I’d respectfully disagree with the idea that in-browser search doesn’t serve simply because the amount of content on a page is not fully shown. Let’s say that a page is paginated instead of scrolling. Does search lose it’s value there? There’s certainly pending content which can’t be searched in-page in both models.
In fact, the value of in-page search actually increases in the infinite scrolling model. At first, the amount of content might be the same between a scrolling model and a paginated one. However, the paginated one has a fixed amount of content that can ever be searched: one page. As you scroll the infinite scrolling page, the amount of content only grows. The more you scroll, the more you can search and the more valuable in-browser search becomes.