BootsFaces doesn't deliver all Bootstrap CSS or JS files. Instead, it tries to optimize the network payload. In other words, the affix.js is only delivered if there's a component using it. I just checked it, there's no such component. So, unless we added it unintentionally, affix.js is never part of your BootsFaces page.
That, in turn, mean it works if you add it manually.
As for your side question: you can examine the JavaScript code delivered by BootsFaces. Open your browser's developer tools, and you'll find a network tab. Inspect the JavaScript files. They are minified file, but usually, we keep the copyright header, giving you an opportunity to find out which files are loaded.
The other option is to clone our GitHub repository and to do a full-text search. That's what I did to find out which components use "affix.js". I simply searched for "affix.js" in my local BootsFaces-OSP project. There was no search result outside the build folders. If you run a different search, such das "dropdown.js", you'll find a Java file importing this JavaScript file as a resource, like so:
In this particular case, we've learned that "dropdown.js" is used by only a single component, <b:selectMultiMenu />
.