I've found an issue with the book activity.
If you set up a page that is hidden at the end, the activity does not mark as completed when the student reaches the end of what is visible to them.

If you then unhide the page, and get the student to view it, it marks the activity as complete.

Steps to replicate:

1. Go to moodle sandbox as someone who can edit the course
2. Create a book activity
3. Fill in the required details
4. Set completion criteria
   1. 'Show as complete when conditions are met
      1. Student must view the activity to complete it
5. Create a chapter, with three subchapter pages, hiding the third
6. Have the student go trough the book and navigate to the end

Observed: Activity is not marked complete
Expected: Completion criteria should ignore hidden pages

This also occurs in 4.0.5 and 4.0.4