Comment 19 for bug 392122

I can confirm that this problem went away after unchecking "reduce backlight brightness" box for "on battery power" as suggested by Justyn in #17.

My symptoms were the same as described where it would dim quicker than the 30 seconds idle and when the mouse moved it would never return the previous brightness. With this option unchecked it fixed this issue. Interestingly this setting seems to interfere in a number of areas and I suspect the problems people are having are due to conflicts with this setting. For example:

If I turn the brightness all the way up using my hardware buttons (with this setting enabled) it goes to full brightness; however, if I open the power management system app it immediately dims to what I presume should be the maximum with the setting enabled.

Possibly this means that the issue has more to do with the brightness buttons ignoring the reduce backlight brightness options and the system getting confused. I'm not sure how the power management is handled (though I plan to look) but perhaps the seemingly too quick dim is actually the system periodicly checking and noticing the brightness is higher than the reduced maximum and adjusting. A normal user would interpret this as an idle event when in fact it is unrelated.

Morale of this story I think the problem is with the brightness buttons allowing the maximum brightness using the "reduce backlight brightness" to be bypassed causing the system to appear to randomly dim the screen and not appropriately restore the brightness when coming out of idle.