import type { RefObject } from 'react'
import * as React from 'react'
import {
  useCallback,
  useContext,
  useEffect,
  useImperativeHandle,
  useRef,
  useState,
  useSyncExternalStore,
} from 'react'
import { useVirtualScroll } from '../hooks/useVirtualScroll.js'
import type { ScrollBoxHandle } from '../ink/components/ScrollBox.js'
import type { DOMElement } from '../ink/dom.js'
import type { MatchPosition } from '../ink/render-to-screen.js'
import { Box } from '../ink.js'
import type { RenderableMessage } from '../types/message.js'
import { TextHoverColorContext } from './design-system/ThemedText.js'
import { ScrollChromeContext } from './FullscreenLayout.js'

// Rows of breathing room above the target when we scrollTo.
const HEADROOM = 3

import { logForDebugging } from '../utils/debug.js'
import { sleep } from '../utils/sleep.js'
import { renderableSearchText } from '../utils/transcriptSearch.js'
import {
  isNavigableMessage,
  type MessageActionsNav,
  type MessageActionsState,
  type NavigableMessage,
  stripSystemReminders,
  toolCallOf,
} from './messageActions.js'

// Fallback extractor: lower + cache here for callers without the
// Messages.tsx tool-lookup path (tests, static contexts). Messages.tsx
// provides its own lowering cache that also handles tool extractSearchText.
const fallbackLowerCache = new WeakMap<RenderableMessage, string>()
function defaultExtractSearchText(msg: RenderableMessage): string {
  const cached = fallbackLowerCache.get(msg)
  if (cached !== undefined) return cached
  const lowered = renderableSearchText(msg)
  fallbackLowerCache.set(msg, lowered)
  return lowered
}

export type StickyPrompt =
  | { text: string; scrollTo: () => void }
  // Click sets this — header HIDES but padding stays collapsed (0) so
  // the content ❯ lands at screen row 0 instead of row 1. Cleared on
  // the next sticky-prompt compute (user scrolls again).
  | 'clicked'

/** Huge pasted prompts (cat file | claude) can be MBs. Header wraps into
 *  2 rows via overflow:hidden — this just bounds the React prop size. */
const STICKY_TEXT_CAP = 500

/** Imperative handle for transcript navigation. Methods compute matches
 *  HERE (renderableMessages indices are only valid inside this component —
 *  Messages.tsx filters and reorders, REPL can't compute externally). */
export type JumpHandle = {
  jumpToIndex: (i: number) => void
  setSearchQuery: (q: string) => void
  nextMatch: () => void
  prevMatch: () => void
  /** Capture current scrollTop as the incsearch anchor. Typing jumps
   *  around as preview; 0-matches snaps back here. Enter/n/N never
   *  restore (they don't call setSearchQuery with empty). Next / call
   *  overwrites. */
  setAnchor: () => void
  /** Warm the search-text cache by extracting every message's text.
   *  Returns elapsed ms, or 0 if already warm (subsequent / in same
   *  transcript session). Yields before work so the caller can paint
   *  "indexing…" first. Caller shows "indexed in Xms" on resolve. */
  warmSearchIndex: () => Promise<number>
  /** Manual scroll (j/k/PgUp/wheel) exited the search context. Clear
   *  positions (yellow goes away, inverse highlights stay). Next n/N
   *  re-establishes via step()→jump(). Wired from ScrollKeybindingHandler's
   *  onScroll — only fires for keyboard/wheel, not programmatic scrollTo. */
  disarmSearch: () => void
}

type Props = {
  messages: RenderableMessage[]
  scrollRef: RefObject<ScrollBoxHandle | null>
  /** Invalidates heightCache on change — cached heights from a different
   *  width are wrong (text rewrap → black screen on scroll-up after widen). */
  columns: number
  itemKey: (msg: RenderableMessage) => string
  renderItem: (msg: RenderableMessage, index: number) => React.ReactNode
  /** Fires when a message Box is clicked (toggle per-message verbose). */
  onItemClick?: (msg: RenderableMessage) => void
  /** Per-item filter — suppress hover/click for messages where the verbose
   *  toggle does nothing (text, file edits, etc). Defaults to all-clickable. */
  isItemClickable?: (msg: RenderableMessage) => boolean
  /** Expanded items get a persistent grey bg (not just on hover). */
  isItemExpanded?: (msg: RenderableMessage) => boolean
  /** PRE-LOWERED search text. Messages.tsx caches the lowered result
   *  once at warm time so setSearchQuery's per-keystroke loop does
   *  only indexOf (zero toLowerCase alloc). Falls back to a lowering
   *  wrapper on renderableSearchText for callers without the cache. */
  extractSearchText?: (msg: RenderableMessage) => string
  /** Enable the sticky-prompt tracker. StickyTracker writes via
   *  ScrollChromeContext (not a callback prop) so state lives in
   *  FullscreenLayout instead of REPL. */
  trackStickyPrompt?: boolean
  selectedIndex?: number
  /** Nav handle lives here because height measurement lives here. */
  cursorNavRef?: React.Ref<MessageActionsNav>
  setCursor?: (c: MessageActionsState | null) => void
  jumpRef?: RefObject<JumpHandle | null>
  /** Fires when search matches change (query edit, n/N). current is
   *  1-based for "3/47" display; 0 means no matches. */
  onSearchMatchesChange?: (count: number, current: number) => void
  /** Paint existing DOM subtree to fresh Screen, scan. Element from the
   *  main tree (all providers). Message-relative positions (row 0 = el
   *  top). Works for any height — closes the tall-message gap. */
  scanElement?: (el: DOMElement) => MatchPosition[]
  /** Position-based CURRENT highlight. Positions known upfront (from
   *  scanElement), navigation = index arithmetic + scrollTo. rowOffset
   *  = message's current screen-top; positions stay stable. */
  setPositions?: (
    state: {
      positions: MatchPosition[]
      rowOffset: number
      currentIdx: number
    } | null,
  ) => void
}

