Port gemini-cli's VirtualizedList + ScrollableList to stock ink 7,
adapting for ink 7's available primitives:

- `overflowY="hidden"` + `marginTop={-scrollTop}` instead of ink-fork's
  `overflowY="scroll"` (ink 7 has proper clip/unclip in render-node-to-output)
- `useBoxMetrics` inside each VirtualizedListItem (Option A) instead of a
  single ResizeObserver WeakMap; reports height changes via onHeightChange
  callback so the parent can update its heights record
- Custom `StaticRender` as `React.memo` with a reference-equality comparator,
  keyed on `itemKey-static-{width}` to freeze completed conversation items
- Character scrollbar column (`│` track / `█` thumb) since ink 7 has no
  native scrollbar prop
- No ScrollProvider / mouse drag (deferred to a follow-up PR)

Wire into MainContent.tsx behind `ui.useTerminalBuffer` setting (Settings
dialog → UI → Virtualized History; default false — opt-in).

Key bindings: Shift+↑/↓ (line), PgUp/PgDn (page), Ctrl+Home/End (top/bottom).

Re-render optimisations:
- renderItem wrapped in useCallback so renderedItems useMemo only recomputes
  when actual deps change (not on every streaming tick)
- Completed history items passed by original object reference so
  VirtualHistoryItem = memo(HistoryItemDisplay) can bail out on stable props
- estimatedItemHeight / keyExtractor / isStaticItem defined as module-level
  constants with no closure deps

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… settings

- keyMatchers.test.ts: 6 new test cases for SCROLL_UP/DOWN, PAGE_UP/DOWN,
  SCROLL_HOME/END commands (41 tests total)
- settingsSchema.test.ts: assert ui.useTerminalBuffer is boolean, default false,
  showInDialog true, requiresRestart false

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In VP mode, pending items are rendered inside VirtualizedList's
overflowY="hidden" container, which uses ink 7's native clipping
as the viewport guard. Remove the availableTerminalHeight JS-
truncation bound from pending items in renderVirtualItem:

- JS truncation at terminal height would silently cut off content
  the user could scroll to read within the virtual viewport.
- ink 7 overflowY="hidden" on the VirtualizedList container is the
  correct clip guard — no JS line-counting workaround needed.
- Remove uiState.constrainHeight from renderVirtualItem deps (no
  longer referenced in the VP rendering path).

The legacy <Static> path is unchanged.

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Replace linear findLastIndex / findIndex scans on the offsets array with
upperBound. Offsets are monotonic by construction, so the lookups inside
the render body and getAnchorForScrollTop drop from O(n) to O(log n).
Material for thousand-turn sessions where the lookup runs on every frame.

Two audit-found bugs in the VP path:

1. `<ShowMoreLines>` was outside the `<OverflowProvider>` that wraps
   `<ScrollableList>` in VP mode. `useOverflowState()` returns
   `undefined` outside the provider, so the component returned `null`
   and the "press ctrl-s to show more lines" affordance silently
   disappeared. Move `<ShowMoreLines>` inside the provider so the hook
   sees the live overflow state, matching the legacy path.

2. `refreshStatic()` and `repaintStaticViewport()` wrote
   `clearTerminal` / `cursorTo+eraseDown` to the host terminal
   unconditionally. In VP mode the React tree owns the visible region
   via ink 7's native `overflowY="hidden"` clipping — the physical
   write is a wasted flash on Ctrl+O / Alt+M / model change / resize.
   Guard both writes on `useTerminalBuffer === false`. The
   `historyRemountKey` bump still fires so the legacy `<Static>`
   fallback would still remount if someone toggled the setting mid-
   session.

Extends the targeted-repaint pattern introduced in #3967 to all
refreshStatic call sites, gated by the VP setting instead of by event
type.

Three audit-found regressions tightened, in order of severity:

1. **Source-copy index offsets missing in VP** — legacy `<Static>` path
   threads per-item `sourceCopyIndexOffsets` so `/copy mermaid N` /
   `/copy latex N` hints stay stable across continuation messages. VP
   `renderVirtualItem` was not passing this prop, so the copy hints
   shown under each diagram drifted on every `gemini_content` chunk
   (the clipboard mechanism itself still worked from raw history; only
   the displayed number was wrong). Add two lookup tables —
   identity-keyed for static items, index-keyed for pending — without
   changing the VirtualizedList data signature, and thread offsets in
   both render branches.

