Commits on Sep 26, 2015

1. Refactor the previous history rewriting logic …

   When the user edits the URL and changes the reference fragment (hash),
   PDF.js intercepts this action, and saves the then-current history state
   in the previous history entry. This is implemented by navigating back,
   editing the history and navigating forward again.
   
   The current logic has a flaw: It assumes that calling history.back() and
   history.forward() immediately updates the history state. This is however
   not guaranteed by the web standards, which states that calling e.g.
   history.back "must traverse the history by a delta -1", which means that
   the browser must QUEUE a task to traverse the session history, per spec:
   http://w3.org/TR/2011/WD-html5-20110113/history.html#dom-history-back
   https://html.spec.whatwg.org/multipage/browsers.html#dom-history-back
   
   Firefox and Internet Explorer deviate from the standards by immediately
   changing the history state instead of queuing the navigation.
   WebKit derived browsers (Chrome, Opera, Safari) and Opera presto do not.
   
   The user-visible consequence of strictly adhering to the standards in
   PDF.js can be shown as follows:
   
   1. Edit the URL.
   2. Append #page=2 for example.
   3. Press Enter.
      -> Presto and WebKit: PDF.js reverts to the previous URL.
      -> Gecko and Trident: PDF.js keeps the new URL, as expected.
   
   To fix the issue, modification of the previous history item happens in
   a few asynchronous steps, guided by the popstate event to detect when
   the history navigation request has been committed.
   
   --
   Some more implementation notes:
   
   I have removed the preventDefault and stopPropagation calls, because
   popstate is not cancelable, and window is already the last target of the
   event propagation.
   
   The previous allowHashChange logic was hard to follow, because it did
   not explain that hashchange will be called twice; once during the
   popstate handler for history.back() (which will reset allowHashChange),
   and again for history.forward() (where allowHashChange will be false).
   The purpose of allowHashChange is now more explicit, by incorporating
   the logic in the replacePreviousHistoryState helper function.

   

   Rob--W committed Sep 26, 2015
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