@zhongfu: If I understood you correctly, that's not a bug, that's how the screensavers works.
The whole point of an overlay is that it, well, overlays. So it's not gonna magically remove part of what's underneath just because it happens to be a nasty UI element ;p.
(Also one of the reasons why I designed the default overlays to mostly match the chrome bars at the top, and the potential blank space at the bottom
).