Xfce Compositing Manager and password windows for root graphical applications

Several Ubuntu Xfce users have reported a delay when root graphical applications (ie launched with gksudo or gksu) ask for the password. The windows hang for several seconds, up to 40. I’ve noticed that even the clock stops ticking.

This bug is probably related to Xfce compositing manager, in particular the shadows under the popups.
The current workaround is to disable “Display shadows under popup windows” or launch the application with gksudo -g.

man gksudo
--disable-grab, -g
Disable the "locking" of the keyboard, mouse,
    and focus done by the program when asking for password.

Edit 071124:
From the UF thread (see below), run gksu-properties and disable capture-mode. I've looked around, but does anyone have more infos about this workaround?

References

UF thread
LP bug report
Xfce bugzilla report