Looks like something that was cloned from some other product and rather crudely shoehorned in. At the very least it should have been introduced after an update and given an option to opt-in to using it, rather than automatically enabling it without any notice.
I'd love for them to add a progress bar and subtitle (think youtube, netflix or prime video) in the PIP window.
If we went by this argument then browsers would have every single feature ever. Which they kinda do...
A great example is Twitch (which I would have assumed was one of the prime use/test cases). Twitch has these things they call "extensions" that streamers can add to the player of anyone watching the stream... and they pop out on hover from the right when you want to access their main settings. So instead of clicking the button to modify the extension suddenly the video is in the corner of my monitor.
That's when I decided to disable it for now :).
Edit: clarification
It is in the right click menu, under "Picture-in-Picture"
It's actually quite convenient to watch a video while you're browsing elsewhere (e.g., a passive video you aren't intentionally watching all of but also trying to do other things). I didn't think I'd use it but every once in a while it's nice.
Using an embedded youtube or other video player that doesn't let you resize? Pop it out!
Want to put a youtube vid in a corner while you do other stuff? Pop it out, switch tabs, and go about your business!
For example, I love it for Netflix so that I can watch a show while doing other things on my laptop.
Most content providers probably don't like this feature.
I was kind of skeptical about it but given that in practice all it does is display a small popout icon on video mouse-over, it's not really that annoying.
One downside is that subtitles (at least with Netflix, don't remember for YouTube) are still displayed in the original tab.
I don't know whether it's right for a browser to add that feature rather than the OS, but Ubuntu doesn't have any plans to add similar functionality so I don't really care.
As far as making it opt-in I dont really understand your reasoning. If people want to use the feature then it's there, if not then they probably won't notice anything is even different. Do you feel similarly about things like reader mode?
A native media player with youtube-dl is still king for power users.
I really dislike Twitter.