Yeah, that was my first reaction as well! With the chapters being this short (which itself is great) it would be really nice to be able to use the arrow keys.

Not sure if the dev will see this, but it would be nice if it was a little easier to get to the table of contents as well. My suggested code change. (quick a dirty example)

  .menu {
    margin-bottom: 1.5em;
    position: sticky;
    top: 0;
    z-index: 1000;
    background: #fff;
    padding: 10px;
  }

FYI, just implemented this on the site. Thanks for the suggestion!

Noted! That would be an easy addition.