Still here. Still not detached... (Part 3)

[STH17]

My decision to file or not file is partly based on my nice guy syndrome, but also because I'm bit worried about how she'll react.

Worried about how she'll react is NGS, not "but also". Non-NGS is: however she reacts, I'll handle it.

Here's what you say to OM if and when you ever see him: Nothing.

That would be a perfect strategy for me, but let suppose that the first time I meet him is in a public setting in front of the kids and he extends a hand to shake. Do I just leave him hanging? Who'd end up looking like the a-hole? Certainly not him...

Absolutely leave him hanging. You don't owe a handshake to him just because a hand is extended to you. Handshake means "nice to meet you." If that's what you want to say to him, then do it. But I think it's a good clear example of boundary setting to not shake his hand. Doesn't need to be a passive aggressive silence/ignoring. You could tell him you're not shaking his hand and then start talking to your kids "Hey who's hungry? Let's get some food!". Same as going dark on a wayward spouse. Cordially(optional?) end the conversation and go be awesome. I also wonder if you could touch his wrist with two fingers and lower his hand back down. "Yeeah I'm gonna pass on that.." Or a wave of your hand with a simple "No thanks". If he insists, you say "I said no thanks". You'll handle it
I wouldn't worry about looking like an a-hole, but I could understand giving consideration for your kids possibly feeling like they have to pick sides with you and him if they see you shunning him. Hopefully they just see an awesome man/father setting healthy boundaries for himself, but they will form their own opinions either way.

You gave her the satisfaction of knowing that her actions still bother you. Why?

I guess it's partly because we've been working together so well with the co-parenting. It's all very cordial. And we had agreed on a really good plan for the holidays and gifts. Then she springs this on me and it riled me up!

I actually think Chris handled this pretty well. You had a plan, she cut you out of it, and you made it clear you are not okay with that behavior. You established a boundary by not making a new plan with her for son's gift, committing to taking care of it by yourself. She had the opportunity to give your son a gift jointly with you, and she has now lost that opportunity. I do agree that the real issue is her being with OM, but I wouldn't bring her choice to be with him into this conflict which is about her supplanting you in the plans you had agreed upon. And while it may not help in DB, you are entitled to voice your opinion about the inappropriateness of OM getting son what was meant to be the crown jewel of the gifts you were giving son. I'd be angry too. He can buy him some socks. I think you did what you could here, and best now to just take care of getting a gift from only yourself as you said you would.