U.S. stocks opened slightly higher Thursday, extending the rebound from the selloff triggered by the U.K. vote to exit the European Union. The main indexes looked set to book monthly losses but modestly higher over the quarter. The S&P 500 opened 3 points, or 0.1%, higher at 2,073. The Dow Jones Industrial Average added 25 points, or 0.1%, to 17,719. The Nasdaq Composite started the session up 7 points, or 0.2% at 4.786.