TreeView: focusing a parent item re-renders its expanded subtree #8718

Part of #8676 (P1). Measure with #8714 before and after.

Problem

TreeView/TreeView.tsx:501 passes a new object to ItemContext.Provider on every Item render. The isFocused state at :312 changes on focus and blur. So focusing or blurring a parent re-renders every descendant that reads ItemContext. TreeView.tsx is excluded from the React Compiler, so nothing memoizes the value automatically.

Measured: summed update-commit time after tabbing to an expanded parent, median of 7 runs:

The cost grows linearly with subtree size.

#8684 (merged) reduced mount and hydration cost but didn't change this. #8682 (open) changes only TreeView CSS.

Proposed fix

Memoize the ItemContext value with useMemo, or enable the React Compiler for TreeView.tsx (#8721). The fix must work on React 18 and 19.

Acceptance criteria

• Focusing an expanded parent doesn't re-render its children. Add a test that counts renders.
• Keyboard navigation, typeahead, expand and collapse, and async loading behave the same as before.
• The PR reports before and after numbers from Add repeatable performance benchmarks for list, tree and button stress cases #8714.