diff --git a/ui-opentui/src/logic/window.ts b/ui-opentui/src/logic/window.ts
new file mode 100644
index 00000000000..3a9056b969d
--- /dev/null
+++ b/ui-opentui/src/logic/window.ts
@@ -0,0 +1,166 @@
+/**
+ * window — pure transcript-windowing math (slice S1 of docs/plans/
+ * opentui-transcript-windowing.md, issue #27). The view (view/transcript.tsx)
+ * replaces out-of-window rows with EXACT-HEIGHT empty boxes (1 yoga node, no
+ * text buffers / native handles), so the mounted set stays ~3 viewports of
+ * rows regardless of transcript length. This module is the testable core:
+ *
+ *  - `computeWindow` — which row keys must be mounted for a given scrollTop:
+ *    rows intersecting [scrollTop − margin, scrollTop + viewport + margin)
+ *    over CUMULATIVE row heights (exact recorded heights; a line-count
+ *    estimate stands in for never-measured rows), plus the never-window rows
+ *    (streaming/live) and the bottom K rows (sticky-bottom region).
+ *  - `shouldRecompute` — the hysteresis gate (≥ ¼ viewport via
+ *    `hysteresisFor`): a computed window only changes once scrollTop has
+ *    moved ≥ hysteresis from the anchor it was computed at, so swaps don't
+ *    thrash at window edges.
+ *  - `correctionIsLegal` — the jank rule for spacer-height corrections:
+ *    a correction may only touch rows fully ABOVE the viewport (the caller
+ *    compensates scrollTop in the same frame — automatic when bottom-anchored
+ *    via the sticky pin) or fully BELOW it (invisible by definition). Anything
+ *    intersecting the viewport would visibly move content: forbidden.
+ *  - `estimateMessageHeight` — the cheap line-count estimate for rows that
+ *    have never been measured (resume history above the viewport). S1 never
+ *    corrects a wrong estimate in place — it is fixed by remount (scrolling
+ *    near) or by the S2 lazy measure pass, both governed by the jank rule.
+ */
+import type { Message, Part } from './store.ts'
+
+/** One transcript row as the window calc sees it. */
+export interface WindowRow<K> {
+  readonly key: K
+  /** Exact recorded height (the row wrapper's last onSizeChange measurement,
+   *  margins included) — or null when the row has never been measured. */
+  readonly height: number | null
+  /** Line-count estimate used while `height` is null (see estimateMessageHeight). */
+  readonly estimate?: number
+  /** Always mounted regardless of the window (streaming/live rows — a remount
+   *  would restart native markdown streaming). */
+  readonly neverWindow: boolean
+}
+
+export interface WindowParams<K> {
+  readonly rows: readonly WindowRow<K>[]
+  readonly scrollTop: number
+  readonly viewportHeight: number
+  /** Mounted band kept above/below the viewport (design: 1 viewport each side). */
+  readonly margin: number
+  /** Stand-in height for null-height rows without their own estimate. */
+  readonly fallbackHeight?: number
+  /** The bottom K rows are always mounted (sticky-bottom region). */
+  readonly bottomK?: number
+}
+
+export interface WindowResult<K> {
+  /** Row keys that must be mounted; everything else renders as a spacer. */
+  readonly mounted: ReadonlySet<K>
+  /** The scrollTop this window was computed at — the next hysteresis anchor. */
+  readonly anchor: number
+}
+
+/** Default stand-in for a null-height row with no estimate (≈ a short row). */
+export const DEFAULT_FALLBACK_HEIGHT = 2
+
+/** Ceiling on a single row's line-count estimate — a pathological wall of text
+ *  must not make the never-mounted region look kilometers tall. */
+const ESTIMATE_MAX_LINES = 500
+
+/** Hysteresis for the window recompute: ≥ ¼ viewport (design rule), never 0. */
+export function hysteresisFor(viewportHeight: number): number {
+  return Math.max(1, Math.ceil(viewportHeight / 4))
+}
+
+/** Whether scrollTop has moved far enough from the last computation anchor to
+ *  justify a new window (no anchor yet → always). */
+export function shouldRecompute(scrollTop: number, anchor: number | null, hysteresis: number): boolean {
+  if (anchor === null) return true
+  return Math.abs(scrollTop - anchor) >= hysteresis
+}
+
+/** Compute the set of row keys that must be mounted for this scroll position. */
+export function computeWindow<K>(params: WindowParams<K>): WindowResult<K> {
+  const fallback = params.fallbackHeight ?? DEFAULT_FALLBACK_HEIGHT
+  const bottomK = params.bottomK ?? 0
+  const windowStart = params.scrollTop - params.margin
+  const windowEnd = params.scrollTop + params.viewportHeight + params.margin
+  const total = params.rows.length
+  const mounted = new Set<K>()
+  let top = 0
+  let index = 0
+  for (const r of params.rows) {
+    const height = r.height ?? r.estimate ?? fallback
+    const bottom = top + height
+    // half-open intersection: a row merely touching a window edge stays out.
