Comment 14 for bug 872652

The problem is caused by a recent change to gnome-settings-daemon (the application of debian/patches/00git_dont_restore_brightness.patch); I'm working on a fix.

I also note that the bug only occurs if the system is plugged in to A/C power at the moment gnome-settings-daemon first detects that the system is idle. When on battery power, the problematic patch does not come into effect, and the backlight works as it should. (Simply unplugging the A/C power after the problem occurs does not help, so this isn't really a useful workaround).