Function useAssistantHistory

src/hooks/useAssistantHistory.ts:72–250

({
  config,
  setMessages,
  scrollRef,
  onPrepend,
}: Props)

 * feature('KAIROS') gate, so build-time elimination is handled there.
 */
export function useAssistantHistory({
  config,
  setMessages,
  scrollRef,
  onPrepend,
}: Props): Result {
  const enabled = config?.viewerOnly === true

  // Cursor state: ref-only (no re-render on cursor change). `null` = no
  // older pages. `undefined` = initial page not fetched yet.
  const cursorRef = useRef<string | null | undefined>(undefined)
  const ctxRef = useRef<HistoryAuthCtx | null>(null)
  const inflightRef = useRef(false)

  // Scroll-anchor: snapshot height + prepended count before setMessages;
  // compensate in useLayoutEffect after React commits. getFreshScrollHeight
  // reads Yoga directly so the value is correct post-commit.
  const anchorRef = useRef<{ beforeHeight: number; count: number } | null>(null)

  // Fill-viewport chaining: after the initial page commits, if content doesn't
  // fill the viewport yet, load another page. Self-chains via the layout effect
  // until filled or the budget runs out. Budget set once on initial load; user
  // scroll-ups don't need it (maybeLoadOlder re-fires on next wheel event).
  const fillBudgetRef = useRef(0)

  // Stable sentinel UUID — reused across swaps so virtual-scroll treats it
  // as one item (text-only mutation, not remove+insert).
  const sentinelUuidRef = useRef(randomUUID())

  function mkSentinel(text: string): SystemInformationalMessage {
    return {
      type: 'system',
      subtype: 'informational',
      content: text,
      isMeta: false,
      timestamp: new Date().toISOString(),
      uuid: sentinelUuidRef.current,
      level: 'info',
    }
  }

  /** Prepend a page at the front, with scroll-anchor snapshot for non-initial.
   *  Replaces the sentinel (always at index 0 when present) in-place. */
  const prepend = useCallback(
    (page: HistoryPage, isInitial: boolean) => {
      const msgs = pageToMessages(page)
      cursorRef.current = page.hasMore ? page.firstId : null

      if (!isInitial) {
        const s = scrollRef.current
        anchorRef.current = s
          ? { beforeHeight: s.getFreshScrollHeight(), count: msgs.length }
          : null
      }

      const sentinel = page.hasMore ? null : mkSentinel(SENTINEL_START)
      setMessages(prev => {
        // Drop existing sentinel (index 0, known stable UUID — O(1)).