Bug in composite?

[Buola Buola]

i don't know if it is a bug in composite, a bug in the drivers i am
using (nvidia closed-source) or a strange normal-behaviour, but I have
the following problem:

when you resize a window that is in off-screen memory, the composite
extension immediatelyd estroys the current pixmap and creates a new
one. So far, so good. The only problem is that this pixmap isn't
initialized to the contents of the previous pixmap, but to a copy of
the rectangle in the screen the window occupies. If the window is on
top and not translucent, this is not a problem, because the contents
is exactly the same, but if it not on top it contains the contents of
the other windows, and if it is translucent it contains the
translucent version, so it starts fading out if you keep resizing.
Does anyone know if this is normal?
You can check if you have the same problem following these steps:
-Start xcompmgr
-Open some application which has multiple subwindows. (gnome-terminal
with a menu bar works for me)
-Make the window translucent with transset
-Resize it so that some of the subwindows don't change size, so they
aren't repainted (resizing from the bottom for gnome-terminal)
If you have the same problem the part which hasn't been repainted
starts disappearing.

Thanks in advance,

     Xavi