Blocking Loading

Do not permit user interaction during loading. This seems like a
good idea, since the true order of pages is not yet known (thanks
to sorting at the end of the loading step). The user might change
the current page and then have it be changed again when the frames
are sorted.
CBPageLayer height limit

Limit the maximal height of a page by imposing a maximal aspect.
Pages that are taller are scaled to the size they would have if their
aspect would be in range. This is important for documents that contain
atypically long pages such as scanned book spines.
This limit is only in place for single pages in a double page layout.