Root cause, measured in Chrome/Brave/Firefox alike (so: spec behaviour, not a
browser bug): a cross-origin embed reads its fullscreen state from ITS OWN
document. While we hold the browser's fullscreen lock — on our wrapper OR on the
iframe element — the YouTube player never learns it is fullscreen, so its own
fullscreen button stays in the "enter" state and clicking it changes nothing
visible. Every other YouTube control works, which is exactly what the user saw.
So we stop taking the lock: F now toggles a CSS layer (`player-stage--max`) that
fills the viewport. The page stays non-fullscreen, so the embed's own fullscreen
button behaves natively, and our own overlays and shortcuts keep working because
our DOM is what's on screen. Escape steps out of maximise before closing.
Verified end to end in real Chrome:
- F -> stage 1920x889, document.fullscreenElement null (no lock taken)
- YouTube's own fullscreen button -> real fullscreen (innerHeight 889 -> 1024,
fullscreenElement becomes the IFRAME) and its icon flips to "exit"
- clicking it again -> back to the maximised view, our class still applied
- wheel volume while maximised: 100 -> 85, with the level flash visible
Drops the "YouTube fullscreen" toolbar button added earlier today: with the lock
gone, YouTube's own button does that job correctly.
peter
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