a11y: roving tabIndex in ComponentTree for large fiber trees (WCAG 2.1.1)

[jamilagibson]

Context

The current ComponentTree gives tabIndex={0} to every visible node header, meaning Tab cycles through all visible nodes. The ARIA Authoring Practices Guide (APG) recommends roving tabIndex for tree widgets — only the focused node has tabIndex={0}; all others are tabIndex={-1}. Arrow keys move focus within the tree.

This is acceptable at current scale (small demo app Fiber tree). It becomes a usability problem when Will's full fiber tree serialization lands and node counts grow significantly.

Implementation

• Only the currently focused node has tabIndex={0}; siblings and cousins get tabIndex={-1}
• ArrowDown / ArrowUp move focus to the next/previous visible node
• ArrowRight expands and moves focus to first child
• ArrowLeft collapses or moves focus to parent

Acceptance criteria

• Roving tabIndex implemented per APG tree widget pattern
• Tab enters the tree at the last focused node (or root if first visit)
• Tab exits the tree in one keystroke
• Arrow key navigation covers all visible nodes
• Existing onKeyDown expand/collapse behavior preserved
• ACCESSIBILITY.md updated

References

• Decision 18 in CLAUDE.md — APG tree widget pattern and roving tabIndex known improvement