Function cutoff_for 

pub fn cutoff_for(
    columns: &[Column<'_>],
    sizing: &Sizing<'_>,
    width: f32,
) -> Priority

The weakest cutoff whose columns fit in width.

Raised until the layout fits, and never past Priority::Essential: the essential columns are what makes a row identify itself, so a window too narrow for them gets them squeezed rather than dropped. Nothing here counts positions, so which column drops is a property of the column.