Dialog disappears when moved within Opengl View

Hi all,
i have a OpenGL enabled View (created with MFC and then done all the GL specific stuff).
Now i created a Dialog and called DoModal() for it.
Everything is fine until i move the dialog in the View the the dialog disappears because the context of the View is drawn in the dialog. So when i move the dialog out of the View it shows a part of the View and is not redrawn anymore…

Why is that?? How can i fix that??