HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>layerY — pageY</title>
  <style>
    #d1 { border: solid blue 1px; padding: 20px; }
  </style>
</head>
<body>
  <div id="d1">Unpositioned — click here</div>
  <pre id="out">Waiting…</pre>
  <script>
    document.getElementById("d1").addEventListener("mousedown", function (e) {
      document.getElementById("out").textContent = JSON.stringify({
        layerY: e.layerY,
        pageY: e.pageY
      }, null, 2);
    });
  </script>
</body>
</html>

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MouseEvent.layerY — vs pageY (Try It)