* This is a built-in function of iOS Safari that didn't work because the
page content was inside absolutely-positioned, scrollable divs.
* The fix is to stop using absolute positioning on `.page-wrapper` and
`.page`, so that the content uses static positioning and flows
naturally down the page.
* Consequently, `.sidebar` and `.nav-chapter` now have to use `position:
fixed` in order to be positioned relative to the viewport.
* This fix also enables Safari's built-in behavior of automatically
hiding the top and bottom toolbars when scrolling down the page.
