Can't change brightness from lock screen

Brightness can not be changed from the lock screen.

This appears to be a bug which can cause slight inconvenience to the user, but there is a chance of big trouble.

Since the lowest brightness level is zero, and a user locks the device (with <Windows + L> or <Command + L>) when he/she turned brightness to complete zero, a successful login is not easy, since the password field is not focused on the lock screen.

Additionally, not a bug, but a design improvement suggestion: the password field should be focused by default for the account which appears on the top (alphabetically).