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/input.cc 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.
See the Power Buttons document for more information about power button behavior.