LONDON—Refinitiv is considering a series of changes to its daily foreign exchange (FX) fixing, including lengthening the trading window used to calculate it, a senior company executive said, amid industry concerns over the global benchmark which used to value trillions of dollars of assets. The 27-year-old WM/R benchmark, which is calculated by its owner Refinitiv, has a five-minute trading window, known as the “fix”, at 4:00 p.m. in London each weekday when investors and banks flood the market with buy and sell orders. The WM/R is by far the most popular benchmark and baked into contracts such as those used by U.S. fund managers wanting to buy European stocks, who would sell dollars and buy euros at exchange rates set at the fixing. One concern among some asset managers, who move billions of dollars through the window, is that more nimble hedge funds can spot and trade off large transactions, …