+    const intersects = bottom > windowStart && top < windowEnd
+    if (intersects || r.neverWindow || index >= total - bottomK) mounted.add(r.key)
+    top = bottom
+    index++
+  }
+  return { mounted, anchor: params.scrollTop }
+}
+
+/**
+ * The jank rule: may a spacer-height correction for the row spanning
+ * [rowTop, rowBottom) be applied at this scroll position without visibly
+ * moving content?
+ *
+ *  - Fully BELOW the viewport → legal (invisible by definition).
+ *  - Fully ABOVE the viewport → legal, PROVIDED the caller compensates
+ *    scrollTop by the height delta in the same frame. When `atBottom`
+ *    (sticky-bottom pinned) the pin performs that compensation automatically
+ *    (bottom-anchored ⇒ zero visual movement); legality is the same either
+ *    way — the flag documents which side owes the compensation.
+ *  - Intersecting the viewport → forbidden; defer until the row scrolls out
+ *    or is remounted for view.
+ */
+export function correctionIsLegal(
+  rowTop: number,
+  rowBottom: number,
+  scrollTop: number,
+  viewportHeight: number,
+  _atBottom: boolean
+): boolean {
+  if (rowTop >= scrollTop + viewportHeight) return true // fully below the viewport
+  if (rowBottom <= scrollTop) return true // fully above — compensate scrollTop in the same frame
+  return false
+}
+
+/** Rendered line count of a text block (1-based; empty text still occupies a row). */
+function lineCount(text: string): number {
+  if (!text) return 1
+  let lines = 1
+  for (let i = 0; i < text.length; i++) if (text.charCodeAt(i) === 10) lines++
+  return lines
+}
+
+/** Estimated rendered lines of one part: text → its line count (view strips
+ *  leading/trailing blanks — mirror that); tool/reasoning → 1 collapsed
+ *  header line (the default render for settled, never-mounted history). */
+function partLines(part: Part): number {
+  if (part.type === 'text') return lineCount(part.text.replace(/^\n+|\n+$/g, ''))
+  return 1 // collapsed tool/reasoning header line
+}
+
+/**
+ * Cheap line-count height estimate for a row that has never been measured
+ * (S1: resume history above the viewport). Deliberately ignores soft wrapping
+ * — it is a placeholder until the row is actually mounted/measured, and a
+ * wrong value may only be corrected per `correctionIsLegal` (or left until
+ * remount). `spacing` is the row's turnSpacing margins; `gap` the inter-part
+ * blank line (0 in /compact).
+ */
+export function estimateMessageHeight(
+  message: Pick<Message, 'text' | 'parts'>,
+  spacing: { readonly top: number; readonly bottom: number },
+  gap: number
+): number {
+  const parts = message.parts
+  let content: number
+  if (parts && parts.length > 0) {
+    content = gap * (parts.length - 1)
+    for (const part of parts) content += partLines(part)
+  } else {
+    content = lineCount(message.text)
+  }
+  return Math.min(ESTIMATE_MAX_LINES, Math.max(1, content)) + spacing.top + spacing.bottom
+}
diff --git a/ui-opentui/src/test/transcriptWindow.test.tsx b/ui-opentui/src/test/transcriptWindow.test.tsx
new file mode 100644
index 00000000000..d2240e10be0
--- /dev/null
+++ b/ui-opentui/src/test/transcriptWindow.test.tsx
@@ -0,0 +1,126 @@
+/**
+ * Transcript windowing S1 — headless integration (view/transcript.tsx +
+ * logic/window.ts behind HERMES_TUI_WINDOWING). Proves, against the REAL
+ * renderer tree:
+ *   - out-of-window rows are actually UNMOUNTED (far fewer live renderables
+ *     than the unwindowed tree — the spacer is 1 box, the row was ~3+ texts),
+ *   - spacers are EXACT-height (total scrollHeight identical ON vs OFF — the
+ *     zero-jank invariant),
+ *   - scrolling far away REMOUNTS spaced-out rows (content paints again),
+ *   - the flag OFF renders the legacy tree (no wrappers, everything mounted).