/**
 * Returns the text of a real user prompt, or null for anything else.
 * "Real" = what the human typed: not tool results, not XML-wrapped payloads
 * (<bash-stdout>, <command-message>, <teammate-message>, etc.), not meta.
 *
 * Two shapes land here: NormalizedUserMessage (normal prompts) and
 * AttachmentMessage with type==='queued_command' (prompts sent mid-turn
 * while a tool was executing — they get drained as attachments on the
 * next turn, see query.ts:1410). Both render as ❯-prefixed UserTextMessage
 * in the UI so both should stick.
 *
 * Leading <system-reminder> blocks are stripped before checking — they get
 * prepended to the stored text for Claude's context (memory updates, auto
 * mode reminders) but aren't what the user typed. Without stripping, any
 * prompt that happened to get a reminder is rejected by the startsWith('<')
 * check. Shows up on `cc -c` resumes where memory-update reminders are dense.
 */
const promptTextCache = new WeakMap<RenderableMessage, string | null>()

function stickyPromptText(msg: RenderableMessage): string | null {
  // Cache keyed on message object — messages are append-only and don't
  // mutate, so a WeakMap hit is always valid. The walk (StickyTracker,
  // per-scroll-tick) calls this 5-50+ times with the SAME messages every
  // tick; the system-reminder strip allocates a fresh string on each
  // parse. WeakMap self-GCs on compaction/clear (messages[] replaced).
  const cached = promptTextCache.get(msg)
  if (cached !== undefined) return cached
  const result = computeStickyPromptText(msg)
  promptTextCache.set(msg, result)
  return result
}

function computeStickyPromptText(msg: RenderableMessage): string | null {
  let raw: string | null = null
  if (msg.type === 'user') {
    if (msg.isMeta || msg.isVisibleInTranscriptOnly) return null
    const block = msg.message.content[0]
    if (block?.type !== 'text') return null
    raw = block.text
  } else if (
    msg.type === 'attachment' &&
    msg.attachment.type === 'queued_command' &&
    msg.attachment.commandMode !== 'task-notification' &&
    !msg.attachment.isMeta
  ) {
    const p = msg.attachment.prompt
    raw =
      typeof p === 'string'
        ? p
        : p.flatMap(b => (b.type === 'text' ? [b.text] : [])).join('\n')
  }
  if (raw === null) return null

  const t = stripSystemReminders(raw)
  if (t.startsWith('<') || t === '') return null
  return t
}

/**
 * Virtualized message list for fullscreen mode. Split from Messages.tsx so
 * useVirtualScroll is called unconditionally (rules-of-hooks) — Messages.tsx
 * conditionally renders either this or a plain .map().
 *
 * The wrapping <Box ref> is the measurement anchor — MessageRow doesn't take
 * a ref. Single-child column Box passes Yoga height through unchanged.
 */
type VirtualItemProps = {
  itemKey: string
  msg: RenderableMessage
  idx: number
  measureRef: (key: string) => (el: DOMElement | null) => void
  expanded: boolean | undefined
  hovered: boolean
  clickable: boolean
  onClickK: (msg: RenderableMessage, cellIsBlank: boolean) => void
  onEnterK: (k: string) => void
  onLeaveK: (k: string) => void
  renderItem: (msg: RenderableMessage, idx: number) => React.ReactNode
}

// Item wrapper with stable click handlers. The per-item closures were the
// `operationNewArrowFunction` leafs → `FunctionExecutable::finalizeUnconditionally`
// GC cleanup (16% of GC time during fast scroll). 3 closures × 60 mounted ×
// 10 commits/sec = 1800 closures/sec. With stable onClickK/onEnterK/onLeaveK
// threaded via itemKey, the closures here are per-item-per-render but CHEAP
// (just wrap the stable callback with k bound) and don't close over msg/idx
// which lets JIT inline them. The bigger win is inside: MessageRow.memo
// bails for unchanged msgs, skipping marked.lexer + formatToken.
//
// NOT React.memo'd — renderItem captures changing state (cursor, selectedIdx,
// verbose). Memoing with a comparator that ignores renderItem would use a
// STALE closure on bail (wrong selection highlight, stale verbose). Including
// renderItem in the comparator defeats memo since it's fresh each render.
function VirtualItem({
  itemKey: k,
  msg,
  idx,
  measureRef,
  expanded,
  hovered,
  clickable,
  onClickK,
  onEnterK,
  onLeaveK,
  renderItem,
}: VirtualItemProps): React.ReactNode {
  return (
    <Box
      ref={measureRef(k)}
      flexDirection="column"
      backgroundColor={expanded ? 'userMessageBackgroundHover' : undefined}
      // bg here masks useVirtualScroll's one-frame offset lag on expand —
      // don't move to the margined Box inside. paddingBottom mirrors the
      // tinted marginTop.
      paddingBottom={expanded ? 1 : undefined}
      onClick={clickable ? e => onClickK(msg, e.cellIsBlank) : undefined}
      onMouseEnter={clickable ? () => onEnterK(k) : undefined}
      onMouseLeave={clickable ? () => onLeaveK(k) : undefined}
    >
      <TextHoverColorContext.Provider
        value={hovered && !expanded ? 'text' : undefined}
      >
        {renderItem(msg, idx)}
      </TextHoverColorContext.Provider>
    </Box>
  )
}

