Cursor down does not always work

[bulrush15]

Firefox v59 on Windows 7.

When I open a book, and click the cursor down key, it will go to the next page in a chapter the first time, but not any other time after that. Perhaps the reading window needs the focus to capture the cursor down key? And the reading pane loses focus?

Otherwise I like the reader as it works in a more narrow window, which I prefer.