A bookdown GitBook content should respond to paging commands immediately after load and should not require click-to-focus.

1. Visit the bookdown book: https://bookdown.org/yihui/bookdown/
2. Attempt to page up/down.
3. Observe that paging does not work.
4. Click anywhere in the content of the book.
5. Attempt to page up/down.
6. Observe that paging now works.

This issue appears to be present in many browsers (Chrome, Safari macOS) many bookdown releases, including 0.9.

By filing an issue to this repo, I promise that

• I have fully read the issue guide at https://yihui.name/issue/.
• I have provided the necessary information about my issue.
  • If I'm asking a question, I have already asked it on Stack Overflow or RStudio Community, waited for at least 24 hours, and included a link to my question there.
  • If I'm filing a bug report, I have included a minimal, self-contained, and reproducible example, and have also included xfun::session_info('bookdown'). I have upgraded all my packages to their latest versions (e.g., R, RStudio, and R packages), and also tried the development version: remotes::install_github('rstudio/bookdown').
  • If I have posted the same issue elsewhere, I have also mentioned it in this issue.
• I have learned the Github Markdown syntax, and formatted my issue correctly.