Forensic render-issue audit (post §4 / §16) — report only, no code changes

[Copilot]

Static survey of the post-§4 / §16 codebase against the 10 anti-patterns in the issue. No code changes; per the issue instructions this PR is the report.

Summary

Codebase is largely clean. The §16 discipline (refs-to-latest, useSyncExternalStore selectors, version-keyed broadcast, EMPTY_* sentinels) is applied consistently. One real cleanup hazard and one edge-case timer found; everything else flagged by a casual scan is intentional and documented in PERF-ARCHITECTURE.md.

HIGH — likely real bug

• showError schedules an uncancelled setTimeout — src/hooks/useCommon.ts:78–84
  • No handle stored; firing after unmount → "setState on unmounted component" in dev.
  • Repeated errors during the window stack timers and clobber display order.

  const showError = (errorString: ErrorString) => {
    if (showErrorMessages) {
      setError(errorString)
      setTimeout(() => setError(null), errorDisplayTime) // ← no cancellation
    }
    console.warn('Error', errorString)
  }

  • Pin-first test: jest.useFakeTimers(), fire two errors, assert getTimerCount() === 2 (current) → fix → flip to === 1. Plus: error → unmount → runAllTimers() → assert no React warning.

MEDIUM — worth pinning behaviourally

• Inline 10 ms setTimeout on <input onFocus> — src/ValueNodes.tsx:225
  • Survives most cases; under Strict Mode / concurrent transitions / fast Tab during search-filter churn the input can unmount before fire → .select() on a detached node.
  • Test: userEvent.tab() across N number inputs while toggling filter; no errors, no leftover timers post-blur.

LOW / intentional (do not touch)

• useKeyboardListener re-attach pattern — balanced add/remove, ref-to-latest absorbs identity churn (ValueNodes.tsx:295–350).
• useAppliedBroadcast deps [version, commands] — deliberate, documented (CollapseProvider.tsx:221–248).
• Inline () => setX(...) callbacks on non-memo'd leaves (StringDisplay, raw <input>, EditButtons) — no memoisation defeated.
• All Provider value={...} sites are stable: EditingProvider uses storeRef.current; CollapseProvider, ThemeProvider, DragSourceProvider are useMemo'd; JsonEditor defaults via module-scoped EMPTY_* constants.
• useState lazy-init audit clean. No index-based keys in mutable hot-path lists. useEditingSelector constrains return to primitive.

Cleanup hazards

File:line	Resource	Cleanup	Severity
src/hooks/useCommon.ts:81	setTimeout	No	Medium (real)
src/ValueNodes.tsx:225	setTimeout	No	Low (edge)
src/JsonEditor.tsx:170–182	setTimeout (search debounce)	Yes	—
src/ValueNodes.tsx:320–327	setTimeout + addEventListener	Yes	—

No setInterval / MutationObserver / ResizeObserver / IntersectionObserver usage in src/ — nothing to leak in those categories.

Recommended next steps

1. Profiler + fake-timer regression test pinning the broken showError behaviour, then fix in a follow-up PR.
2. Lightweight test for the NumberValue focus-timeout edge case; fix opportunistically when the file is next touched.
3. No action on L-tier items; consider a one-line comment near showError once fixed, referencing the cancellation requirement.

Out of scope (per the issue): large-fixture wall-clock benchmarks.

[CarlosNZ]

Out of date after #330