export function VirtualMessageList({
  messages,
  scrollRef,
  columns,
  itemKey,
  renderItem,
  onItemClick,
  isItemClickable,
  isItemExpanded,
  extractSearchText = defaultExtractSearchText,
  trackStickyPrompt,
  selectedIndex,
  cursorNavRef,
  setCursor,
  jumpRef,
  onSearchMatchesChange,
  scanElement,
  setPositions,
}: Props): React.ReactNode {
  // Incremental key array. Streaming appends one message at a time; rebuilding
  // the full string array on every commit allocates O(n) per message (~1MB
  // churn at 27k messages). Append-only delta push when the prefix matches;
  // fall back to full rebuild on compaction, /clear, or itemKey change.
  const keysRef = useRef<string[]>([])
  const prevMessagesRef = useRef<typeof messages>(messages)
  const prevItemKeyRef = useRef(itemKey)
  if (
    prevItemKeyRef.current !== itemKey ||
    messages.length < keysRef.current.length ||
    messages[0] !== prevMessagesRef.current[0]
  ) {
    keysRef.current = messages.map(m => itemKey(m))
  } else {
    for (let i = keysRef.current.length; i < messages.length; i++) {
      keysRef.current.push(itemKey(messages[i]!))
    }
  }
  prevMessagesRef.current = messages
  prevItemKeyRef.current = itemKey
  const keys = keysRef.current
  const {
    range,
    topSpacer,
    bottomSpacer,
    measureRef,
    spacerRef,
    offsets,
    getItemTop,
    getItemElement,
    getItemHeight,
    scrollToIndex,
  } = useVirtualScroll(scrollRef, keys, columns)
  const [start, end] = range

  // Unmeasured (undefined height) falls through — assume visible.
  const isVisible = useCallback(
    (i: number) => {
      const h = getItemHeight(i)
      if (h === 0) return false
      return isNavigableMessage(messages[i]!)
    },
    [getItemHeight, messages],
  )
  useImperativeHandle(cursorNavRef, (): MessageActionsNav => {
    const select = (m: NavigableMessage) =>
      setCursor?.({
        uuid: m.uuid,
        msgType: m.type,
        expanded: false,
        toolName: toolCallOf(m)?.name,
      })
    const selIdx = selectedIndex ?? -1
    const scan = (
      from: number,
      dir: 1 | -1,
      pred: (i: number) => boolean = isVisible,
    ) => {
      for (let i = from; i >= 0 && i < messages.length; i += dir) {
        if (pred(i)) {
          select(messages[i]!)
          return true
        }
      }
      return false
    }
    const isUser = (i: number) => isVisible(i) && messages[i]!.type === 'user'
    return {
      // Entry via shift+↑ = same semantic as in-cursor shift+↑ (prevUser).
      enterCursor: () => scan(messages.length - 1, -1, isUser),
      navigatePrev: () => scan(selIdx - 1, -1),
      navigateNext: () => {
        if (scan(selIdx + 1, 1)) return
        // Past last visible → exit + repin. Last message's TOP is at viewport
        // top (selection-scroll effect); its BOTTOM may be below the fold.
        scrollRef.current?.scrollToBottom()
        setCursor?.(null)
      },
      // type:'user' only — queued_command attachments look like prompts but have no raw UserMessage to rewind to.
      navigatePrevUser: () => scan(selIdx - 1, -1, isUser),
      navigateNextUser: () => scan(selIdx + 1, 1, isUser),
      navigateTop: () => scan(0, 1),
      navigateBottom: () => scan(messages.length - 1, -1),
      getSelected: () => (selIdx >= 0 ? (messages[selIdx] ?? null) : null),
    }
  }, [messages, selectedIndex, setCursor, isVisible])
  // Two-phase jump + search engine. Read-through-ref so the handle stays
  // stable across renders — offsets/messages identity changes every render,
  // can't go in useImperativeHandle deps without recreating the handle.
  const jumpState = useRef({
    offsets,
    start,
    getItemElement,
    getItemTop,
    messages,
    scrollToIndex,
  })
  jumpState.current = {
    offsets,
    start,
    getItemElement,
    getItemTop,
    messages,
    scrollToIndex,
  }

  // Keep cursor-selected message visible. offsets rebuilds every render
  // — as a bare dep this re-pinned on every mousewheel tick. Read through
  // jumpState instead; past-overscan jumps land via scrollToIndex, next
  // nav is precise.
  useEffect(() => {
    if (selectedIndex === undefined) return
    const s = jumpState.current
    const el = s.getItemElement(selectedIndex)
    if (el) {
      scrollRef.current?.scrollToElement(el, 1)
    } else {
      s.scrollToIndex(selectedIndex)
    }
  }, [selectedIndex, scrollRef])

