Book: missing "next chapter" button on the main page

[Kinrany]

If you open book.async.rs without the /introduction.html part, the "Introduction" chapter will not be highlighted and the "next page" button on the right side will not be available.

Most links go to the main page (with no chapter selected), so this results in an extra click every time the book is opened. And it's arguably even more confusing to beginners (like me 🙂) who are not yet used to this quirk.

This seems to be a problem with mdBook, but their main page is fine. Perhaps the bug has already been fixed in a newer version?