I shared an idea with my coworker during lunch about how to streamline our workflow. Next meeting, they presented it as their own idea to the boss and got praised for it. When I confronted them privately, they said "ideas are shared in a team environment" and that I'm being petty. This has happened three times now.
We brainstorm together all the time. The idea came up in a casual conversation and I developed it further before presenting it. They mentioned a rough concept, I did all the actual work of turning it into a proposal. If they wanted credit, they should have presented it themselves instead of just talking about it at lunch.