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The situation I'm takling about at my workplace is as such: I do have a decent knowledge of a software base and have many times proposed improvements to my manager. He declined them for time reasons. Fast forward a few month later, and I see a colleague who has overheard said propositions and has implemented them "on the side"; I know for a fact the he was pretending to be oversaturated with work but he used that time to covertly, without sharing with his team, implement those features.
I now discover that the manager is liking the features, and even presents them to me as inventive; I also discover that he has been sold the implementation of said features as stellar, when I've noticed that they are harshly implemented and that they in instances reuse core components I've myself introduced in the past.
I'm all for improving and sharing as a team. I do think stealing ideas and using allocated time, which we divide between devs in the team, to implement those stolen ideas, is not in order; at least, not in pure career interested advancements.
How can I react constructively to the situation, by explaining the situation to my manager in a professional manner ?