Noticed this today. Minor but thought I'd mention:
In the attached code, if you maximize the frame,
there will be a faint darker gray square where
the small notebook page previously was.
This issue has been around for a long time and has something to do with the interaction wxNotebook has with the XP Theme. I don't know if anybody has tried very hard to track it down or not, but when you have other content in the panel the problem usually isn't very noticeable.