Trader Richard Cohen works on the floor of the New York Stock Exchange. Stock indexes ended mixed on Wednesday after Nvidia, Palantir and other superstar stocks pared most of their steep losses from the morning. The S&P 500 dipped 0.2% after trimming a loss that reached 1.1% earlier in the day and remains near its all-time high set last week. The Dow Jones Industrial Average added 16 points, or less than 0.1%, and the Nasdaq composite fell 0.7%. The day’s action centered again around stocks caught up in the mania around artificial-intelligence technology. Advertisement Nvidia, whose chips are powering much...