GridGroupColumnFn

A callback invoked once per synthesized group column, receiving a GroupDescriptor, and returning the settings to apply to that column.

In 'multipleColumns' mode there is one synthesized column per active group level, so the callback runs once per level, each time receiving that level's own GroupDescriptor. In 'singleColumn' mode there is only ever one synthesized column for all levels, so the callback runs exactly once, receiving the first group level's GroupDescriptor — deeper levels aren't passed to it even though the same column renders their header/footer rows.