+ */
+import { ScrollBoxRenderable, type Renderable } from '@opentui/core'
+import { useRenderer } from '@opentui/solid'
+import { afterEach, describe, expect, test } from 'vitest'
+
+import { createSessionStore } from '../logic/store.ts'
+import { ThemeProvider } from '../view/theme.tsx'
+import { Transcript } from '../view/transcript.tsx'
+import { renderProbe, type RenderProbe } from './lib/render.ts'
+
+type Store = ReturnType<typeof createSessionStore>
+
+const ENV_KEY = 'HERMES_TUI_WINDOWING'
+const envBefore = process.env[ENV_KEY]
+afterEach(() => {
+  if (envBefore === undefined) delete process.env[ENV_KEY]
+  else process.env[ENV_KEY] = envBefore
+})
+
+/** Seed `n` settled one-line system rows (flat text — no async markdown). */
+function seedRows(n: number): Store {
+  const store = createSessionStore()
+  store.apply({ type: 'gateway.ready' })
+  for (let i = 0; i < n; i++) store.pushSystem(`row-${i} marker`)
+  return store
+}
+
+function walk(node: Renderable, visit: (n: Renderable) => void): void {
+  visit(node)
+  for (const child of node.getChildren()) walk(child, visit)
+}
+
+interface Mounted {
+  probe: RenderProbe
+  count: () => number
+  scrollbox: () => ScrollBoxRenderable
+}
+
+async function mountTranscript(store: Store, windowing: '1' | '0'): Promise<Mounted> {
+  process.env[ENV_KEY] = windowing
+  let root: Renderable | undefined
+  function Grab() {
+    root = useRenderer().root
+    return null
+  }
+  const probe = await renderProbe(
+    () => (
+      <ThemeProvider theme={() => store.state.theme}>
+        <Grab />
+        <Transcript store={store} />
+      </ThemeProvider>
+    ),
+    { width: 50, height: 12 }
+  )
+  // several passes: layout (heights recorded) → frame tick (window computed)
+  // → swap render — the driver is the renderer frame callback.
+  for (let i = 0; i < 6; i++) await probe.settle()
+  const count = () => {
+    let n = 0
+    if (root) walk(root, () => n++)
+    return n
+  }
+  const scrollbox = () => {
+    let sb: ScrollBoxRenderable | undefined
+    if (root) {
+      walk(root, node => {
+        if (node instanceof ScrollBoxRenderable) sb ??= node
+      })
+    }
+    if (!sb) throw new Error('no scrollbox in the mounted tree')
+    return sb
+  }
+  return { probe, count, scrollbox }
+}
+
+const ROWS = 120
+
+describe('transcript windowing (HERMES_TUI_WINDOWING) — S1 machinery', () => {
+  test('out-of-window rows unmount into exact-height spacers; OFF keeps the full tree', async () => {
+    const on = await mountTranscript(seedRows(ROWS), '1')
+    const off = await mountTranscript(seedRows(ROWS), '0')
+    try {
+      // sticky-bottom: both variants sit pinned at the bottom showing the tail.
+      expect(on.probe.frame()).toContain(`row-${ROWS - 1} marker`)
+      expect(off.probe.frame()).toContain(`row-${ROWS - 1} marker`)
+
+      // ZERO-JANK INVARIANT: spacers are exact-height — the windowed content
+      // is precisely as tall as the fully-mounted content.
+      expect(on.scrollbox().scrollHeight).toBe(off.scrollbox().scrollHeight)
+      expect(on.scrollbox().scrollHeight).toBeGreaterThan(100) // sanity: way past the viewport
+
+      // The window actually sheds renderables: ~viewport±margin + bottom-30
+      // stay mounted out of 120 rows; the rest are 1-box spacers. The legacy
+      // tree keeps every row's text renderables alive.
+      expect(on.count()).toBeLessThan(off.count() * 0.6)
+    } finally {
+      on.probe.destroy()
+      off.probe.destroy()
+    }
+  })
+
+  test('scrolling far from the bottom remounts spaced-out rows (and the frame paints them)', async () => {
+    const on = await mountTranscript(seedRows(ROWS), '1')
+    try {
+      const sb = on.scrollbox()
+      // row-0 is far outside the bottom window: not painted while pinned.
+      expect(on.probe.frame()).not.toContain('row-0 marker')
+      sb.scrollTo(0)
+      for (let i = 0; i < 6; i++) await on.probe.settle()
+      expect(sb.scrollTop).toBe(0)
+      expect(on.probe.frame()).toContain('row-0 marker')
+    } finally {
+      on.probe.destroy()
+    }
+  })
+})
diff --git a/ui-opentui/src/test/window.test.ts b/ui-opentui/src/test/window.test.ts
new file mode 100644
index 00000000000..a6b033a8b89
--- /dev/null
+++ b/ui-opentui/src/test/window.test.ts
@@ -0,0 +1,237 @@
+/**
+ * window.ts — pure transcript-windowing math (design: docs/plans/
+ * opentui-transcript-windowing.md, slice S1). Table-tests the window calc
+ * (viewport ± margin intersection over cumulative exact heights), the
+ * hysteresis recompute gate, the never-window / bottom-K rules, the
+ * null-height estimate fallback, and the correction-legality jank rule.
