Comment 2 for bug 1453403

> The current header behaviour is described in the app design
> guidelines[1] (see "Overlay" section), and it's used also in the image
> viewer of gallery-app[2].

The guidelines don't specify when to use each. But, I assume tapping is
appropriate when there is no scrollable content. The gallery displays an
image which is not something that is scrolled through as you view the
image.

> (in a standard list view, the
> elements provided are somehow in a hierarchy - e.g. relevance, date, etc
> - so scrolling up the view is a less important action than scrolling
> down, but pages in a PDF documents has not that strong hierarchy).

The hierarchy you mention sounds to me like linearity. You scroll to view
something in a linear way. Reading a book is a linear process, the
hierarchy is page numbers/content. In fact, I would argue that scrolling
through a PDF is exactly the same as scrolling through an article in the
web browser, which slides the header out when you scroll down the page,
therefore I would use this behaviour to be consistent with the rest of the
platform.