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Rihanna attended a Roc Nation charity basketball game on Thursday evening at the Barclays Center in New York City, and when her ex-boyfriend Chris Brown made a surprise appearance on the court, she didn't even flinch -- in fact, she totally stared him down! No one's got a stink eye like Bad Gal RiRi ... and you know she's thinking to herself how stupid his bleached blonde hair looks! At least we wanna believe she is...

Ri was joined by BFF Cara Delevingne, and at one point she got up from her seat and strutted around in a pair of barely-there cutoffs with a smile on her face. It's not clear if the R&B superstar exchanged any words with her ex, but she held her ground and kept her cool.

Funny thing is, Chris' on-again gf Karrueche Tran was NOT there -- she was en route to Jamaica, where she's hosting a party at a nightclub in Kingston. Think Chris would have brought Karrueche if she was in town?