GroupBy projection

GroupBy is a row‑model transformation, not a UI feature.

1) Canonical pipeline

rows -> filter -> sort -> groupBy -> flattenTree -> virtualization

2) Row kinds

• group — virtual group row
• leaf — source row

3) GroupBy contract

interface DataGridGroupBySpec {
  fields: string[]
  expandedByDefault?: boolean
}

• setGroupBy(spec | null)
• toggleGroup(groupKey)

4) Selection semantics

Selection operates on the flattened stream. For adapters, use createGridSelectionContextFromFlattenedRows({ rows, colCount }).

5) UI meta

Use getDataGridRowRenderMeta(rowNode) for level/isGroup/isExpanded.