fix($anchorScroll): make sure scrolling takes place when readyState === 'complete'

When navigating to a page with a hash in the URL, Firefox and IE do their
native scrolling AFTER $anchorScroll, which cancels the applied `yOffset`.
Waiting for `document.readyState` to be 'complete' solves the problem in
Firefox.
(In IE, the issue still appears when navigating to a URL (with a hash),
but not when reloading the same URL.)

Note, that with or without this fix subsequent calls to `$anchorScroll()`
or changes to `$location.hash()` work as expected in all supported
browsers.

Author: gkalpak

src/ng/anchorScroll.js

@@ -168,6 +168,7 @@ function $AnchorScrollProvider() {
    */
   this.$get = ['$window', '$location', '$rootScope', function($window, $location, $rootScope) {
     var document = $window.document;
+    var scrollScheduled = false;
 
     // Helper function to get first anchor from a NodeList
     // (using `Array#some()` instead of `angular#forEach()` since it's more performant
@@ -234,6 +235,20 @@ function $AnchorScrollProvider() {
       }
     }
 
+    function scrollWhenReady() {
+      if (document.readyState === 'complete') {
+        $rootScope.$evalAsync(scroll);
+      } else if (!scrollScheduled) {
+        scrollScheduled = true;
+        document.addEventListener('readystatechange', function unbindAndScroll() {
+          if (document.readyState === 'complete') {
+            document.removeEventListener('readystatechange', unbindAndScroll);
+            $rootScope.$evalAsync(scroll);
+          }
+        });
+      }
+    }
+
     function scroll() {
       var hash = $location.hash(), elm;
 
@@ -255,7 +270,7 @@ function $AnchorScrollProvider() {
     if (autoScrollingEnabled) {
       $rootScope.$watch(function autoScrollWatch() {return $location.hash();},
         function autoScrollWatchAction() {
-          $rootScope.$evalAsync(scroll);
+          scrollWhenReady();
         });
     }