From the bookdown guide about bs4_book: "Thoughtful typography to make the contents as easy as possible to read, regardless of the size of your device. A sticky header gets out of your way when reading, but is easily accessible if you need it.”

The bs4_book menu is not sticky on small screens. (This might be a bug report, given what the documentation says?)

If I'm reading in the middle of a long page, and I want to go to another part of the book, I have no obvious way on the screen to do so. I have to remember that there was a hamburger menu at the top of the page, and I have to know what it does already. It would be really handy to have a sticky menu on mobile, especially when you are reading a long page.

In particular, "sticky" implies to me that the hamburger menu for the TOC floats on the screen, or if it disappears, it shows back up the moment I start scrolling back up toward the top of the page.