After a resolution change of screen or adding a second head (using xrandr), new GTK+ windows having a transparent (rgba) background are created without any decoration and ignoring any other window for initial position, cannot be moved or resized.
Killing xcompmgr makes the decoration visible back, but restarting it afterwards makes them disappear again. xcompmgr 1.1.6 running without any option.
Restarting X server restores to normal behaviour until next resolution change.
Global transparency of window set in Fluxbox app file is not impacted.
Occurs on a Gentoo system, FluxBox version 1.3.5 compiled with useflags :
imlib nls slit toolbar truetype vim-syntax xinerama -bidi
Reproduced with RXVT-unicode and a small C program "GTK+ alpha demo" (not written by me), in attachment.