Minimal FSE theme showing that a Grid layout with both columnCount and minimumColumnWidth ignores a theme.json blockGap preset and computes its
columns against a 0.5em fallback. Two columns with a gap larger than 0.5em don't fit, so they collapse to one column.
theme.json sets styles.spacing.blockGap to var:preset|spacing|32. The templates/index.html template renders the same grid twice: once relying on the theme gap (broken) and once with blockGap set on the block (control).
This gist represents a very minimal FSE theme, if you rename templates__index.html to templates/index.html.