I've seen that on pages with text editors that don't load all the text or where search is expanded with more features, like for example on Github. In these cas

I've seen that on pages with text editors that don't load all the text or where search is expanded with more features, like for example on Github. In these cases I usually click outside the editor and then control+f works as usual. If not, you can also try F3, and if that's hijacked too browsers usually have a shortcut to "find in page" in the hamburguer/three-dots menu