2. **`renderVirtualItem` callback invalidated on every streaming tick**
   — its deps included `activePtyId` / `embeddedShellFocused` /
   `isEditorDialogOpen`, all of which flip mid-stream when a shell
   tool runs or a dialog opens. Each flip rebuilt the callback,
   invalidated `VirtualizedList.renderedItems`'s useMemo, and forced
   every static item to re-render through `<StaticRender>` — defeating
   the very memoization the design relies on. Move the three pending-
   only fields into a ref read inside the callback. Static-item closure
   now depends only on inputs that legitimately affect static output
   (terminalWidth, slashCommands, getCompactLabel, …). Pending items
   still re-render correctly because their item identity changes per
   tick, so the callback is called fresh each time and reads the
   latest ref.

3. **`pending` items now honour `constrainHeight`** in VP, matching the
   legacy path. Previously VP unconditionally passed `undefined` for
   `availableTerminalHeight` on pending, relying on the viewport
   `overflowY="hidden"` clip to limit visible size — but that hid the
   `<ShowMoreLines>` affordance from the user. Now that ShowMoreLines
   is correctly wired (previous commit), restore parity.

4. **Heights map memory leak** in `VirtualizedList` — `setHeights` only
   grew. Each `/clear` left orphan `h-N` keys; each pending → completed
   transition left orphan `p-N` keys. Add a `useLayoutEffect` that
   prunes entries whose keys are not in the current `data`. Runs in
   layout phase so the prune commits in the same paint as the data
   change — no stale-offsets frame.

Completion-pass artifacts driven by the multi-agent audit:

- Settings description rewritten to enumerate the symptoms VP fixes so
  users with active flicker reports can find the toggle without reading
  the design doc.
- `absorbedCallIds` returns a module-level constant Set when compact mode
  is off, instead of a fresh `new Set()` per render. Fixes a hidden
  cascade: `activePtyId` flip mid-stream → useMemo runs → returns a new
  empty Set → `isSummaryAbsorbed` rebuilds → `renderVirtualItem`
  rebuilds → `VirtualizedList.renderedItems` recomputes → every static
  item re-renders. With the constant, the cascade dies at the source.
  Helps both VP and legacy paths.
- VP-path unit tests for MainContent (4 cases): ScrollableList mounts
  and Static does not when `useTerminalBuffer: true`; ShowMoreLines is
  reachable in VP mode (regression of the OverflowProvider mis-wrap);
  source-copy index offsets thread into renderItem for static items;
  renderItem callback identity is stable across `activePtyId` flips
  (proves the ref-based read keeps StaticRender memo effective).

…une + tighten ShowMoreLines test

Round-2 audit follow-ups. Three real findings addressed; one flagged
false positive documented separately.

1. **absorbedCallIds Set identity now content-stable when compact mode is
   on.** The earlier EMPTY constant only short-circuited the compactMode=
   false path; when compact mode is enabled (some users default-on it),
   activePtyId / embeddedShellFocused flips during streaming still
   produced fresh Sets per render even when membership was unchanged,
   restarting the same cascade the pendingStateRef fix was meant to
   avoid. Compare-and-reuse via a ref: if the new Set has identical
   membership to the previous one, return the previous reference.

2. **`heights` map prune in `VirtualizedList` is gated.** Previously
   every streaming tick rebuilt an N-key Set and walked all heights,
   even on the steady-state path where nothing changes. Now only fires
   when the heights record has clearly outpaced live data
   (`size > max(8, 2 × data.length)`) — covers `/clear` and accumulated
   pending → completed transitions, skips the 30-Hz hot path entirely.

3. **VP ShowMoreLines test now actually verifies overflow connectivity.**
   Previous mock unconditionally rendered "SHOW_MORE", so the test only
   proved the JSX mounted — it would still pass if a future refactor
   moved `<OverflowProvider>` out of the VP tree again. The mock now
   reads `useOverflowState()` and emits "OVERFLOW_DISCONNECTED" when the
   context is missing. The VP test asserts both presence of "SHOW_MORE"
   and absence of the disconnected marker, so the regression is now
   caught.

