Window: focus event

addEventListener("focus", (event) => { })
onfocus = (event) => { }

<p id="log">Click on this document to give it focus.</p>

.paused {
  background: #dddddd;
  color: #555555;
}

const log = document.getElementById("log");
function pause() {
  document.body.classList.add("paused");
  log.textContent = "FOCUS LOST!";
}
function play() {
  document.body.classList.remove("paused");
  log.textContent =
    "This document has focus. Click outside the document to lose focus.";
}
window.addEventListener("blur", pause);
window.addEventListener("focus", play);