AKA: "Wide Youtube", AKA: "Wide video container" - Uses the page space on youtube more efficiently (especially good for high resolutions)
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That seems like a browser/youtube bug. I just tried zooming back and forth a bunch on Firefox and Chrome and I saw the same problem, but it only stayed like that for half a second before the browser adjusted it to fit properly. Also, it happened with and without my script.
Did you test without my script to see if it happens anyways?
Also, you may have disabled all other userscripts, but it does seem like you have some youtube addons that mess with the page since yours doesn't look normal.
Hey, I have my browser default zoom to 125% as I use a 21:9 monitor, but on my 16:9 monitor the entire video doesn't properly adjust to fit correctly. At 100%. 175% and 200% it looks fine but 110% and 125% it looks like the picture, I of course tested it with everything disabled except your script. I've added two pictures to show the difference between 100% and 125%.
https://i.imgur.com/a6HzjQD.png (125% zoom)
https://i.imgur.com/VBOhkze.png (100% zoom)
Edit: I should add that in fullscreen it works at any % of zoom.