The intended power button behavior differs across Chrome OS devices.
Convertible Chromebooks have 360-degree hinges that allow them to be used in both “clamshell” (i.e. traditional laptop) and tablet modes. As such, they typically feature power buttons located on their sides that are accessible in all configurations.
Regardless of whether a user is logged in or not, tapping the power button once turns the screen off. If a user is logged in and the “Show lock screen when waking from sleep” screen lock setting is enabled, the screen is additionally locked. Tapping the power button a second time after the screen has been turned off turns the screen back on. This is intended to give users an easy way to turn the screen off before carrying the device while it's in tablet mode.
If the power button is held for two seconds or longer, the system shuts down. This delay is 3.5 seconds if the screen was already turned off when the button was held.
The power button behaves identically regardless of whether the device is in tablet or clamshell mode.
Clamshell-only Chromebooks have hinges that stop before reaching 180 degrees, supporting only traditional laptop form factors. Their power buttons are typically located at the top-right corner of the keyboard (i.e. to the right of Volume Up and just above Backspace).
While a user is logged in and the screen is unlocked:
When no user is logged in, the guest user is logged in, or the screen is already locked, holding the power button for one second or longer powers off the Chromebook.
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.
Although the Pixelbook is a convertible device with a side-mounted power button, its power button behavior is closer to that of clamshell Chromebooks. The one difference is that it turns its screen off just 3 seconds after it has been locked using the power button (rather than the 40 seconds described in Inactivity Delays) in order to make it easier to carry the device while it's in tablet mode. See bug 756601 and bug 758574 for more details.
Chromeboxes, Chromebases, and related devices that lack integrated keyboards typically feature ACPI-style power buttons. Power button release events are not reported properly to powerd, so the system instead takes action immediately when the power button is pressed.
While a user is logged in and the screen is unlocked, pressing the power button locks the screen. If the screen is already locked, no user is logged in, or a guest user is logged in, pressing the power button shuts the system down.
Chromebits don't have power buttons.
Some power button behavior is consistent across all Chrome OS devices to date:
Various mechanisms are used to determine which behavior should be used:
touchview USE flag in a Portage overlay to pass the --enable-touchview flag to Chrome, which tells ash::PowerButtonController that it should initialize ash::TabletPowerButtonController when an accelerometer event is received.--force-clamshell-power-button flag to Chrome.legacy_power_button USE flag in a Portage overlay, which causes powerd's legacy_power_button pref to be set and the --aura-legacy-power-button flag to be passed to Chrome.powerd receives power button events from the kernel's input subsystem and reports them to Chrome via D-Bus as described in the Input document.
User-facing behavior power button behavior is spread across multiple classes in Chrome:
chromeos::PowerManagerClient receives D-Bus notifications about power button events from powerd.ash::PowerButtonController initiates action in response to power button events received from powerd on clamshell devices and on the Pixelbook.ash::TabletPowerButtonController initiates action on convertible devices.ash::LockStateController contains the high-level logic for transitioning between different animations and performing actions when they complete.ash::SessionStateAnimator displays animations and contains durations.ash::PowerButtonDisplayController contains logic related to forcing the display off in response to power button events.