the blocked area (= the area outside of a modal dialog) doesn't cover the whole page.
the following events increase the effect: onresize and onscroll
users are able to manipulate the uncovered area (unintentionally).
the size of the uncovered area depends on the:
- structure of the page in the background
- user activities (window resizing, scrolling, ...)
(the provided patch also offers an improvement of this issue. however, it isn't the final solution.)