+ */
+import { describe, expect, test } from 'vitest'
+
+import type { Message } from '../logic/store.ts'
+import {
+  computeWindow,
+  correctionIsLegal,
+  estimateMessageHeight,
+  hysteresisFor,
+  shouldRecompute,
+  type WindowRow
+} from '../logic/window.ts'
+
+function row(
+  key: number,
+  height: number | null,
+  opts?: { neverWindow?: boolean; estimate?: number }
+): WindowRow<number> {
+  const base = { key, height, neverWindow: opts?.neverWindow ?? false }
+  return opts?.estimate === undefined ? base : { ...base, estimate: opts.estimate }
+}
+
+/** n rows of uniform height h, keyed 0..n-1 (row i spans [i*h, (i+1)*h)). */
+function uniform(n: number, h: number): WindowRow<number>[] {
+  return Array.from({ length: n }, (_, i) => row(i, h))
+}
+
+function mountedKeys(result: { mounted: ReadonlySet<number> }): number[] {
+  return [...result.mounted].sort((a, b) => a - b)
+}
+
+describe('hysteresisFor', () => {
+  test('≥ ¼ viewport, rounded up', () => {
+    expect(hysteresisFor(40)).toBe(10)
+    expect(hysteresisFor(5)).toBe(2)
+    expect(hysteresisFor(4)).toBe(1)
+  })
+
+  test('never below 1 row (degenerate viewports)', () => {
+    expect(hysteresisFor(0)).toBe(1)
+    expect(hysteresisFor(2)).toBe(1)
+  })
+})
+
+describe('shouldRecompute', () => {
+  test('no prior anchor → always recompute', () => {
+    expect(shouldRecompute(0, null, 10)).toBe(true)
+    expect(shouldRecompute(500, null, 10)).toBe(true)
+  })
+
+  test('movement below hysteresis → keep the current window', () => {
+    expect(shouldRecompute(108, 100, 10)).toBe(false)
+    expect(shouldRecompute(92, 100, 10)).toBe(false)
+    expect(shouldRecompute(100, 100, 10)).toBe(false)
+  })
+
+  test('movement at/above hysteresis (either direction) → recompute', () => {
+    expect(shouldRecompute(110, 100, 10)).toBe(true)
+    expect(shouldRecompute(90, 100, 10)).toBe(true)
+    expect(shouldRecompute(250, 100, 10)).toBe(true)
+  })
+})
+
+describe('computeWindow — viewport ± margin intersection', () => {
+  // 100 rows × 10 → content height 1000. Viewport 40, margin 40 (1 viewport),
+  // scrollTop 480 → window [440, 560). Row i spans [10i, 10i+10).
+  const base = { viewportHeight: 40, margin: 40, scrollTop: 480 }
+
+  test('mounts exactly the rows intersecting [scrollTop − margin, scrollTop + viewport + margin)', () => {
+    const result = computeWindow({ rows: uniform(100, 10), ...base })
+    expect(mountedKeys(result)).toEqual([44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55])
+  })
+
+  test('rows merely TOUCHING the window edge are not mounted', () => {
+    const result = computeWindow({ rows: uniform(100, 10), ...base })
+    // row 43 spans [430, 440) — its bottom touches windowStart 440: out.
+    expect(result.mounted.has(43)).toBe(false)
+    // row 56 spans [560, 570) — its top touches windowEnd 560: out.
