Odd menu issue w/ Book upgrade

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I've been using an older version of the Hugo-Book theme for many years and recently updated the theme. I'm having some odd behavior that seems related to the "hamburger" menu that doesn't happen when I use the older version of Hugo-Book.

If I view the site with a fully maximized browser, it looks as if the header is non-existent:

If I shrink the browser to mimic a smaller form factor the "hamburger" menu shows some odd behavior (clickable boxes??) and doesn't work:

Any ideas where to go in order to start diagnosing this?