回複：Screensaver at work?Which 小按紐?

No. It does not look like screen saver. Not the screen saver I set - 3D Flower Box. When it happens, typically I am still using the machine, run programs etc, it just set the screen to almost dark. But you can still see the screen content vaguely.
The little button is located on the top of keyboard, right hand side of the power button, which is in the upper left corner of my laptop's keyboard. That little button is dedicated to light up, not wake up the screen. I didn't know its other purpose.