I just found a Chrome/Firefox extension for true 21/9 fullscreen on YouTube and Netflix

This extension basically detects black bars and fix it (you know, the annoying bars on the four sides of the screen while watching a 21/9 video like a movie trailer on Youtube for example). I got a 21/9 screen for 3 years and just discovered this today!

Chrome: https://chrome.google.com/webstore/detail/ultrawidify/dndehlekllfkaijdlokmmicgnlanfjbi

Firefox: https://addons.mozilla.org/en-US/firefox/addon/ultrawidify/

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Demo here: https://www.youtube.com/watch?v=j2xn1WpbtCQ

Works like a charm, you got nothing to do.

(I don't know the dev or anything)