Semi-transparent GdkWindows?

[Christian Neumair <chris gnome-de org>]

I'm currently trying to implement a semi-transparent drag window for
Nautilus. I'm trying to do this by creating a window widget and
connecting to its "expose-event", and then drawing onto it this handler.
Unfortunately, gdk_window_shape_combine_mask takes a bilevel alpha mask,
i.e. full alpha vs. no alpha, probably since the X extension it uses
just sets the window shape, so all I can do is draw a offscreen drawable
on the windows and set its bilevel alpha mask.

I played around with cairo, but it also seems to draw onto the window
and there doesn't seem to be any way to do something like
gdk_window_fill (0x00000000).

So my sad and hopefully wrong conclusion is that GdkWindows and the
underlying Windows don't seem to have a full alpha channel. Can cairo
help me here? Do we need a new, more powerful, X extension?

Thanks for your replies in advance!

-- 
Christian Neumair <chris gnome-de org>