JSFiddle - React, Tailwind, and code Playground

HTML

<textarea>FOCUS OUTSIDE</textarea>

JavaScript

var iframe = document.createElement('iframe');
iframe.src = '';
document.body.appendChild(iframe);
var textarea = iframe.contentDocument.body.appendChild(document.createElement('textarea'));
textarea.textContent = 'FOCUS INSIDE';
textarea.addEventListener('click', logOnClick);
document.querySelector('textarea').addEventListener('click', logOnClick);

function logOnClick() {
    var log = document.body.appendChild(document.createElement('div'));
    log.textContent = 'document.activeElement = ' + document.activeElement + ', iframe.contentDocument.activeElement = ' + iframe.contentDocument.activeElement;
}