I have just built and installed from the debian-unstable branch off git
(https://anonscm.debian.org/cgit/pkg-xorg/driver/xserver-xorg-video-ati.git/)
which is directly tracking upstream - confirmed 7.9 is loaded, the problem is
still present.

Hmm, so XFCE4 Display could be responsible for blacking out the other monitors
momentarily too?