After FDH's not-quite-breakup with me, he asked if I could be "hands off" with FSD3 going forward. I asked what he meant and he couldn't articulate it to me (and this is why we're in counseling) -- so I tried some of the stuff I've read here on StepTalk about disengagement.

TRIED, didn't necessarily succeed. As I understand disengagement (and PLEASE correct me if I'm wrong), it means I don't...
--Go out of my way to interact with her (no seeking playtime)
--Go out of my way to do parent functions for her (no dressing her, helping out in the bathroom, or cooking meals)
--Clean up after her
--Correct her behavior
--Administer punishment
--Make purchasing decisions pertaining to her

I do...
--Leave the parenting to him, her PARENT
--Leave the room when it gets to be overwhelming
--Make sure my needs are met
--Make sure the child isn't actively running into the street
--Make sure the parent isn't actively murdering the child

I failed in the "no making food" part for sure*, and in the "no cleaning up after her part,"** and maybe a little bit in the interaction part because it's hard to ignore someone when they clasp your hand and tell you they love you. And it was SO HARD to keep my mouth shut in the behavior/discipline parts that I may have failed there a little, too. But FDH said I did fine by him... thought at this point, I think not murdering his kid is about all it takes to be on FDH's approved-to-parent-my-brat list.
*I was up first in the morning and I wanted pancakes, so I made them -- can I help it if she beat FDH to the table?
**We had places to be and things to do and they'd been going at it for TWO HOURS about a lost goddamn sock. So I shoved some clothes in a drawer and tossed some toys in a bucket to unearth the little bugger where she ALWAYS hides her damn socks. I'd count this as "making sure my needs are met" rather than "cleaning up after her."

FSD3 is at the button-pushing age and FDH has been under an abnormal amount of stress. The result is a LOT of screaming and yelling between the two of them. And I've watched them -- FSD3 is just plain mean to him sometimes. Hitting, spitting, whining, snatching, manipulating, screaming insults at him... ALL things I would've shoved her butt in timeout for without batting an eye (or raising my voice). FDH did indeed have talking-tos with her a couple of times, even stuck her in timeout once. Certainly yelled himself hoarse at least twice. But I'm watching him, too. He's a mess. He cannot keep his temper with her over even the littlest stuff and I can tell he's afraid of what it's doing to his parenting plan with her since he can't even stand to be in the same room as her when she starts in on him about how he's doing something wrong ("NO DADDY, THE SHOE GOES THIS WAY!!!!" Sheesh, she sounds like a stereotype of a mother-in-law).

I left the room a lot. I worry about that a bit because I don't want FDH to feel rejected. But, yeah, his kid is being a little jerk and it's hard to watch her beat on the man I love without saying something. Once, when FDH left the room in a huff because she back-talked him, I told her I thought she was being mean and I didn't want to play with her anymore... That's not really disengaged, is it?

Oh, well. FDH seemed to think my behavior was an improvement. So now, he's only mad at his progeny. Which is fine, I guess, because as my father was fond of telling me: "Hey! I can kill you and make another one that looks just like you. DON'T TEST ME."