+    expect(result.mounted.has(56)).toBe(false)
+  })
+
+  test('anchor echoes the scrollTop the window was computed at', () => {
+    expect(computeWindow({ rows: uniform(100, 10), ...base }).anchor).toBe(480)
+    expect(computeWindow({ rows: [], scrollTop: 7, viewportHeight: 40, margin: 40 }).anchor).toBe(7)
+  })
+
+  test('scrolled to the top: window clamps naturally (no negative-row weirdness)', () => {
+    const result = computeWindow({ rows: uniform(100, 10), scrollTop: 0, viewportHeight: 40, margin: 40 })
+    // window [-40, 80) → rows 0..7
+    expect(mountedKeys(result)).toEqual([0, 1, 2, 3, 4, 5, 6, 7])
+  })
+
+  test('empty transcript → empty window', () => {
+    const result = computeWindow({ rows: [], scrollTop: 0, viewportHeight: 40, margin: 40 })
+    expect(result.mounted.size).toBe(0)
+  })
+
+  test('everything fits in the window → everything mounted', () => {
+    const result = computeWindow({ rows: uniform(5, 2), scrollTop: 0, viewportHeight: 40, margin: 40 })
+    expect(mountedKeys(result)).toEqual([0, 1, 2, 3, 4])
+  })
+
+  test('works with non-numeric keys (generic)', () => {
+    const rows: WindowRow<string>[] = [
+      { key: 'a', height: 50, neverWindow: false },
+      { key: 'b', height: 50, neverWindow: false },
+      { key: 'c', height: 50, neverWindow: false }
+    ]
+    const result = computeWindow({ rows, scrollTop: 60, viewportHeight: 30, margin: 0 })
+    // window [60, 90) → only 'b' ([50, 100)) intersects
+    expect([...result.mounted]).toEqual(['b'])
+  })
+})
+
+describe('computeWindow — never-window and bottom-K rules', () => {
+  test('neverWindow rows stay mounted however far outside the window', () => {
+    const rows = uniform(100, 10)
+    rows[90] = row(90, 10, { neverWindow: true })
+    const result = computeWindow({ rows, scrollTop: 0, viewportHeight: 40, margin: 40 })
+    expect(result.mounted.has(90)).toBe(true)
+    expect(result.mounted.has(89)).toBe(false)
+  })
+
+  test('the bottom K rows are always mounted (sticky-bottom region)', () => {
+    const result = computeWindow({ rows: uniform(100, 10), scrollTop: 0, viewportHeight: 40, margin: 40, bottomK: 5 })
+    expect(mountedKeys(result)).toEqual([0, 1, 2, 3, 4, 5, 6, 7, 95, 96, 97, 98, 99])
+  })
+
+  test('bottomK larger than the transcript mounts everything', () => {
+    const result = computeWindow({ rows: uniform(10, 10), scrollTop: 0, viewportHeight: 4, margin: 0, bottomK: 50 })
+    expect(result.mounted.size).toBe(10)
+  })
+})
+
+describe('computeWindow — null heights use the estimate', () => {
+  test('a per-row estimate stands in for a never-measured height (and shifts later offsets)', () => {
+    // row 0 estimated at 100 → row 1 starts at 100; window [100, 110) hits only row 1.
+    const rows = [row(0, null, { estimate: 100 }), row(1, 10)]
+    const result = computeWindow({ rows, scrollTop: 100, viewportHeight: 10, margin: 0 })
+    expect(mountedKeys(result)).toEqual([1])
+  })
+
+  test('a recorded height wins over the estimate', () => {
+    const rows = [row(0, 10, { estimate: 100 }), row(1, 10)]
+    const result = computeWindow({ rows, scrollTop: 100, viewportHeight: 10, margin: 0 })
+    // row 0 is REALLY 10 tall → row 1 spans [10, 20): nothing in [100, 110).
+    expect(result.mounted.size).toBe(0)
+  })
+
+  test('null height with no estimate falls back to fallbackHeight', () => {
+    const rows = [row(0, null), row(1, 10)]
+    const result = computeWindow({ rows, scrollTop: 0, viewportHeight: 10, margin: 0, fallbackHeight: 100 })
+    // row 0 assumed [0, 100) → mounted; row 1 [100, 110) → out of [0, 10).
+    expect(mountedKeys(result)).toEqual([0])
+  })
+})
+
+describe('correctionIsLegal — the jank rule', () => {
+  // viewport shows [100, 140)
+  const scrollTop = 100
+  const viewportHeight = 40
+
+  test.each([true, false])('fully ABOVE the viewport is legal (compensation applies) — atBottom=%s', atBottom => {
+    expect(correctionIsLegal(20, 60, scrollTop, viewportHeight, atBottom)).toBe(true)
+    // boundary: row bottom touching the viewport top is still fully above
+    expect(correctionIsLegal(50, 100, scrollTop, viewportHeight, atBottom)).toBe(true)
+  })
+
+  test.each([true, false])('fully BELOW the viewport is legal (invisible) — atBottom=%s', atBottom => {
+    expect(correctionIsLegal(150, 170, scrollTop, viewportHeight, atBottom)).toBe(true)
+    // boundary: row top touching the viewport bottom is still fully below
+    expect(correctionIsLegal(140, 160, scrollTop, viewportHeight, atBottom)).toBe(true)
