Function split_h_flex 

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

Split area into columns left-to-right, like split_h, but with explicit control over how leftover space is placed via Flex.

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