Workaround for bug in Qt 5.4 that causes multiple close events to be delivered when using a keyboard shortcut to quit on OS X. Fixes #1440444 [ebook-viewer forgets state of ToC if closed by keyboard (Cmd-Q)](https://bugs.launchpad.net/calibre/+bug/1440444)

src/calibre/gui2/viewer/main.py

@@ -80,6 +80,7 @@ def __init__(self, pathtoebook=None, debug_javascript=False, open_at=None,
                  start_in_fullscreen=False):
         MainWindow.__init__(self, debug_javascript)
         self.view.magnification_changed.connect(self.magnification_changed)
+        self.closed = False
         self.show_toc_on_open = False
         self.current_book_has_toc = False
         self.iterator          = None
@@ -252,7 +253,11 @@ def quit(self):
             QApplication.instance().quit()
 
     def closeEvent(self, e):
+        if self.closed:
+            e.ignore()
+            return
         if self.shutdown():
+            self.closed = True
             return MainWindow.closeEvent(self, e)
         else:
             e.ignore()