The subject nobody is allowed to raise, and the one Carl has not finished with.
Chaco does not appear. He arrives instead as a piece of unfinished business: Mordecai emerges from his room, holds up his hands, and gets out ahead of it — he knows losing his temper with Chaco was a mistake, it will not happen again, and no, they are not going to talk about it. Carl reminds him what happens if he does it again, gets a promise, and lets it drop for lack of time.
Privately he notes that he is not finished with Mordecai over that bullshit with Chaco. Roughly ninety seconds after the promise, Mordecai smacks Carl in the head.
The absent cause of Mordecai's suspension and of the party's unresolved argument.