GitHub search is notoriously poor, though—it can only match whole words (e.g. the query "FooBar" does not match the text "FooBarBaz"), and it doesn't support regexes.
So searching for `fn foobar` will return any file containing the words `fn` and `foobar` not caring about you looking for `fn foobar`.
Then if you find what you want opening the preview doesn't jump to the right line in the file so I have to search again using "find in page".