Shoutbox

Provided the user has enabled "change my status to away when I'm inactive for x minutes" in the Messenger options:

By default, the special away status (named 'Idle') is only triggered when the user's status is 'Online'. When the user has any other status, the idle status will not be triggered.

It stays active as long as the user doesn't do anything. This includes not sending any IM to any of his contacts, not moving the mouse, etc. So yes, sending an IM also counts as an activity.

When the user does some activity while he has the Idle status, his/her status will always be reverted back to 'Online'.

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But note that there are addons, plugins and/or Messenger Plus! scripts which can change all this behaviour.

So it isn't impossible that you see an 'Idling' contact chatting with you. Nor is it impossible to also set the idle status after a period of inactivity regardless of what your current status is (even with 'Appearing Offline'). And the return status, which will be set when you do some activity again, can also be changed with such tools...