What could she mean here? Is it like when when the lawyers approach the bench and chat with the judge, out of hearing of the public and jury? But how could that apply here?
(This is the same Eric Scott as represented in the Back in the Day collection earlier this morning.)
Comics I Don't Understand 
Because he’s holding the SIDE of his face maybe? But what then is the BAR?
maybe he is holding a phone, having a private convo?
Or it’s like calling a time-out. (But oddly using that courtroom expression for it.)
Not a time out. like a court side-bar, the man wants to discuss the way the discussion is going. e.g. “should this topic that has just come up be allowed to be part of the current discussion”
I think this is the definition implied:
sidebar conversation:
informal A conversation held privately between two people while one or more other people are already speaking.
I was trying to focus on the presentation, but these two people in front of me started having a sidebar conversation and completely distracted me!
Please don’t hold sidebar conversations during our meetings—it’s so rude!
The odd part is he seems to be having the sidebar conversation with either himself, or with the viewer (4th wall)
He is holding his hand up to the side of his mouth as people do when they are trying to avoid others hearing what they are saying to someone next to them.
What is the point of a side bar when there are only two people? What sense does a side bar even make when not in a courtroom? That’s the joke. Yes, we realize you don’t think it is funny, but that is still the joke.
Aha, that definition supplied by chemgal is helpful, and something I was unaware of. But even with that support for a daily-real-life use of the phrase, there are unanswered questions…
Maybe she’s accusing him of having a private sidebar with the reader(s). It’s certainly not a comic I’d add to my list.
I came here to agree with Boise Ed
Think of the TV show, “The Office”, or “Abbott Elementary”. These are pretend documentaries, and the characters periodically just talk to the documentary camera, That seems to be near the definition chemgal gave of a sidebar.
So he’s pretending to address a nonexistent camera. Or God. Or the ghost of his mother. Or just muttering to himself.
It’s funnier to say “Sidebar? Again with the sidebar? than to say “Are you muttering to yourself again?”
Yes, he turned to have a confidential conversation with the reader.
Forget which movie (my father would be upset about that) but in some Marx Brothers movie Groucho walks to the camera and talks to it away from the others in the scene – a side bar conversation with the audience.
Animal Crackers, as I recall.
Groucho Marx speaking directly to the camera was a deliberate parody of Eugene O’Neill’s play “Strange Interlude.” In fact Groucho introduces his soliloquy by saying “Pardon me while I have a strange interlude.”
Yes, thank you both. I do remember that it was a joke off “Strange Interlude” now.