好，我找到了答案。这是装饰层的问题。
我们的WPF专家已经遇到它并提供了一些解决方案。
请参见 Karl Shifflett的帖子。

Ok, I found kind of an answer. That's problem with adorner layer. Our WPF gurus already encountered it and provided some solution. See Karl Shifflett's post.