Function split_v_flex 

pub fn split_v_flex(
    area: Rect,
    constraints: &[Constraint],
    flex: Flex,
) -> Vec<Rect> 

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

Never panics, for the same reason as split_v: every offset is computed with saturating_add/saturating_sub.