(Reported by Tony over email.)

On a convertible Chromebook:

1. Turn off the automatic-screen-lock setting (or start a guest session).
2. Put the device into tablet mode.
3. Open the system tray and see the screen brightness slider.
4. Tap the power button to turn the screen off.
5. Tap the power button again to turn the screen back on.

After step 5, you can see the brightness slider animating from 0% back to its original level.

This makes sense given how things work. powerd notifies Chrome about backlight brightness changes via BrightnessChanged D-Bus signals, and Chrome depends on receiving these. The signals include a "user_initiated" bool argument (see chromeos::PowerManagerClient::Observer::BrightnessChanged in Chrome), and Chrome uses this to avoid popping up the onscreen brightness bubble when an automated change is received while in laptop mode. It still animates the slider in the system tray, though.

I agree that it's distracting, though. Maybe Chrome should avoid animating the slider when the brightness changes to or from 0% in response to an automated change. This seems like it'd be easy to handle within ash/system/brightness/tray_brightness.cc, but I'm not sure there's any way to tell views::Slider to avoid animating when SetValue is called.