Function grid_template_columns 

pub fn grid_template_columns(
    columns: &[Column<'_>],
    sizing: &Sizing<'_>,
    cutoff: Priority,
) -> String

The grid-template-columns value for the columns kept at cutoff.

Emitting only the surviving tracks is what keeps the track list and the hiding in agreement. An app that hides a cell with display: none but leaves its track in place gets a column of empty space, which is the other half of goingson’s mobile bug: its narrow rule drops to four tracks by hand and has to be edited in step with the nth-child cut.