“The default value for
flex-growis 0, which means that growing is opt-in.”
Most Flexbox confusion comes from not knowing which way the primary axis runs. Comeau makes every property draggable, so you can watch flex-basis get overridden and auto margins eat the leftover space. Ten years of Stack Overflow answers telling people to add “display: flex” and pray. This is the explanation that should have shipped with the spec.