+  })
+
+  test.each([true, false])('any intersection with the viewport is FORBIDDEN — atBottom=%s', atBottom => {
+    expect(correctionIsLegal(90, 110, scrollTop, viewportHeight, atBottom)).toBe(false) // clips the top edge
+    expect(correctionIsLegal(130, 150, scrollTop, viewportHeight, atBottom)).toBe(false) // clips the bottom edge
+    expect(correctionIsLegal(110, 120, scrollTop, viewportHeight, atBottom)).toBe(false) // inside
+    expect(correctionIsLegal(90, 150, scrollTop, viewportHeight, atBottom)).toBe(false) // spans the whole viewport
+  })
+})
+
+describe('estimateMessageHeight — line-count estimate for never-mounted rows', () => {
+  const spacing = { top: 2, bottom: 1 }
+
+  test('flat row: newline count + turn spacing', () => {
+    expect(estimateMessageHeight({ text: 'hello' }, spacing, 1)).toBe(1 + 3)
+    expect(estimateMessageHeight({ text: 'a\nb\nc' }, spacing, 1)).toBe(3 + 3)
+  })
+
+  test('empty text still occupies at least one row', () => {
+    expect(estimateMessageHeight({ text: '' }, { top: 0, bottom: 0 }, 0)).toBe(1)
+  })
+
+  test('parts row: text lines + 1 per collapsed tool/reasoning + inter-part gaps', () => {
+    const message: Pick<Message, 'text' | 'parts'> = {
+      text: '',
+      parts: [
+        { type: 'text', id: 'p1', text: 'line1\nline2' },
+        { type: 'tool', id: 't1', name: 'terminal', state: 'complete' },
+        { type: 'reasoning', id: 'p2', text: 'thought\nover\nlines' }
+      ]
+    }
+    // 2 (text) + 1 (tool header) + 1 (collapsed reasoning) + 2 gaps + 3 spacing
+    expect(estimateMessageHeight(message, spacing, 1)).toBe(2 + 1 + 1 + 2 + 3)
+  })
+
+  test('text parts strip leading/trailing blank lines (the view does the same)', () => {
+    const message: Pick<Message, 'text' | 'parts'> = {
+      text: '',
+      parts: [{ type: 'text', id: 'p1', text: '\n\nhello\n' }]
+    }
+    expect(estimateMessageHeight(message, { top: 0, bottom: 0 }, 1)).toBe(1)
+  })
+
+  test('compact mode (gap 0, no margins) collapses the chrome', () => {
+    const message: Pick<Message, 'text' | 'parts'> = {
+      text: '',
+      parts: [
+        { type: 'text', id: 'p1', text: 'one' },
+        { type: 'tool', id: 't1', name: 'terminal', state: 'complete' }
+      ]
+    }
+    expect(estimateMessageHeight(message, { top: 0, bottom: 0 }, 0)).toBe(2)
+  })
+
+  test('a pathological wall of text is clamped', () => {
+    const text = Array.from({ length: 10_000 }, (_, i) => `l${i}`).join('\n')
+    expect(estimateMessageHeight({ text }, { top: 0, bottom: 0 }, 0)).toBeLessThanOrEqual(500)
+  })
+})
diff --git a/ui-opentui/src/view/transcript.tsx b/ui-opentui/src/view/transcript.tsx
index 15dbc3efee4..d837408765c 100644
--- a/ui-opentui/src/view/transcript.tsx
+++ b/ui-opentui/src/view/transcript.tsx
@@ -13,20 +13,76 @@
  *
  * A `ScrollAnchorProvider` gives collapse/expand toggles (tool/thinking) a handle
  * to hold the viewport in place so expanding doesn't yank to the bottom (#4).
+ *
+ * ── Windowing (S1 of docs/plans/opentui-transcript-windowing.md, #27) ──────
+ * Behind `HERMES_TUI_WINDOWING` (unset → ON; 0/false/no/off → OFF), each row is
+ * wrapped in a measuring box (`onSizeChange` records its exact laid-out height,
+ * margins included) and rows outside [scrollTop − viewport, scrollTop +
+ * 2·viewport) swap to an EXACT-HEIGHT empty spacer `<box height={recorded}/>`
+ * — 1 yoga node, no text buffers, no native handles — so the mounted set stays
+ * ~3 viewports of rows regardless of transcript length (the 671MB→Ink-parity
+ * memory fix). The Solid `<Show>` unmount destroys the row's renderables
+ * (@opentui/solid `_removeNode` → `destroyRecursively()` once unparented).
+ *
+ * Driver: a renderer frame callback (`setFrameCallback` — scroll always
+ * triggers a render, so every scroll movement is observed; no extra timer)
+ * compares `scrollTop` to the last computation anchor with ≥ ¼-viewport
+ * hysteresis (logic/window.ts) and publishes the mounted-key set through one
+ * signal + `createSelector`, so only rows whose mounted-ness actually flipped
+ * re-render. Never windowed: streaming rows, the last row while a turn runs,
+ * and the bottom BOTTOM_ALWAYS_MOUNTED rows (see its doc). Rows the window has
+ * never adjudicated default to MOUNTED (new live rows must paint instantly).
+ * While a mouse selection is live the window FREEZES (no swaps — a swap would
+ * destroy highlighted renderables out from under the native selection walk).
