Is watching a video on HDD using VLC, or a youtube video in a browser considered as 'computer idle'?

view story

http://askubuntu.com – I have Ubuntu 11.04 (natty). I have read 'Screen goes blank when VLC is playing video' My question is similar, but also I have this problem when viewing a video on youtube. I am not quite clear about the meaning of 'computer idle'. Is it considered idle only when the keyboard or mouse is not being used? Because, when watching a video stored on HDD using VLC, or when watching a youtube video in a browser, there is some disk activity. And I was thinking that this means that the computer is not idle. In system settings -> control center -> screensaver preferences, I have a check mar (HowTos)