How to completely disable YouTube iframe fullscreen? - Youtube Iframe API

I'm already creating the YouTube player with

playerVars: {
    controls: 0,
    fs: 0,
    disablekb: 1
}

but on double click, it still goes to fullscreen, although it pauses then continues the playback. So if I could disable just the fullscreen part, it would be working fine.

(How) is that possible?

By creating the iframe by myself, I've managed to resolve the issue. Here is the code:

const createYtPlayer = (videoId, container = inv) => new Promise(resolve => {
    const el = document.createElement('iframe');
    el.frameBorder = 0;
    el.setAttribute('allow', 'accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture');
    el.setAttribute('donotallowfullscreen', '');
    el.src = 'https://www.youtube.com/embed/' + videoId + '?disablekb=1&enablejsapi=1&controls=0&origin=' + encodeURIComponent(window.origin);
    container.appendChild(el);

    const player = new YT.Player(el, {
        height: 200,
        width: 200,
        videoId,
        playerVars: {
            controls: 0,
            fs: 0,
            disablekb: 1
        },
        events: {
            'onReady': () => {
                setBestSoundQuality(player);
                player.setVolume(100);
                resolve(player);
            }
        }
    });
});

})();

---

OLD ANSWER:

It turned out that its probably a bug. I did a workaround with the help of @Paul Fitzgerald's answer, because that worked.

For some reason, if the allowfullscreen is present in any way, the donotallowfullscreen attribute has no effect (and the allowfullscreen is added automatically).

So I added

const iframe = player.getIframe();
iframe.removeAttribute('allowfullscreen');
iframe.setAttribute('donotallowfullscreen', '');

to the onReady event, and it works.

EDIT: it only seems to work in Firefox.