As you've probably learned, pwBPD have very strong emotions and often have a deep-seeded fear of abandonment. It sounds to me like at the movie theater, she felt ignored (re: abandoned), which hurt her feelings. But, as a pwBPD, that hurt was more like a third-degree burn than a sunburn. In her hurt and anger, she lashed out at the safe target (you) and in a tangential way. Does that sound right?
My H would do things like that. A doctor's office would be slow getting back to him. He'd take it as a personal insult, then turn it on me and talk about how awful doctors are and, as an extension, my family (since some of them are doctors). Made no sense, really, but I eventually learned to see through to the core of what he was upset about.
Incidents like this are a way of life. You can't stop them from happening. Nor can you shield her from them.
What you can do is listen to her and validate the feelings behind what she's saying. You were able to stay calm. That's good. But when you listened and responded, did you really listen and respond? Or was it more about trying to get it all to stop?
Here are a couple of articles that I've found very helpful:
https://bpdfamily.com/content/listen-with-empathyhttps://bpdfamily.com/content/communication-skills-dont-be-invalidatingI'm not trying to be critical. Just to get you to look at your reactions and how they might play into the dynamic. I know in my case, once I changed how I responded to my H (with calm strength and empathy as opposed to desperate apologies or playing along just to get him to stop), things actually improved. Not saying this will definitely happen in your case, but building your own confidence and strength will do you a world of good, regardless of what happens in your relationship.
Give those a read and let me know what you think!
If you were to do this over, what would you do or say different?