Comment 116 for bug 257827

(In reply to comment #14)
> Yves-Alexis,
>
> Thank you so much for researching this solution. I can confirm that it works
> on my Thinkpad X301, giving 16 brightness levels, provided that the ACPI
> brightness switch is enabled.
>
> And, the progress bar on the brightness OSD is blank (i.e., shows zero level),
> which is probably what you were referring to.

Exactly, there's another hidden xfconf key you might want to tune: /xfce4-power-manager/show-brightness-popup (it disable completely the notifications)