Hello,
I can completely relate to an argument like that one... .although I have to admit I still do not understand them. I, like you, would have thought it no big deal, picked it up and went about my business, but people with BPD... .just not going to happen. I'm still trying to figure this all out (after being married 27 years) and sometimes feel like those battered wives who, when someone asks why they stay, they just say, "... .because I love him". Sounds so pathetic, but it's true. And, in the many times that he's not losing him mind over something simple, he really is a great guy!
Know you are definitely not alone... .there are obviously many of us going thru the same things, which in itself helps just a little

Hopefully we'll all find our way thru this soon.