<nerd-sniped>
I'm assuming the script in question is https://github.com/Zren/ResizeYoutubePlayerToWindowSize/blob/master/153699.user.js
Margin-top is coming from the userscript, as are classes ytwp-window-player, ytwp-viewing-video, and ytwp-scroll-top.
I can trigger the behavior reliably by patching the script so that ytwp.isWatchUrl always returns true and then running it from JS console.
From ytwp.isWatchUrl:
url.match(/https?:\/\/(www\.)?youtube.com\/(c|channel|user)\/[^\/]+\/live/)
url.match(/https?:\/\/(www\.)?youtube.com\/watch\?/)
These should both be /^https:.../. As written, they match https://example.com/?q=https://youtube.com/watch?. (It's a valid URL; ':', '/', and '?' are all specifically allowed unescaped in query strings.)
If you try to go to https://kagi.com/summarizer/index.html?url=https://youtube.com/watch?v we redirect use JS to rewrite the contents of the URL bar and as a side effect the [:/?]s in the query string get escaped.
Theory: Intermittency is due to a race condition. When you go to https://kagi.com/summarizer/index.html?url=https://youtube.com/watch?v, if the userscript runs first, the URL matches and styles are applied, but if our JS runs first, it rewrites the URL so the userscript doesn't apply styles when it runs.
Question then becomes how the script is firing on our pages. Script says // @include http*://youtube.com/*. I'm guessing it's matching like https://kagi.com/summarizer/index.html?url=https://youtube.com/watch?v. I don't follow the userscript extension ecosystem that closely; Tampermonkey on Firefox doesn't seem to match that way, but maybe a different browser/extension combination would.
</nerd-sniped>
