After latest dist-upgrade, when enabling a second monitor via randr and
Xinerama, windows, menus and pop-up elements has their shadow displayed
above the window contents, rendering the content hard to see.

Successive windows add their respective shadows "at the top" of all the
windows.

I believe the problem lies with the built-in compositor. Disabling it via
gconftool-2 --toggle /apps/metacity/general/compositing_manager returns
the usability by getting rid of the shadows.

This was working perfectly with the previous version.

This bug affect metacity for 1 year.
I've the same problem with my notebook (intel 945) but I can't reproduce this
bug on another notebook with a radeon x600 (free drivers, not fglrx). On the
second computer everythings works well connecting a second monitor.

The only solution to the problem is disable gnome compositing.