1. Define html page as header, body and footer parts.

2. Root container is loaded in body-div as part of page:
RootPanel.get("div_body").add(new MyContainer());
3. Define a Window and setModal(true);

4. Show window, the shadow does not cover browser client area, only top area in IE.

in FF, the shadow covers full client area, but if browser has scroll bars, user can scroll down to see unmasked area.