Now seriously OP, I hope things are better this morning.
Sounds like he doesn’t deal with things very nicely or healthily.
You know best which battle to pick, but here are my thoughts on attempting to discuss things with your partner without getting sucked into an argument - it sounds like he likes to manipulate things by engineering an argument so he never gets his behaviour addressed!
Can you try to stick to the point and keep repeating it in a calm voice without getting turned aside?
Or say something like ‘yes that may be the case, but we’re not talking about that now, we’re talking about what happened last night...’ etc.
Or even ‘I can hear that made you feel sad/ frustrated/ whatever, but but we’re not talking about that now, we’re talking about what happened last night...’ etc
Or ‘i love you and I’m not trying to have an argument. This is important to me, please can we talk it through?’
Although the first few times your DP may try and push past your refusal to engage, as it’s a power thing I think. However it is rather hard to argue for long with someone who keeps saying annoyingly rational and calm things like ‘I love you and I dont want an argument with you...’ but without giving in either.
Or, if nothing else works, be really blunt,
‘I can see you’re angry but I’m not going to argue with you. My feelings count too.
‘I understand that it might make you defensive / angry, but it makes me feel really unheard when you change the subject like this’.
Or if he’s determined to argue and behave nastily, then walk away. You don’t have to stay and be stuck in an argument which is doing no good.