>
  <button type="button" id="smooth">Smooth scrollBy</button>
  <p id="out">top: 0, left: 0</p>
  <script>
    var element = document.getElementById("box");
    var out = document.getElementById("out");
    element.addEventListener("scroll", function () {
      out.textContent = "top: " + element.scrollTop + ", left: " + element.scrollLeft;
    });
    document.getElementById("smooth").addEventListener("click", function () {
      element.scrollBy({ top: 100, left: 100, behavior: "smooth" });
    });
  </script>
</body>
</html>

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Element.scrollBy — smooth options (Try It)