  // Pending seek request. jump() sets this + bumps seekGen. The seek
  // effect fires post-paint (passive effect — after resetAfterCommit),
  // checks if target is mounted. Yes → scan+highlight. No → re-estimate
  // with a fresher anchor (start moved toward idx) and scrollTo again.
  const scanRequestRef = useRef<{
    idx: number
    wantLast: boolean
    tries: number
  } | null>(null)
  // Message-relative positions from scanElement. Row 0 = message top.
  // Stable across scroll — highlight computes rowOffset fresh. msgIdx
  // for computing rowOffset = getItemTop(msgIdx) - scrollTop.
  const elementPositions = useRef<{
    msgIdx: number
    positions: MatchPosition[]
  }>({ msgIdx: -1, positions: [] })
  // Wraparound guard. Auto-advance stops if ptr wraps back to here.
  const startPtrRef = useRef(-1)
  // Phantom-burst cap. Resets on scan success.
  const phantomBurstRef = useRef(0)
  // One-deep queue: n/N arriving mid-seek gets stored (not dropped) and
  // fires after the seek completes. Holding n stays smooth without
  // queueing 30 jumps. Latest press overwrites — we want the direction
  // the user is going NOW, not where they were 10 keypresses ago.
  const pendingStepRef = useRef<1 | -1 | 0>(0)
  // step + highlight via ref so the seek effect reads latest without
  // closure-capture or deps churn.
  const stepRef = useRef<(d: 1 | -1) => void>(() => {})
  const highlightRef = useRef<(ord: number) => void>(() => {})
  const searchState = useRef({
    matches: [] as number[], // deduplicated msg indices
    ptr: 0,
    screenOrd: 0,
    // Cumulative engine-occurrence count before each matches[k]. Lets us
    // compute a global current index: prefixSum[ptr] + screenOrd + 1.
    // Engine-counted (indexOf on extractSearchText), not render-counted —
    // close enough for the badge; exact counts would need scanElement on
    // every matched message (~1-3ms × N). total = prefixSum[matches.length].
    prefixSum: [] as number[],
  })
  // scrollTop at the moment / was pressed. Incsearch preview-jumps snap
  // back here when matches drop to 0. -1 = no anchor (before first /).
  const searchAnchor = useRef(-1)
  const indexWarmed = useRef(false)

  // Scroll target for message i: land at MESSAGE TOP. est = top - HEADROOM
  // so lo = top - est = HEADROOM ≥ 0 (or lo = top if est clamped to 0).
  // Post-clamp read-back in jump() handles the scrollHeight boundary.
  // No frac (render transform didn't respect it), no monotone clamp
  // (was a safety net for frac garbage — without frac, est IS the next
  // message's top, spam-n/N converges because message tops are ordered).
  function targetFor(i: number): number {
    const top = jumpState.current.getItemTop(i)
    return Math.max(0, top - HEADROOM)
  }

  // Highlight positions[ord]. Positions are MESSAGE-RELATIVE (row 0 =
  // element top, from scanElement). Compute rowOffset = getItemTop -
  // scrollTop fresh. If ord's position is off-viewport, scroll to bring
  // it in, recompute rowOffset. setPositions triggers overlay write.
  function highlight(ord: number): void {
    const s = scrollRef.current
    const { msgIdx, positions } = elementPositions.current
    if (!s || positions.length === 0 || msgIdx < 0) {
      setPositions?.(null)
      return
    }
    const idx = Math.max(0, Math.min(ord, positions.length - 1))
    const p = positions[idx]!
    const top = jumpState.current.getItemTop(msgIdx)
    // lo = item's position within scroll content (wrapper-relative).
    // viewportTop = where the scroll content starts on SCREEN (after
    // ScrollBox padding/border + any chrome above). Highlight writes to
    // screen-absolute, so rowOffset = viewportTop + lo. Observed: off-by-
    // 1+ without viewportTop (FullscreenLayout has paddingTop=1 on the
    // ScrollBox, plus any header above).
    const vpTop = s.getViewportTop()
    let lo = top - s.getScrollTop()
    const vp = s.getViewportHeight()
    let screenRow = vpTop + lo + p.row
    // Off viewport → scroll to bring it in (HEADROOM from top).
    // scrollTo commits sync; read-back after gives fresh lo.
    if (screenRow < vpTop || screenRow >= vpTop + vp) {
      s.scrollTo(Math.max(0, top + p.row - HEADROOM))
      lo = top - s.getScrollTop()
      screenRow = vpTop + lo + p.row
    }
    setPositions?.({ positions, rowOffset: vpTop + lo, currentIdx: idx })
    // Badge: global current = sum of occurrences before this msg + ord+1.
    // prefixSum[ptr] is engine-counted (indexOf on extractSearchText);
    // may drift from render-count for ghost messages but close enough —
    // badge is a rough location hint, not a proof.
    const st = searchState.current
    const total = st.prefixSum.at(-1) ?? 0
    const current = (st.prefixSum[st.ptr] ?? 0) + idx + 1
    onSearchMatchesChange?.(total, current)
    logForDebugging(
      `highlight(i=${msgIdx}, ord=${idx}/${positions.length}): ` +
        `pos={row:${p.row},col:${p.col}} lo=${lo} screenRow=${screenRow} ` +
        `badge=${current}/${total}`,
    )
  }
  highlightRef.current = highlight

  // Seek effect. jump() sets scanRequestRef + scrollToIndex + bump.
  // bump → re-render → useVirtualScroll mounts the target (scrollToIndex
  // guarantees this — scrollTop and topSpacer agree via the same
  // offsets value) → resetAfterCommit paints → this passive effect
  // fires POST-PAINT with the element mounted. Precise scrollTo + scan.
  //
  // Dep is ONLY seekGen — effect doesn't re-run on random renders
  // (onSearchMatchesChange churn during incsearch).
  const [seekGen, setSeekGen] = useState(0)
  const bumpSeek = useCallback(() => setSeekGen(g => g + 1), [])

  useEffect(() => {
    const req = scanRequestRef.current
    if (!req) return
    const { idx, wantLast, tries } = req
    const s = scrollRef.current
    if (!s) return
    const { getItemElement, getItemTop, scrollToIndex } = jumpState.current
    const el = getItemElement(idx)
    const h = el?.yogaNode?.getComputedHeight() ?? 0

