If you're sitting around complaining about someone at a volume of voice where you can be heard, is the person eavesdropping if they hear you?

No. You’re not eavesdropping if someone is talking loudly around you and you hear them.

Most people that talk to themselves do so quietly. This person obviously wants to be overheard.

You’re eavesdropping if you are trying to overhear.

I wasn’t trying to overhear, but couldn’t help hearing what they were saying. I did go talk to them about it, which upset them (no doubt they were embarrassed to find out I heard what they said).

^Maybe they didn’t think you could hear them or that you were there. If it wasn’t intentional it won’t be called eavesdropping from your perspective. From their perspective it will depend on what they supposed you were doing at that moment.