[nstepien]

TreeDataGrid passes its group rows to DataGrid as R, so when a group row, or a cell of one, is active, onCellKeyDown and onActivePositionChange receive a GroupRow typed as R: a property access like args.row.name.toUpperCase() type-checks and throws at runtime. Their row is now typed R | GroupRow<R>, and isGroupRow is exported so handlers can narrow it. Like #4219, this makes the types say what happens.

Builds on #4264.

Breaking change

- function onCellKeyDown(args: CellKeyDownArgs<Row>, event: CellKeyboardEvent) {
-   console.log(args.row.name);
+ function onCellKeyDown(args: TreeCellKeyDownArgs<Row>, event: CellKeyboardEvent) {
+   if (isGroupRow(args.row)) return;
+   console.log(args.row.name);
  }

Affects TreeDataGrid's onCellKeyDown and onActivePositionChange handlers: their row is typed R | GroupRow<R>, which is what the grid passed all along when a group row was active. isGroupRow is exported to narrow it. DataGrid handlers are unchanged.

Changes

• CellKeyDownArgs and PositionChangeArgs get a third type parameter, TActiveRow, which defaults to TRow, so DataGrid handler types are unchanged. It only applies to the ACTIVE variant of CellKeyDownArgs: EditCell is only rendered through Row, never through GroupedRow, so EDIT args never carry a group row.
• New exported aliases, TreeCellKeyDownArgs<R, SR> and TreePositionChangeArgs<R, SR>, keep handlers short. The third type parameter stays public and is documented next to them.
• TreeDataGridProps omits the two inherited props and redeclares them with the aliases; TreeDataGrid's own handleKeyDown uses TreeCellKeyDownArgs. Passing them on to DataGrid type-checks because function parameters are contravariant.
• isGroupRow is exported from TreeDataGrid.tsx, where it stays with its WeakSet: DataGrid doesn't use it, so exporting it from there creates no import cycle. If the group-row edit-mode bug below gets fixed in DataGrid, that PR can move both to a shared module.
• The rows as R[] cast stays, and its TODO comment now explains which callbacks receive group rows.
• README: isGroupRow under "Other", the aliases and the third type parameter under "Types", onCellKeyDown and onActivePositionChange under TreeDataGridProps, and a "Group rows in callbacks" paragraph with an example under <TreeDataGrid />. The CellKeyDownArgs and PositionChangeArgs headings gain TActiveRow, so their anchors changed and the links to them are updated.
• Re-verified that the other callbacks never receive group rows: the cell mouse events (group cells don't fire them), rowClass (not applied to group rows), onCellCopy and onCellPaste (filtered), renderers.renderRow and renderCell (wrapped), rowHeight (RowHeightArgs), and onRowsChange (indexes mapped back).

Follow-ups

• Typing on a cell of a group row in a column with renderEditCell sets mode: 'EDIT' with the group row; no editor renders, and the arrow keys stop working until a click. That fix gets its own PR.

Verification

vp run ci passes on the top of the stack (#4272). Added a browser test that activates a group row, then a cell of it, then a cell of a regular row, and asserts which rows onCellKeyDown and onActivePositionChange received, using isGroupRow. A consumer type-check of the built lib/index.d.ts, with and without exactOptionalPropertyTypes, uses TreeCellKeyDownArgs, TreePositionChangeArgs, isGroupRow, GroupRow, expandedGroupIds, and renderGroupCell.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.27%. Comparing base (4f19b31) to head (66866bf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4265   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files          44       44           
  Lines        2213     2213           
  Branches      971      971           
=======================================
  Hits         2197     2197           
  Misses         16       16           

Files with missing lines	Coverage Δ
src/TreeDataGrid.tsx	98.66% <100.00%> (ø)

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