    if (!el || h === 0) {
      // Not mounted after scrollToIndex. Shouldn't happen — scrollToIndex
      // guarantees mount by construction (scrollTop and topSpacer agree
      // via the same offsets value). Sanity: retry once, then skip.
      if (tries > 1) {
        scanRequestRef.current = null
        logForDebugging(`seek(i=${idx}): no mount after scrollToIndex, skip`)
        stepRef.current(wantLast ? -1 : 1)
        return
      }
      scanRequestRef.current = { idx, wantLast, tries: tries + 1 }
      scrollToIndex(idx)
      bumpSeek()
      return
    }

    scanRequestRef.current = null
    // Precise scrollTo — scrollToIndex got us in the neighborhood
    // (item is mounted, maybe a few-dozen rows off due to overscan
    // estimate drift). Now land it at top-HEADROOM.
    s.scrollTo(Math.max(0, getItemTop(idx) - HEADROOM))
    const positions = scanElement?.(el) ?? []
    elementPositions.current = { msgIdx: idx, positions }
    logForDebugging(`seek(i=${idx} t=${tries}): ${positions.length} positions`)
    if (positions.length === 0) {
      // Phantom — engine matched, render didn't. Auto-advance.
      if (++phantomBurstRef.current > 20) {
        phantomBurstRef.current = 0
        return
      }
      stepRef.current(wantLast ? -1 : 1)
      return
    }
    phantomBurstRef.current = 0
    const ord = wantLast ? positions.length - 1 : 0
    searchState.current.screenOrd = ord
    startPtrRef.current = -1
    highlightRef.current(ord)
    const pending = pendingStepRef.current
    if (pending) {
      pendingStepRef.current = 0
      stepRef.current(pending)
    }
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [seekGen])

  // Scroll to message i's top, arm scanPending. scan-effect reads fresh
  // screen next tick. wantLast: N-into-message — screenOrd = length-1.
  function jump(i: number, wantLast: boolean): void {
    const s = scrollRef.current
    if (!s) return
    const js = jumpState.current
    const { getItemElement, scrollToIndex } = js
    // offsets is a Float64Array whose .length is the allocated buffer (only
    // grows) — messages.length is the logical item count.
    if (i < 0 || i >= js.messages.length) return
    // Clear stale highlight before scroll. Between now and the seek
    // effect's highlight, inverse-only from scan-highlight shows.
    setPositions?.(null)
    elementPositions.current = { msgIdx: -1, positions: [] }
    scanRequestRef.current = { idx: i, wantLast, tries: 0 }
    const el = getItemElement(i)
    const h = el?.yogaNode?.getComputedHeight() ?? 0
    // Mounted → precise scrollTo. Unmounted → scrollToIndex mounts it
    // (scrollTop and topSpacer agree via the same offsets value — exact
    // by construction, no estimation). Seek effect does the precise
    // scrollTo after paint either way.
    if (el && h > 0) {
      s.scrollTo(targetFor(i))
    } else {
      scrollToIndex(i)
    }
    bumpSeek()
  }

  // Advance screenOrd within elementPositions. Exhausted → ptr advances,
  // jump to next matches[ptr], re-scan. Phantom (scan found 0 after
  // jump) triggers auto-advance from scan-effect. Wraparound guard stops
  // if every message is a phantom.
  function step(delta: 1 | -1): void {
    const st = searchState.current
    const { matches, prefixSum } = st
    const total = prefixSum.at(-1) ?? 0
    if (matches.length === 0) return

    // Seek in-flight — queue this press (one-deep, latest overwrites).
    // The seek effect fires it after highlight.
    if (scanRequestRef.current) {
      pendingStepRef.current = delta
      return
    }

    if (startPtrRef.current < 0) startPtrRef.current = st.ptr

    const { positions } = elementPositions.current
    const newOrd = st.screenOrd + delta
    if (newOrd >= 0 && newOrd < positions.length) {
      st.screenOrd = newOrd
      highlight(newOrd) // updates badge internally
      startPtrRef.current = -1
      return
    }

    // Exhausted visible. Advance ptr → jump → re-scan.
    const ptr = (st.ptr + delta + matches.length) % matches.length
    if (ptr === startPtrRef.current) {
      setPositions?.(null)
      startPtrRef.current = -1
      logForDebugging(
        `step: wraparound at ptr=${ptr}, all ${matches.length} msgs phantoms`,
      )
      return
    }
    st.ptr = ptr
    st.screenOrd = 0 // resolved after scan (wantLast → length-1)
    jump(matches[ptr]!, delta < 0)
    // screenOrd will resolve after scan. Best-effort: prefixSum[ptr] + 0
    // for n (first pos), prefixSum[ptr+1] for N (last pos = count-1).
    // The scan-effect's highlight will be the real value; this is a
    // pre-scan placeholder so the badge updates immediately.
    const placeholder =
      delta < 0 ? (prefixSum[ptr + 1] ?? total) : prefixSum[ptr]! + 1
    onSearchMatchesChange?.(total, placeholder)
  }
  stepRef.current = step

