If the person who is in a bad mood is pouting and moping around and waiting for their partner to ask instead of being honest about how they feel, then they're being toxic.

Again, I don't disagree with you. However, where is it said that this is specifically what they mean? I feel like we must be reading something different.

especially when i was younger, but sometimes still nowadays, one of us would sometimes have a really bad day for a really inane reason that seemed like the world at the time, and learning to communicate that is difficult. especially with autism and dissociative identity disorder. sometimes memories aren't available, sometimes it's difficult to talk about. sometimes the partner did something by accident that upset the entire day. sometimes the bad thing is that your partner ate all your ice cream in the freezer and you really wanted some mango ice cream, except it was just a dream. what the hell. when you're emotionally charged, things just get more difficult. nonviolent communication can be a great tool, but it requires some amount of emotional availability and fluency and patience that takes effort to build up over years. talking through can take hours and days and weeks even now. nowadays it's mostly automatic that we talk about everything, but in the beginning exuding a bad day was natural for me because i grew up in a dysfunctional family, and my partner also had to learn a lot. a therapist helped a ton, though of course therapists vary wildly in quality, suitability and of course price depending on the region. i think people can change a lot too. there's always some variability with people.