Window: focus イベント

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

.paused {
  background: #ddd;
  color: #555;
}

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.";
}
const log = document.getElementById("log");
window.addEventListener("blur", pause);
window.addEventListener("focus", play);