In a healthy environment, being upset is really very unnecessary lol. Most problems can be resolved without antagonism if you put some empathy and thought into it. If I notice something really bothering me though, I'll decide if I have to deal with it right away, and if so, I'll hash things out, apologize or get apologies/concessions where I need to, etc., but it does take time. And if needed, I'll put my foot down without hostility, which takes some practice. Being a chill guy means that people can mistake assertiveness for aggression, especially in a tense situation. But on the other hand, it means you've saved up a lot of capital for when you need to be forceful.

If it's not urgent, I'll think on it and have a measured response for later. That's nice because you have time to think thoroughly, but the downside to putting things off for later is that once I know what I'll say, the conflict is essentially resolved for me. It's not an act, there's really no reason to be upset. So, sometimes if there's been some time in-between I actually forget what the exact problem was, while the other person might assume mutual anxiety over the issue I'd already untangled... it's hard not to jokingly feel like the other party is an idiot for this, but not everyone is a type 9 I guess. With other healthy 9 friends (as well as many non-9 friends), things tide over very well and when we talk again, we often find we've reached the same resolution. Otherwise, it's best to nip things in the bud, or set a specific time to bring it up later.

That said, there are always problems that can't be resolved, and I remind myself it's nothing personal, just the way things are. But without generalizing, I do have to remind myself not to be so tolerant. That means renegotiating high conflict relationships, doing what it takes to progress out of high stress environments, etc. It takes a lot of effort to confront things head on, and pre-emptively, but it's still necessary, albeit more taxing to do alone. It's important to have the right support around you to recharge and get going, because sometimes you don't have that energy or don't want to spend it. Speaking for myself, it's just more efficient to deal with sympatico people who share a positive, cooperative, intelligent attitude so that there's less emotional labor involved.

I didn't mean for this to be so long, but the only other comment seemed to pathologize and stereotype 9's too much for my liking. Personality types are a great tool for discussing patterns, but there's a temptation of dogmatically superimposing a description for Type XYZ onto a person's actions and going "oh, you're being a Type XYZ" instead of more deeply inhabiting their broader motivations + possible growth. The healthier and more vibrant a person becomes, I think the more difficult it becomes to conform them to a certain type, and similarly I think it's regressive to let the enneagram speak for what people "are" -- it's just a guide that'll always be less complete than our actual realities. We shouldn't use the enneagram to start "tracing" over a person's psychology, and especially not our own. With that in mind, finding calm doesn't equate to narcoticization or sweeping things under the rug, and it's fairly misrepresentative and impractical to jump to that as an explanation.