chore(rules): document render-phase ref-seed choice + no-render-in-render caveat

.claude/rules/sim-components.md

@@ -28,5 +28,6 @@ react-doctor diagnostics are hypotheses, not verdicts — confirm against the co
 - `no-barrel-import` — barrel imports are the repo convention (see sim-imports.md, "Barrel Exports"). Keep them.
 - `js-tosorted-immutable` — in `'use client'` code, keep `[...arr].sort(cmp)`; `toSorted` is unpolyfilled and crashes Safari <16 / iOS 15 (see "List-render performance" above). Only apply it in server-only modules.
 - `rerender-state-only-in-handlers` / "state set but never rendered" — a false positive when the `useState` is consumed by a `useEffect`/`useLayoutEffect` dependency (the effect must re-run on change). Only convert to a ref when nothing reads the value reactively.
+- `no-render-in-render` — a helper *called inline* (`{renderRow()}`) is reconciled by position and does **not** remount, so extracting it to a component is usually pure churn and can regress behavior (prop-drilling many closures, focus/scroll loss on the inner `<input>`). Apply it only when the helper is genuinely a *component defined during render*, or when the move is mechanical (a stateless, ref-free helper whose closures become a small, explicit prop set).
 - `async-await-in-loop` on an upload/progress loop where sequential execution is intentional (per-item progress, server backpressure) — leave it.
 - Broad refactors (`prefer-useReducer` for many `useState`, `no-giant-component` splits) — out of scope for a perf pass; note, don't churn.

.claude/rules/sim-hooks.md

@@ -51,16 +51,24 @@ export function useFeature({ id, onSelect }: UseFeatureProps) {
 
 ## State shape
 
-Never mirror a prop into state with `useState(prop)` + a syncing `useEffect` — a prop change clobbers in-progress local edits. Use the prop directly, reset via a remount `key`, or — when you must seed local state from a prop only on a transition (e.g. a modal opening) — reset during render with the `prevX` ref idiom:
+Never mirror a prop into state with `useState(prop)` + a syncing `useEffect` — a prop change clobbers in-progress local edits. Use the prop directly, reset via a remount `key`, or — when you must seed local state from a prop only on a transition (e.g. a modal opening) — adjust it **during render** with a `prev`-value tracker held in `useState`:
 
 ```typescript
-const prevOpenRef = useRef(open)
-if (prevOpenRef.current !== open) {
-  prevOpenRef.current = open
+const [prevOpen, setPrevOpen] = useState(open)
+if (prevOpen !== open) {
+  setPrevOpen(open)
   if (open) setName(initialName) // closed → open only
 }
 ```
 
+React re-renders immediately with the corrected state without committing the stale value. Rules: the `if (prev !== current)` guard is mandatory (an unconditional `setState` in render loops forever), the tracker is set **inside** the guard, and you may only set the currently-rendering component's state this way. Hold the tracker in `useState`, **not a `useRef`** — React forbids reading/writing `ref.current` during render (react.dev, useRef → "Do not write _or read_ `ref.current` during rendering"; the `react-hooks` `refs` lint flags it), and a `useState` tracker is concurrent-safe where a mutated ref is not (a discarded render rolls state back, not a ref).
+
+**The tracker's initial value decides mount behavior — choose it deliberately.** The example seeds `useState(open)` because the modal mounts closed, so the first render's guard is `false` and nothing resets on mount (correct — `name` is already at its initial value). When the effect you're replacing did real work **on mount** — opening a panel because a prop already matches, seeding editable state from an already-present value, or a component that can mount in the active state — seed a **sentinel** the live value can't equal (e.g. `useState<T | null>(null)`), otherwise the guard is `false` on the first render and that mount action is silently dropped. Place the block before any early `return`.
+
+> Some existing components use a `useRef` prev-tracker (`if (prevRef.current !== x) { prevRef.current = x; … }`). It works but reads/writes a ref during render — prefer the `useState` form above for new code.
+
+Only convert a `useState` to a `useRef` when the value is **never read during render/JSX and is never a hook dependency** — a value in a `useEffect`/`useMemo`/`useCallback` dep array must re-run the hook on change, so it stays state (see also `rerender-state-only-in-handlers` in `sim-components.md`). Convert only set-only values read solely inside handlers or effect bodies (e.g. a prompt-history index, a pending-upload URL). If a ref feeds render, mutating it won't re-render and the UI goes stale.
+
 Model mutually-exclusive flags as ONE `status` enum, not several contradictory booleans. `isLoading`/`isVerified`/`isInvalidOtp` describing one machine collapse to `status: 'idle' | 'verifying' | 'verified' | 'error'` (+ `errorMessage`); derive any boolean a consumer still needs (`status === 'error'`).
 
 Derive busy/success from the mutation object — never duplicate `mutation.isPending`/`mutation.isSuccess` into local `useState`. Read them directly (`mutation.isSuccess`) and reset with `mutation.reset()`. A distinct phase the mutation doesn't cover — e.g. a pre-submit captcha/Turnstile gate that runs before `mutate()` — is not a duplicate; keep that flag.