E-book viewer: Fix page position incorrect after startup bookmark

E-book viewer: Fix a bug that could cause the reported position to be
incorrect immediately after opening a previously opened book.

This also fixes the Back button not working if a link is clicked on the
page immediately after opening the book.

src/calibre/ebooks/oeb/display/indexing.coffee

@@ -50,6 +50,8 @@ class BookIndexing
         this.last_check = [null, null]
 
     cache_valid: (anchors) ->
+        if not anchors
+            return false
         for a in anchors
             if not Object.prototype.hasOwnProperty.call(this.cache, a)
                 return false
@@ -65,6 +67,8 @@ class BookIndexing
             return this.cache
 
         ans = {}
+        if not anchors
+            return ans
         for anchor in anchors
             elem = document.getElementById(anchor)
             if elem == null

src/calibre/gui2/viewer/main.py

@@ -772,12 +772,14 @@ class EbookViewer(MainWindow, Ui_EbookViewer):
             self.scrolled(self.view.scroll_fraction)
 
     def internal_link_clicked(self, frac):
+        self.update_page_number()  # Ensure page number is accurate as it is used for history
         self.history.add(self.pos.value())
 
     def link_clicked(self, url):
         path = os.path.abspath(unicode(url.toLocalFile()))
         frag = None
         if path in self.iterator.spine:
+            self.update_page_number()  # Ensure page number is accurate as it is used for history
             self.history.add(self.pos.value())
             path = self.iterator.spine[self.iterator.spine.index(path)]
             if url.hasFragment():
@@ -913,6 +915,14 @@ class EbookViewer(MainWindow, Ui_EbookViewer):
         else:
             self.view.document.page_position.restore()
         self.view.document.after_resize()
+        # For some reason scroll_fraction returns incorrect results in paged
+        # mode for some time after a resize is finished. No way of knowing
+        # exactly how long, so we update it in a second, in the hopes that it
+        # will be enough *most* of the time.
+        QTimer.singleShot(1000, self.update_page_number)
+
+    def update_page_number(self):
+        self.set_page_number(self.view.document.scroll_fraction)
 
     def close_progress_indicator(self):
         self.pi.stop()