https://drafts.csswg.org/css-grid-3/#track-sizing Track sizing works the same as in CSS Grid, except that that the algorithm is run only with the subset of items that have a known grid axis placement. This subset contains the items placed at the first implicit line in the masonry axis in Grid Item Placement above, plus the items that have a specified definite placement in the grid axis, plus the items that span all grid axis tracks (which obviously will be placed at the first implicit line since there is no other way to place them). Other items will be placed in the grid axis by the § 2.4 Masonry Layout Algorithm below and don’t contribute to intrinsic track sizing (because that would create circular dependencies).