Not addressed:
- Audit P0-1 claim that `renderMode` (Alt+M) / model-change updates
  don't reach VP static items: false positive. `renderMode` is a React
  Context (`RenderModeContext`), and Context propagation traverses the
  tree past `memo` boundaries — MarkdownDisplay's `useRenderMode()`
  consumer re-renders on context change regardless of whether
  `StaticRender` bails out. Verified by reading
  `packages/cli/src/ui/contexts/RenderModeContext.tsx` and
  `MarkdownDisplay.tsx:172`. No code change.
- Audit P1-2 pendingStateRef write-during-render race: speculative,
  relies on a multi-pass render path React 18+ does not currently use.
  Documented assumption in the existing inline comment.

…ct-mode mergedHistory stability

Round-3 audit follow-ups. Three real findings; the rest verified clean.

1. **`renderItem` errors no longer crash the CLI.** Previously a throw
   inside a per-item render propagated through `VirtualizedList`'s
   useMemo into React's commit phase, tearing down the whole Ink tree —
   one bad history record could nuke the session. Wrap each call in a
   try/catch and substitute a small red `[render error] …` text box on
   failure. The row stays in the viewport so the user can scroll past
   it.

2. **Defensive height coerce in offset accumulation.** A buggy
   `estimatedItemHeight` returning NaN / negative / Infinity would
   poison every downstream offset and break the `upperBound` /
   `findLastLE` binary search (which assumes monotonic offsets). Clamp
   to `Number.isFinite(raw) && raw > 0 ? raw : 0`. No-op for the
   in-tree estimators that return 3; insurance against future
   consumers.

3. **`mergedHistory` is content-stable when compact mode is on.** The
   Round-2 absorbedCallIds stability fix didn't reach this path:
   `mergeCompactToolGroups` always allocates a fresh array, and
   `mergedHistory`'s useMemo lists `activePtyId` / `embeddedShellFocused`
   as deps, so every streaming tick mid-shell-tool produced a new array
   even when items aligned. Cascade went `mergedHistory` → offsets map
   → `renderVirtualItem` → every static item re-rendered. Pair-wise
   compare new vs previous and return the previous reference when items
   align. Restores StaticRender memo effectiveness for compact-mode
   users.

Not addressed (audit findings deemed not worth fixing in this PR):
- `scrollToItem` silently no-ops when item is not in data — no current
  caller checks the return value, low impact.
- `allVirtualItems` array spread is O(n) per streaming tick — real but
  not a crash; revisit in a perf-focused follow-up.
- `itemRefs.current` is dead surface (never read) — cosmetic.
- StrictMode-only-in-DEBUG double-invoke paths verified safe.

… coverage + cleanups

Addresses wenshao's CHANGES_REQUESTED review on PR #3941.

- Add focused unit tests for `VirtualizedList` (9 cases) covering empty
  data, `renderStatic` full-render, `initialScrollIndex` with
  `SCROLL_TO_ITEM_END`, `targetScrollIndex` anchoring, imperative
  `scrollToEnd` / `scrollToIndex`, per-item `renderItem` error isolation,
  NaN/negative estimator coercion, and out-of-range `initialScrollIndex`
  clamping.
- Add `useBatchedScroll` unit tests (4 cases) covering initial reads,
  pending-value reads in the same tick, post-commit pending reset, and
  callback identity stability across rerenders.
- Remove dead `itemRefs` / `onSetRef` plumbing (declared, written, never
  read; `useCallback` with empty deps was also a stale-closure trap).
- Remove unused `isStatic?: boolean` from `VirtualizedListProps`
  (only `isStaticItem` is actually consumed).
- Tighten the render-phase setState block: each setter is now guarded
  by an equality check so React bails out of redundant updates, and a
  comment documents that this is the React-endorsed "adjusting state
  while rendering" pattern (the synchronous update avoids a one-frame
  flash at the previous position when `targetScrollIndex` changes).

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…wup)

Declared and written in a `useLayoutEffect` on every `data` change but
never read anywhere in the component. Flagged in wenshao's round-4 review
of PR #3941.

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…on-ink7

# Conflicts:
#	packages/cli/src/ui/components/MainContent.test.tsx