Window: focus event

<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);