Tencent Holdings Ltd. plans to more than double its stock buyback program to at least $12.8 billion in 2024, mollifying investors concerned about a gradual dissipation of growth during a Chinese economic downturn.China’s largest internet company joins rivals from Alibaba Group Holding Ltd. to JD.com Inc. in ramping up dividends or share repurchases, a symptom of maturity in businesses that once led the nation in growth. Tencent on Wednesday reported a lower-than-expected 7% rise in revenue after...