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powerd uses user input to determine when the system should turn its backlight off or suspend in response to inactivity. powerd does not listen for this input directly; rather, it receives periodic HandleUserActivity D-Bus method calls from Chrome while the user is active. These method calls include a UserActivityType enum describing the type of activity that was observed, allowing powerd to e.g. avoid turning the screen back on if the user presses the Brightness Down key while the screen is already off.

powerd/system/ uses the kernel's input subsystem to observe power button and lid switch events. (ACPI power button events may be additionally received by Chrome as standard keyboard input, but they are ignored there since button releases are not reported correctly.) These events are reported to Chrome via InputEvent D-Bus signals containing InputEvent protocol buffers; Chrome uses the power button notifications to display screen-lock and shutdown animations.

Power button behavior

The behavior of the power button is largely controlled by software and may change, but the general expectations for devices with keyboard-integrated power buttons (i.e. Chromebooks) are:
  • An initial press of the power button turns the system on.
  • Holding the power button at the login screen or in guest mode begins a brief, interruptible-by-releasing-the-button animation where the screen fades to white, after which the system is shut down.
  • Holding the power button while signed in starts a screen-lock animation and then (if the button is still held) shuts the system down.
  • Holding the power button while the screen is locked shuts the system down.
  • Tapping the power button while the system is suspended wakes it.
  • If the display is off due to user inactivity or manually setting the screen brightness to zero, the power button turns the display back on rather than locking the screen or shutting down the system.
Devices with power buttons that are separate from their keyboards (e.g. Chromeboxes and Chromebases) generally behave similarly but lack the interactive animations described above: just tapping the power button locks or shuts down the system.