While Changing the driver of 'intel' to 'modesetting', *backlight* control does not work. package need 'xorg-server' which includes 'xf86-video-modesetting' file changed: /etc/X11/xorg.conf.d/20-intel.conf Section "Device" Identifier "Intel" Driver "modesetting" EndSection
Xfce unrelated, so you can close this bug report and join the effort here: https://bugs.freedesktop.org/show_bug.cgi?id=96572
As poma indicated, this is not related to the power manager (or Xfce) directly.