Interesting issue. So, assuming that the csster is simply not as sharp as usual while casting, and an opponent gets an AOO on the apparently loafing mage, I still have one concern.
Who determines that a character has an AOO? I have played with a DM recently who expected players to speak up and say they take an AOO; but in this case, how would they know their character could do so? Using this general definition, how would a character ever know, in fact, except by trail and error? So, should players scream “AOO!” all the time in case there is one?!
Obviously I mean that to be rhetorical. So, do most DMs mention AOO, or expect players to do so?
What would a player be told in this specific case? Something like; “The guy next to you isn’t paying full attention, it seems like you can strike him freely while still pressing your other attack (on whomever)”?
This kind of suggests casting, but does not reveal it explicitly. Or would a DM say “AOO due to casting”, or say nothing at all, but just describe a still, quiet opponent?
Cheers