Clear history when opening new book

src/calibre/gui2/viewer/main.py

@@ -790,6 +790,7 @@ class EbookViewer(MainWindow):
             self.save_current_position()
             self.iterator.__exit__()
         self.iterator = EbookIterator(pathtoebook)
+        self.history.clear()
         self.open_progress_indicator(_('Loading ebook...'))
         worker = Worker(target=partial(self.iterator.__enter__, view_kepub=True))
         worker.start()

src/calibre/gui2/viewer/ui.py

@@ -111,6 +111,9 @@ class History(list):  # {{{
         self.forward_pos = None
         self.set_actions()
 
+    def clear(self):
+        del self[:]
+
     def set_actions(self):
         if self.action_back is not None:
             self.action_back.setDisabled(self.back_pos is None)