On my Thinkpad Yoga 14 20FY0002US running Ubuntu 17.04, when automatic brightness is enabled (the default), GNOME increases my backlight brightness every time I start playing a video in Totem, VLC, or Chrome.  This was driving me nuts for several weeks until I eventually traced the problem to a division by zero in iio_proxy_changed:

        brightness = manager->priv->ambient_last_absolute * 100.f / manager->priv->ambient_norm_value;
        brightness = MIN (brightness, 100.f);
        brightness = MAX (brightness, 0.f);
        manager->priv->ambient_accumulator = (GSD_AMBIENT_SMOOTH * brightness) +
                (1.0 - GSD_AMBIENT_SMOOTH) * manager->priv->ambient_accumulator;

By inserting g_debug statements I found that both ambient_last_absolute and ambient_norm_value are 0 there, so brightness becomes NaN and then 100.  So every time this code path executes, ambient_accumulator creeps upward.

I’m pretty sure there’s something wrong with the setup of this moving average system if depends on ratios of quantities that can be zero or near zero.