[DataGrid] Fix row tree reset when a data source grid becomes visible

[JCQuintas]

Summary

Fixes #23262.

A data source grid with treeData + lazyLoading inside a React <Activity> boundary lost its lazy-loaded children and skeleton rows, and had childrenExpanded reset on every group node, whenever the boundary went hidden → visible.

<Activity> re-runs effects when it becomes visible again, even though no dependency changed. The props-sync effect at the bottom of useGridRows was not resilient to that, because its only guard against re-applying rows is an identity comparison:

const currentRows = props.dataSource ? gridDataRowsSelector(apiRef) : props.rows;
const areNewRowsAlreadyInState =
  apiRef.current.caches.rows.rowsBeforePartialUpdates === currentRows;

rowsBeforePartialUpdates holds the raw array passed to the last setRows(), while gridDataRowsSelector builds a new array on every recompute. Under a data source those are never the same reference, so areNewRowsAlreadyInState is effectively always false and the effect falls through to a full throttledRowsChange rebuild. That normally goes unnoticed, because the effect only runs when a dependency actually changed — but <Activity> re-fires it with identical dependencies, and the rebuild drops the skeleton rows (they are not data rows) and resets the expansion state.

With a data source the rows live in the state rather than in props, so there is never a new set of rows to apply here. The fix makes that explicit instead of asking a question that cannot be answered by reference identity. Only a new getRowId still requires a rebuild, to re-key the rows already in the state — tracked with a ref, the same way the effect already tracks rowCount a few lines above.

This is a second, distinct site from #22603. That PR stopped the <Activity> re-show from re-fetching in useGridDataSourceBase; with it merged, the row tree was still destroyed by this effect. Verified in both directions on top of master at 77536d2:

	without this fix	with this fix
children	50 → 50	50 → 50
childrenExpanded	true → false	true → true
skeleton rows	50 → 0	50 → 50

Tests

• dataSourceTreeData.DataGridPro.test.tsx — the loaded children, the skeleton rows and the expansion state survive an <Activity> hide/show. Fails on master.
• dataSourceTreeData.DataGridPro.test.tsx — a real rowCount prop update is still applied, covering the fall-through path that the early return must not swallow.
• dataSource.DataGridPro.test.tsx — a getRowId change still re-keys the rows of a data source grid. This path had no coverage, and an earlier version of this fix silently broke it while the whole suite stayed green.

Changelog

Fixed the Data Grid dropping lazy-loaded rows, skeleton rows and the group expansion state when a data source grid re-appears inside a React <Activity> boundary.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-23288--material-ui-x.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/x-data-grid	🔺+103B(+0.03%)	🔺+41B(+0.03%)
@mui/x-data-grid-pro	🔺+100B(+0.02%)	🔺+41B(+0.03%)
@mui/x-data-grid-premium	🔺+103B(+0.01%)	🔺+43B(+0.02%)
@mui/x-charts	0B(0.00%)	0B(0.00%)
@mui/x-charts-pro	0B(0.00%)	0B(0.00%)
@mui/x-charts-premium	0B(0.00%)	0B(0.00%)
@mui/x-date-pickers	0B(0.00%)	0B(0.00%)
@mui/x-date-pickers-pro	0B(0.00%)	0B(0.00%)
@mui/x-tree-view	0B(0.00%)	0B(0.00%)
@mui/x-tree-view-pro	0B(0.00%)	0B(0.00%)
@mui/x-scheduler	0B(0.00%)	0B(0.00%)
@mui/x-scheduler-premium	0B(0.00%)	0B(0.00%)
@mui/x-chat	0B(0.00%)	0B(0.00%)
@mui/x-license	0B(0.00%)	0B(0.00%)

Details of bundle changes

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