Will a browser give an iframe a separate thread for JavaScript?

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Recently tested if JavaScript running in a iFrame would block JavaScript from running in the parent window.

iFrame on same domain as parent:

  • Chrome 68.0.3440.84: Blocks
  • Safari 11.0.2 (13604.4.7.1.3): Blocks
  • Safari 15.1 on iOS: Blocks
  • Firefox 96: Blocks

iFrame on different domain as parent

  • Chrome 68.0.3440.84: Doesn’t block
  • Safari 11.0.2 (13604.4.7.1.3): Blocks (outdated, but I don’t have a macbook)
  • Safari 15.1 on iOS: Doesn’t block
  • Firefox 96: Doesn’t block
  • Chrome for Android 96: sometimes Blocks and sometimes Doesn’t block (There are some complex rules in Chrome for Android that determine when Chrome for Android does and doesn’t isolate a process, see chrome://process-internals and chrome://flags)

parent.html:

    <body>
    <div id="count"></div>
    <iframe src="./spin.html"></iframe>     
    <script>
        let i = 0;
        let div = document.getElementById("count");
        setInterval(() => {
            div.innerText = i++;
        }, 100);
    </script>
    </body>

spin.html:

    <body>
    <button id="spin">spin</button>
    <script>
        const spin = document.getElementById("spin");
        spin.addEventListener('click', () => {
            const start = Date.now();
            while (Date.now() - start < 1000) { }
        })
    </script>
    </body>

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