This tests that a modal dialog's containing block is in the initial containing block and that it is unaffected by ancestor elements with overflow or opacity.

This dialog should be unaffected by its ancestor with opacity.
This dialog should be unaffected by its ancestor with overflow. It should not be clipped.
This dialog should be onscreen with a width of 90% of the page. It is the child of an narrow element positioned off screen, but the containing block of a top layer element is the initial containing block, so its position and percent lengths are relative to that.