+ * S1 never corrects a spacer height in place — wrong estimates (possible only
+ * for never-measured resume history above the viewport) are fixed by remount;
+ * `correctionIsLegal` governs anything smarter (S2).
  */
-import type { ScrollBoxRenderable } from '@opentui/core'
-import { createMemo, createSignal, For, Show } from 'solid-js'
+import type { BoxRenderable, ScrollBoxRenderable } from '@opentui/core'
+import { useRenderer } from '@opentui/solid'
+import { createMemo, createSelector, createSignal, For, onCleanup, onMount, Show } from 'solid-js'
 
-import type { SessionStore } from '../logic/store.ts'
+import { envFlag } from '../logic/env.ts'
+import type { Message, SessionStore } from '../logic/store.ts'
+import { computeWindow, estimateMessageHeight, hysteresisFor, shouldRecompute } from '../logic/window.ts'
 import { DisplayProvider } from './display.tsx'
 import { HomeHint } from './homeHint.tsx'
-import { MessageLine } from './messageLine.tsx'
+import { MessageLine, turnSpacing } from './messageLine.tsx'
 import { ScrollAnchorProvider } from './scrollAnchor.tsx'
 import { useTheme } from './theme.tsx'
 
+/**
+ * The bottom K rows are ALWAYS mounted (the sticky-bottom region the user
+ * lives in; also the zone where swap turbulence would be most visible). 30 is
+ * a fixed, documented pick (the design's alternative — ceil(viewport/avg-row)
+ * — buys little: rows under the viewport+margin are mounted by the window calc
+ * anyway, so K only backstops sticky re-pins and burst appends).
+ */
+const BOTTOM_ALWAYS_MOUNTED = 30
+
+/** The published window state: which keys are mounted, and which keys the
+ *  computation has SEEN (unseen keys default to mounted — see isMounted). */
+interface WinState {
+  readonly mounted: ReadonlySet<number>
+  readonly known: ReadonlySet<number>
+}
+
+function sameSet(a: ReadonlySet<number>, b: ReadonlySet<number>): boolean {
+  if (a.size !== b.size) return false
+  for (const k of a) if (!b.has(k)) return false
+  return true
+}
+
+/** Signal equality for WinState — identical sets must not re-notify selectors. */
+function sameWinState(a: WinState | undefined, b: WinState | undefined): boolean {
+  if (!a || !b) return a === b
+  return sameSet(a.mounted, b.mounted) && sameSet(a.known, b.known)
+}
+
 export function Transcript(props: { store: SessionStore }) {
   const [scroll, setScroll] = createSignal<ScrollBoxRenderable | undefined>()
   const theme = useTheme()
+  const renderer = useRenderer()
   const dropped = () => props.store.state.dropped
   const sid = () => props.store.state.sessionId
   // The NEWEST assistant answer's index — gold is earned (design pass): only
@@ -38,6 +94,128 @@ export function Transcript(props: { store: SessionStore }) {
     }
     return -1
   })
+
+  // ── windowing state (S1) ───────────────────────────────────────────────
+  // Read once per transcript: the flag is an A/B + escape hatch, not live config.
+  const windowing = envFlag(process.env.HERMES_TUI_WINDOWING, true)
+  // Stable row keys: messages carry no id and the store relies on reference
+  // identity (<For> keys by item reference; solid-js/store proxies are cached
+  // per underlying object, so the reference is stable across reads/mutations).
+  // A WeakMap-assigned monotonic number gives the window math a primitive key
+  // without restructuring the store or mutating Message objects.
+  const rowKeys = new WeakMap<Message, number>()
+  let rowSeq = 0
+  const keyOf = (message: Message): number => {
+    let key = rowKeys.get(message)
+    if (key === undefined) {
+      key = ++rowSeq
+      rowKeys.set(message, key)
+    }
+    return key
+  }
+  // key → last exact height measured while the REAL row was mounted (the
+  // wrapper's onSizeChange value; includes the row's margins). Non-reactive:
+  // spacers read it once at swap time, the frame driver reads it per compute.
+  const heights = new Map<number, number>()
+  const [winState, setWinState] = createSignal<WinState | undefined>(undefined, { equals: sameWinState })
+  // Non-reactive mirror of the latest winState for event callbacks (onSizeChange
+  // must not subscribe; createSelector reads are for tracked scopes).
+  let liveWin: WinState | undefined
+  // Per-row mounted-ness: only rows whose answer FLIPPED re-run their <Show>.
+  // Keys the window has never adjudicated (no state yet / not in `known`, e.g.
+  // a message appended since the last compute) default to MOUNTED.
