HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>screenX — delta</title>
</head>
<body>
  <p>Move the mouse. Try dragging the browser window if you can.</p>
  <pre id="out">—</pre>
  <script>
    document.addEventListener("mousemove", function (e) {
      var approxWindowLeft = e.screenX - e.clientX;
      document.getElementById("out").textContent = [
        "screenX: " + e.screenX,
        "clientX: " + e.clientX,
        "approx window left: " + approxWindowLeft
      ].join("\n");
    });
  </script>
</body>
</html>

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MouseEvent.screenX — screen vs client (Try It)