Fix diff standalone window being a dialog window instead of a normal window

src/calibre/gui2/tweak_book/diff/main.py

@@ -490,8 +490,10 @@ def main(args=sys.argv):
     d = Diff(show_as_window=True)
     func = getattr(d, attr)
     QTimer.singleShot(0, lambda : func(left, right))
-    d.exec_()
+    d.show()
+    app.exec_()
     return 0
 
+
 if __name__ == '__main__':
     main()