Function row 

pub fn row(
    theme: &Theme,
    columns: &[Column],
    first: bool,
    selected: bool,
    cells: Vec<AnyElement>,
) -> Div

One body row, its cells zipped onto the columns.

The zip is the whole point — a cell is never sized where it is written, so a row cannot drift from the header. cells shorter than columns is a bug in the caller rather than a shape to render, and the assert says so in debug builds; release truncates rather than panicking at a user.