  useImperativeHandle(
    jumpRef,
    () => ({
      // Non-search jump (sticky header click, etc). No scan, no positions.
      jumpToIndex: (i: number) => {
        const s = scrollRef.current
        if (s) s.scrollTo(targetFor(i))
      },
      setSearchQuery: (q: string) => {
        // New search invalidates everything.
        scanRequestRef.current = null
        elementPositions.current = { msgIdx: -1, positions: [] }
        startPtrRef.current = -1
        setPositions?.(null)
        const lq = q.toLowerCase()
        // One entry per MESSAGE (deduplicated). Boolean "does this msg
        // contain the query". ~10ms for 9k messages with cached lowered.
        const matches: number[] = []
        // Per-message occurrence count → prefixSum for global current
        // index. Engine-counted (cheap indexOf loop); may differ from
        // render-count (scanElement) for ghost/phantom messages but close
        // enough for the badge. The badge is a rough location hint.
        const prefixSum: number[] = [0]
        if (lq) {
          const msgs = jumpState.current.messages
          for (let i = 0; i < msgs.length; i++) {
            const text = extractSearchText(msgs[i]!)
            let pos = text.indexOf(lq)
            let cnt = 0
            while (pos >= 0) {
              cnt++
              pos = text.indexOf(lq, pos + lq.length)
            }
            if (cnt > 0) {
              matches.push(i)
              prefixSum.push(prefixSum.at(-1)! + cnt)
            }
          }
        }
        const total = prefixSum.at(-1)!
        // Nearest MESSAGE to the anchor. <= so ties go to later.
        let ptr = 0
        const s = scrollRef.current
        const { offsets, start, getItemTop } = jumpState.current
        const firstTop = getItemTop(start)
        const origin = firstTop >= 0 ? firstTop - offsets[start]! : 0
        if (matches.length > 0 && s) {
          const curTop =
            searchAnchor.current >= 0 ? searchAnchor.current : s.getScrollTop()
          let best = Infinity
          for (let k = 0; k < matches.length; k++) {
            const d = Math.abs(origin + offsets[matches[k]!]! - curTop)
            if (d <= best) {
              best = d
              ptr = k
            }
          }
          logForDebugging(
            `setSearchQuery('${q}'): ${matches.length} msgs · ptr=${ptr} ` +
              `msgIdx=${matches[ptr]} curTop=${curTop} origin=${origin}`,
          )
        }
        searchState.current = { matches, ptr, screenOrd: 0, prefixSum }
        if (matches.length > 0) {
          // wantLast=true: preview the LAST occurrence in the nearest
          // message. At sticky-bottom (common / entry), nearest is the
          // last msg; its last occurrence is closest to where the user
          // was — minimal view movement. n advances forward from there.
          jump(matches[ptr]!, true)
        } else if (searchAnchor.current >= 0 && s) {
          // /foob → 0 matches → snap back to anchor. less/vim incsearch.
          s.scrollTo(searchAnchor.current)
        }
        // Global occurrence count + 1-based current. wantLast=true so the
        // scan will land on the last occurrence in matches[ptr]. Placeholder
        // = prefixSum[ptr+1] (count through this msg). highlight() updates
        // to the exact value after scan completes.
        onSearchMatchesChange?.(
          total,
          matches.length > 0 ? (prefixSum[ptr + 1] ?? total) : 0,
        )
      },
      nextMatch: () => step(1),
      prevMatch: () => step(-1),
      setAnchor: () => {
        const s = scrollRef.current
        if (s) searchAnchor.current = s.getScrollTop()
      },
      disarmSearch: () => {
        // Manual scroll invalidates screen-absolute positions.
        setPositions?.(null)
        scanRequestRef.current = null
        elementPositions.current = { msgIdx: -1, positions: [] }
        startPtrRef.current = -1
      },
      warmSearchIndex: async () => {
        if (indexWarmed.current) return 0
        const msgs = jumpState.current.messages
        const CHUNK = 500
        let workMs = 0
        const wallStart = performance.now()
        for (let i = 0; i < msgs.length; i += CHUNK) {
          await sleep(0)
          const t0 = performance.now()
          const end = Math.min(i + CHUNK, msgs.length)
          for (let j = i; j < end; j++) {
            extractSearchText(msgs[j]!)
          }
          workMs += performance.now() - t0
        }
        const wallMs = Math.round(performance.now() - wallStart)
        logForDebugging(
          `warmSearchIndex: ${msgs.length} msgs · work=${Math.round(workMs)}ms wall=${wallMs}ms chunks=${Math.ceil(msgs.length / CHUNK)}`,
        )
        indexWarmed.current = true
        return Math.round(workMs)
      },
    }),
    // Closures over refs + callbacks. scrollRef stable; others are
    // useCallback([]) or prop-drilled from REPL (stable).
    // eslint-disable-next-line react-hooks/exhaustive-deps
    [scrollRef],
  )

  // StickyTracker goes AFTER the list content. It returns null (no DOM node)
  // so order shouldn't matter for layout — but putting it first means every
  // fine-grained commit from its own scroll subscription reconciles THROUGH
  // the sibling items (React walks children in order). After the items, it's
  // a leaf reconcile. Defensive: also avoids any Yoga child-index quirks if
  // the Ink reconciler ever materializes a placeholder for null returns.
  const [hoveredKey, setHoveredKey] = useState<string | null>(null)
  // Stable click/hover handlers — called with k, dispatch from a ref so
  // closure identity doesn't change per render. The per-item handler
  // closures (`e => ...`, `() => setHoveredKey(k)`) were the
  // `operationNewArrowFunction` leafs in the scroll CPU profile; their
  // cleanup was 16% of GC time (`FunctionExecutable::finalizeUnconditionally`).
  // Allocating 3 closures × 60 mounted items × 10 commits/sec during fast
  // scroll = 1800 short-lived closures/sec. With stable refs the item
  // wrapper props don't change → VirtualItem.memo bails for the ~35
  // unchanged items, only ~25 fresh items pay createElement cost.
  const handlersRef = useRef({ onItemClick, setHoveredKey })
  handlersRef.current = { onItemClick, setHoveredKey }
  const onClickK = useCallback(
    (msg: RenderableMessage, cellIsBlank: boolean) => {
      const h = handlersRef.current
      if (!cellIsBlank && h.onItemClick) h.onItemClick(msg)
    },
    [],
  )
  const onEnterK = useCallback((k: string) => {
    handlersRef.current.setHoveredKey(k)
  }, [])
  const onLeaveK = useCallback((k: string) => {
    handlersRef.current.setHoveredKey(prev => (prev === k ? null : prev))
  }, [])