+  const isMounted = createSelector(winState, (key: number, s: WinState | undefined) => {
+    return !s || !s.known.has(key) || s.mounted.has(key)
+  })
+  const estimateFor = (message: Message): number => {
+    const compact = props.store.state.compact
+    return estimateMessageHeight(message, turnSpacing(message.role, compact), compact ? 0 : 1)
+  }
+
+  // ── window driver: per-frame scrollTop check (no scroll signal exists; the
+  // frame callback fires on every rendered frame, and scrolling always renders).
+  let anchor: number | null = null
+  let lastCount = -1
+  const tick = (): void => {
+    const sb = scroll()
+    if (!sb) return
+    // Selection freeze: the native selection walks the LIVE tree — swapping a
+    // row out (destroying its renderables) mid-selection would corrupt the
+    // highlight/copy. Frozen ≠ broken: unseen new rows default to mounted.
+    if (renderer.getSelection()?.isActive) return
+    const viewportHeight = sb.viewport.height
+    if (viewportHeight <= 0) return
+    const messages = props.store.state.messages
+    const scrollTop = sb.scrollTop
+    const countChanged = messages.length !== lastCount
+    if (!countChanged && !shouldRecompute(scrollTop, anchor, hysteresisFor(viewportHeight))) return
+    const running = props.store.state.info.running ?? false
+    const rows = messages.map((message, i) => {
+      const key = keyOf(message)
+      return {
+        key,
+        height: heights.get(key) ?? null,
+        estimate: estimateFor(message),
+        // Never window: a streaming row (remount would restart native markdown
+        // streaming) and the last row while a turn runs (deltas land there).
+        // A row with an expanded tool/reasoning body is NOT detectable from
+        // here (the override lives in component-local signals — toolPart.tsx/
+        // reasoningPart.tsx); expanded rows far above the viewport may
+        // re-collapse on remount. Accepted for S1.
+        neverWindow: (message.streaming ?? false) || (running && i === messages.length - 1)
+      }
+    })
+    const result = computeWindow({
+      rows,
+      scrollTop,
+      viewportHeight,
+      margin: viewportHeight, // 1 viewport each side (design §Mechanism 1)
+      bottomK: BOTTOM_ALWAYS_MOUNTED
+    })
+    anchor = result.anchor
+    lastCount = messages.length
+    const known = new Set(rows.map(r => r.key))
+    // The store cap splices old rows out — drop their recorded heights too.
+    if (countChanged) for (const key of heights.keys()) if (!known.has(key)) heights.delete(key)
+    liveWin = { mounted: result.mounted, known }
+    setWinState(liveWin)
+  }
+  onMount(() => {
+    if (!windowing) return
+    const frame = (_deltaTime: number): Promise<void> => {
+      tick()
+      return Promise.resolve()
+    }
+    renderer.setFrameCallback(frame)
+    onCleanup(() => renderer.removeFrameCallback(frame))
+  })
+
+  /** One windowed row: a measuring wrapper around the real MessageLine or an
+   *  exact-height spacer. The wrapper stays mounted either way (1 box), so its
+   *  `onSizeChange` keeps the height record fresh while the row is real. */
+  const WindowedRow = (rowProps: { message: Message; index: () => number }) => {
+    const key = keyOf(rowProps.message)
+    let wrapper: BoxRenderable | undefined
+    const record = (): void => {
+      if (!wrapper) return
+      // Only record while the REAL row is mounted — a spacer's (or estimate's)
+      // height must never overwrite the exact measurement.
+      if (liveWin && liveWin.known.has(key) && !liveWin.mounted.has(key)) return
+      const h = wrapper.height
+      if (h > 0) heights.set(key, h)
+    }
+    return (
+      <box ref={el => (wrapper = el)} style={{ flexDirection: 'column', flexShrink: 0 }} onSizeChange={record}>
+        <Show
+          when={isMounted(key)}
+          fallback={<box style={{ height: heights.get(key) ?? estimateFor(rowProps.message), flexShrink: 0 }} />}
+        >
+          <MessageLine message={rowProps.message} latest={rowProps.index() === latestAssistant()} />
+        </Show>
+      </box>
+    )
+  }
+
   return (
     <box style={{ flexGrow: 1, minHeight: 0 }}>
       <scrollbox ref={setScroll} style={{ flexGrow: 1, minHeight: 0 }} stickyScroll stickyStart="bottom">
@@ -58,7 +236,13 @@ export function Transcript(props: { store: SessionStore }) {
               </text>
             </Show>
             <For each={props.store.state.messages}>
-              {(message, i) => <MessageLine message={message} latest={i() === latestAssistant()} />}
+              {(message, i) =>
+                windowing ? (
+                  <WindowedRow message={message} index={i} />
+                ) : (
+                  <MessageLine message={message} latest={i() === latestAssistant()} />
+                )
+              }
             </For>
           </DisplayProvider>
         </ScrollAnchorProvider>