Function split_v_n_flex 

pub fn split_v_n_flex<const N: usize>(
    area: Rect,
    constraints: [Constraint; N],
    flex: Flex,
) -> [Rect; N]

Split area into stacked rows top-to-bottom, like split_v_n, but with explicit control over how leftover space is placed via Flex, like split_v_flex.

Never allocates, for any N; never panics, for the same reason as split_v_flex.