It's always been that way, that's pretty much a standard for every video player both web and heavy clients
This is a pretty dumb question - I would always expect a double-click to fullscreen, and a single click to pause/play on any video in any video player.
The decision was probably made many years ago for the very simple reason that clicking specific targets is much more difficult, so enabling people to use scroll wheel and mouse buttons to do things is a very efficient method of control.
Why does double clicking a video make it fullscreen?
I mean, why not? What other feature would you use a double-click to trigger?
I mean, there are options. It could mute/unmute, close the stream, copy one of various track information such as track name or playback position, send the video to different screen etc. But I can't think of anything that makes much sense, although mute/unmute is at least somewhat sensible.
But, yeah, it's one of those interactions that has just always worked like that, long before browsers could play video even. It's similar to how Ctrl+C copies stuff, Ctrl+Z undoes previous operation, right-click opens a context menu etc.; they don't have to do those things, but users have learned to expect it, so it would be pretty dumb to change that with no reason.
It is pretty common, most video players on Windows for instance - this is the default shortcut.
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