  return (
    <>
      <Box ref={spacerRef} height={topSpacer} flexShrink={0} />
      {messages.slice(start, end).map((msg, i) => {
        const idx = start + i
        const k = keys[idx]!
        const clickable = !!onItemClick && (isItemClickable?.(msg) ?? true)
        const hovered = clickable && hoveredKey === k
        const expanded = isItemExpanded?.(msg)
        return (
          <VirtualItem
            key={k}
            itemKey={k}
            msg={msg}
            idx={idx}
            measureRef={measureRef}
            expanded={expanded}
            hovered={hovered}
            clickable={clickable}
            onClickK={onClickK}
            onEnterK={onEnterK}
            onLeaveK={onLeaveK}
            renderItem={renderItem}
          />
        )
      })}
      {bottomSpacer > 0 && <Box height={bottomSpacer} flexShrink={0} />}
      {trackStickyPrompt && (
        <StickyTracker
          messages={messages}
          start={start}
          end={end}
          offsets={offsets}
          getItemTop={getItemTop}
          getItemElement={getItemElement}
          scrollRef={scrollRef}
        />
      )}
    </>
  )
}

const NOOP_UNSUB = () => {}

/**
 * Effect-only child that tracks the last user-prompt scrolled above the
 * viewport top and fires onChange when it changes.
 *
 * Rendered as a separate component (not a hook in VirtualMessageList) so it
 * can subscribe to scroll at FINER granularity than SCROLL_QUANTUM=40. The
 * list needs the coarse quantum to avoid per-wheel-tick Yoga relayouts; this
 * tracker is just a walk + comparison and can afford to run every tick. When
 * it re-renders alone, the list's reconciled output is unchanged (same props
 * from the parent's last commit) — no Yoga work. Without this split, the
 * header lags by ~one conversation turn (40 rows ≈ one prompt + response).
 *
 * firstVisible derivation: item Boxes are direct Yoga children of the
 * ScrollBox content wrapper (fragments collapse in the Ink DOM), so
 * yoga.getComputedTop is content-wrapper-relative — same coordinate space as
 * scrollTop. Compare against scrollTop + pendingDelta (the scroll TARGET —
 * scrollBy only sets pendingDelta, committed scrollTop lags). Walk backward
 * from the mount-range end; break when an item's top is above target.
 */
function StickyTracker({
  messages,
  start,
  end,
  offsets,
  getItemTop,
  getItemElement,
  scrollRef,
}: {
  messages: RenderableMessage[]
  start: number
  end: number
  offsets: ArrayLike<number>
  getItemTop: (index: number) => number
  getItemElement: (index: number) => DOMElement | null
  scrollRef: RefObject<ScrollBoxHandle | null>
}): null {
  const { setStickyPrompt } = useContext(ScrollChromeContext)
  // Fine-grained subscription — snapshot is unquantized scrollTop+delta so
  // every scroll action (wheel tick, PgUp, drag) triggers a re-render of
  // THIS component only. Sticky bit folded into the sign so sticky→broken
  // also triggers (scrollToBottom sets sticky without moving scrollTop).
  const subscribe = useCallback(
    (listener: () => void) =>
      scrollRef.current?.subscribe(listener) ?? NOOP_UNSUB,
    [scrollRef],
  )
  useSyncExternalStore(subscribe, () => {
    const s = scrollRef.current
    if (!s) return NaN
    const t = s.getScrollTop() + s.getPendingDelta()
    return s.isSticky() ? -1 - t : t
  })

  // Read live scroll state on every render.
  const isSticky = scrollRef.current?.isSticky() ?? true
  const target = Math.max(
    0,
    (scrollRef.current?.getScrollTop() ?? 0) +
      (scrollRef.current?.getPendingDelta() ?? 0),
  )

  // Walk the mounted range to find the first item at-or-below the viewport
  // top. `range` is from the parent's coarse-quantum render (may be slightly
  // stale) but overscan guarantees it spans well past the viewport in both
  // directions. Items without a Yoga layout yet (newly mounted this frame)
  // are treated as at-or-below — they're somewhere in view, and assuming
  // otherwise would show a sticky for a prompt that's actually on screen.
  let firstVisible = start
  let firstVisibleTop = -1
  for (let i = end - 1; i >= start; i--) {
    const top = getItemTop(i)
    if (top >= 0) {
      if (top < target) break
      firstVisibleTop = top
    }
    firstVisible = i
  }

  let idx = -1
  let text: string | null = null
  if (firstVisible > 0 && !isSticky) {
    for (let i = firstVisible - 1; i >= 0; i--) {
      const t = stickyPromptText(messages[i]!)
      if (t === null) continue
      // The prompt's wrapping Box top is above target (that's why it's in
      // the [0, firstVisible) range), but its ❯ is at top+1 (marginTop=1).
      // If the ❯ is at-or-below target, it's VISIBLE at viewport top —
      // showing the same text in the header would duplicate it. Happens
      // in the 1-row gap between Box top scrolling past and ❯ scrolling
      // past. Skip to the next-older prompt (its ❯ is definitely above).
      const top = getItemTop(i)
      if (top >= 0 && top + 1 >= target) continue
      idx = i
      text = t
      break
    }
  }

  const baseOffset =
    firstVisibleTop >= 0 ? firstVisibleTop - offsets[firstVisible]! : 0
  const estimate = idx >= 0 ? Math.max(0, baseOffset + offsets[idx]!) : -1

