Nvidia's (NVDA.O) move to buy back $25 billion of its shares after its stock has more than tripled this year caught some investors off-guard, even as they cheered a stellar second-quarter report. Shares of Nvidia touched a record high on Thursday, a day after the company blew past expectations with its quarterly revenue forecast as an artificial-intelligence boom fueled demand for its chips. Nvidia shares, which had run up in the days leading up to its report, climbed more than 6% on Thursday but pared gains to end the day little changed. However, Nvidia's stock buyback - the fifth-biggest repurchase...