Open a book
Hit Page Down and notice that opening the book has (correctly) put the focus in the newly opened book so you can immediately do keyboard commands in that window.
Now drag the tab of that book to another place in your layout.
Hit Page Down again.
Nothing happens.
Problem: the focus has been lost from the book's window to the tab area of the place where it was left behind. Notice you can hit <Tab> and move the focus highlight around among the UI elements of the tab that was beneath the book you just moved.
Expect: dragging a book to move it to a new place would set the focus back to that book just as when you first open it.
