idling

Too funny :3

You can't do that with IM, if by offline mode you mean Steam offline mode, and not Steam community (your status) being offline. If you mean community only, then yes it's possible.

It can't work with Steam in truly offline mode - in this case the Steam network is not even receiving your games being played request, and not dropping any cards. You probably mean Steam community (your status) being offline - yes, that works as intended, but by offline I understand that there is no connection between you and Steam network, if you just switch your Steam community status to offline, then you're shown as offline, but not being so.

Unless Volvo fucked up something and after going online it not only registers your hours played, but also dropping cards. This is however not idling, this is receiving cards after becoming online, so it's not truly offline mode. And I doubt it's even possible, it'd be a gigantic loophole.