In Google calendar you can have a large dialog in the center of the screen, which has a close button but isn't 'modal' - and is light dismissible. But then when a meeting is happening it gives you a small dialog notification which is not centered but is modal. It is not at all clear which of these is on 'top' (neither has a backdrop I can see) or why clicking around or clicking the X isn't dismissing the large one, or you can't even scroll.
