Any way to capture keyboard/mouse in an extension?

[Grzegorz Nosek <root localdomain pl>]

Basically, I have the same question (and for the same underlying reason) as:

http://mail.gnome.org/archives/gnome-shell-list/2011-April/msg00360.html

which unfortunately remained unanswered.

I want to alter windows' opacities after alt+scroll over them (or 
meta+scroll, or whatever). Any pointers?

Best regards,
 Grzegorz Nosek