Add YouTube Background Play userscript

[VeigaPunk]

What

Adds a new userscript, youtubeBackgroundPlay/youtube-background-play.user.js, that keeps YouTube and YouTube Music playing when the tab is hidden or the screen is locked.

It is a port of the well-known "YouTube Background Playback" Firefox fix, adapted for userscript managers (tested with AdGuard's userscript engine / Dguard).

How it works

1. Spoofs the Page Visibility API — document.hidden, visibilityState (incl. vendor-prefixed variants) and hasFocus() always report visible/focused.
2. Neutralizes visibility/focus events — visibilitychange, blur, focusout, pagehide, freeze are stopped at capture phase, and listener registration for them on window/document is swallowed so site pause logic never hooks them.
3. Filters synthetic pause calls — wraps the player's pauseVideo() and HTMLMediaElement.pause() so programmatic pauses without a recent real user gesture (~1.5s window) are ignored, while genuine user pauses keep working.
4. Keeps Media Session alive — lock-screen controls remain functional.
5. Suppresses idle-watch heartbeat beacons that drive "Are you still watching?" style pauses.

Runs at document-start, @grant none, no external dependencies. Matches *.youtube.com, *.youtube-nocookie.com and music.youtube.com.

Metadata

@downloadURL/@updateURL follow the repo convention (raw GitHub URLs on master), so once merged, users get automatic updates from this repo.

Also published on Greasyfork for testing: https://greasyfork.org/en/scripts/589117-youtube-background-play-firefox-fix-port

License: MIT (compatible with the repo's GPLv3 as a contribution).