JSFiddle - React, Tailwind, and code Playground

HTML

<button id="btn">open popup</button>
<pre id="log"></pre>

JavaScript

btn.onclick = e => {
  const popup = window.open('https://google.com');
  waitForClose(popup, e => log.textContent += 'closed');
};

function waitForClose(win, cb) {
  function poll() {
    if(win.closed) {
      cb();
    }
    else {
      requestAnimationFrame(poll);
    }
  }
  poll();
}