  // For click-jumps to items not yet mounted (user scrolled far past,
  // prompt is in the topSpacer). Click handler scrolls to the estimate
  // to mount it; this anchors by element once it appears. scrollToElement
  // defers the Yoga-position read to render time (render-node-to-output
  // reads el.yogaNode.getComputedTop() in the SAME calculateLayout pass
  // that produces scrollHeight) — no throttle race. Cap retries: a /clear
  // race could unmount the item mid-sequence.
  const pending = useRef({ idx: -1, tries: 0 })
  // Suppression state machine. The click handler arms; the onChange effect
  // consumes (armed→force) then fires-and-clears on the render AFTER that
  // (force→none). The force step poisons the dedup: after click, idx often
  // recomputes to the SAME prompt (its top is still above target), so
  // without force the last.idx===idx guard would hold 'clicked' until the
  // user crossed a prompt boundary. Previously encoded in last.idx as
  // -1/-2/-3 which overlapped with real indices — too clever.
  type Suppress = 'none' | 'armed' | 'force'
  const suppress = useRef<Suppress>('none')
  // Dedup on idx only — estimate derives from firstVisibleTop which shifts
  // every scroll tick, so including it in the key made the guard dead
  // (setStickyPrompt fired a fresh {text,scrollTo} per-frame). The scrollTo
  // closure still captures the current estimate; it just doesn't need to
  // re-fire when only estimate moved.
  const lastIdx = useRef(-1)

  // setStickyPrompt effect FIRST — must see pending.idx before the
  // correction effect below clears it. On the estimate-fallback path, the
  // render that mounts the item is ALSO the render where correction clears
  // pending; if this ran second, the pending gate would be dead and
  // setStickyPrompt(prevPrompt) would fire mid-jump, re-mounting the
  // header over 'clicked'.
  useEffect(() => {
    // Hold while two-phase correction is in flight.
    if (pending.current.idx >= 0) return
    if (suppress.current === 'armed') {
      suppress.current = 'force'
      return
    }
    const force = suppress.current === 'force'
    suppress.current = 'none'
    if (!force && lastIdx.current === idx) return
    lastIdx.current = idx
    if (text === null) {
      setStickyPrompt(null)
      return
    }
    // First paragraph only (split on blank line) — a prompt like
    // "still seeing bugs:\n\n1. foo\n2. bar" previews as just the
    // lead-in. trimStart so a leading blank line (queued_command mid-
    // turn messages sometimes have one) doesn't find paraEnd at 0.
    const trimmed = text.trimStart()
    const paraEnd = trimmed.search(/\n\s*\n/)
    const collapsed = (paraEnd >= 0 ? trimmed.slice(0, paraEnd) : trimmed)
      .slice(0, STICKY_TEXT_CAP)
      .replace(/\s+/g, ' ')
      .trim()
    if (collapsed === '') {
      setStickyPrompt(null)
      return
    }
    const capturedIdx = idx
    const capturedEstimate = estimate
    setStickyPrompt({
      text: collapsed,
      scrollTo: () => {
        // Hide header, keep padding collapsed — FullscreenLayout's
        // 'clicked' sentinel → scrollBox_y=0 + pad=0 → viewportTop=0.
        setStickyPrompt('clicked')
        suppress.current = 'armed'
        // scrollToElement anchors by DOMElement ref, not a number:
        // render-node-to-output reads el.yogaNode.getComputedTop() at
        // paint time (same Yoga pass as scrollHeight). No staleness from
        // the throttled render — the ref is stable, the position read is
        // deferred. offset=1 = UserPromptMessage marginTop.
        const el = getItemElement(capturedIdx)
        if (el) {
          scrollRef.current?.scrollToElement(el, 1)
        } else {
          // Not mounted (scrolled far past — in topSpacer). Jump to
          // estimate to mount it; correction effect re-anchors once it
          // appears. Estimate is DEFAULT_ESTIMATE-based — lands short.
          scrollRef.current?.scrollTo(capturedEstimate)
          pending.current = { idx: capturedIdx, tries: 0 }
        }
      },
    })
    // No deps — must run every render. Suppression state lives in a ref
    // (not idx/estimate), so a deps-gated effect would never see it tick.
    // Body's own guards short-circuit when nothing changed.
    // eslint-disable-next-line react-hooks/exhaustive-deps
  })

  // Correction: for click-jumps to unmounted items. Click handler scrolled
  // to the estimate; this re-anchors by element once the item appears.
  // scrollToElement defers the Yoga read to paint time — deterministic.
  // SECOND so it clears pending AFTER the onChange gate above has seen it.
  useEffect(() => {
    if (pending.current.idx < 0) return
    const el = getItemElement(pending.current.idx)
    if (el) {
      scrollRef.current?.scrollToElement(el, 1)
      pending.current = { idx: -1, tries: 0 }
    } else if (++pending.current.tries > 5) {
      pending.current = { idx: -1, tries: 0 }
    }
  })

  return null
}