No need to go to the replay: The stock market is grinding out wins like an NFL juggernaut.

In August, the average U.S.-stock mutual fund or exchange-traded fund posted a total return of 2.9%, according to LSEG data, to push the year-to-date advance to 8.5%. International-stock funds were also strong, up 3.5% in August and 21.2% for the year to date. (See funds-data tables including Mutual-Fund Yardsticks.)