Make the child grow to share the container's leftover space, with the given
weight (default 1). A readable alias for proportion(weight): two
stretch() children split the slack evenly, stretch(2) versus stretch(1)
splits it 2:1. So add(list).stretch() reads as "the list takes the slack."
Make the child grow to share the container's leftover space, with the given weight (default 1). A readable alias for proportion(weight): two stretch() children split the slack evenly, stretch(2) versus stretch(1) splits it 2:1. So add(list).stretch() reads as "the list takes the slack."