diff --git a/apps/desktop/src/components/assistant-ui/thread/timeline-data.test.ts b/apps/desktop/src/components/assistant-ui/thread/timeline-data.test.ts
index 0c2918aea59a..b24f5391a879 100644
--- a/apps/desktop/src/components/assistant-ui/thread/timeline-data.test.ts
+++ b/apps/desktop/src/components/assistant-ui/thread/timeline-data.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 
-import { activeTimelineIndex, deriveTimelineEntries, timelinePreview } from './timeline-data'
+import { activeTimelineIndex, deriveTimelineEntries, sameTimelineEntries, timelinePreview } from './timeline-data'
 
 describe('timelinePreview', () => {
   it('collapses whitespace to a single line', () => {
@@ -39,6 +39,33 @@ describe('deriveTimelineEntries', () => {
   })
 })
 
+describe('sameTimelineEntries', () => {
+  const rail = [
+    { id: 'u1', preview: 'first' },
+    { id: 'u2', preview: 'second' }
+  ]
+
+  it('treats an identical derivation as unchanged, so the memo can reuse it', () => {
+    expect(sameTimelineEntries(rail, [...rail.map(e => ({ ...e }))])).toBe(true)
+  })
+
+  it('detects a changed preview, id, or length', () => {
+    expect(sameTimelineEntries(rail, [rail[0], { id: 'u2', preview: 'edited' }])).toBe(false)
+    expect(sameTimelineEntries(rail, [rail[0], { id: 'u9', preview: 'second' }])).toBe(false)
+    expect(sameTimelineEntries(rail, [rail[0]])).toBe(false)
+  })
+
+  it('is stable when a filtered-out prompt joins the transcript', () => {
+    const withNoise = deriveTimelineEntries([
+      { id: 'u1', role: 'user', text: 'first' },
+      { id: 'u2', role: 'user', text: 'second' },
+      { id: 'u3', role: 'user', text: '[IMPORTANT: Background process 7 finished]' }
+    ])
+
+    expect(sameTimelineEntries(rail, withNoise)).toBe(true)
+  })
+})
+
 describe('activeTimelineIndex', () => {
   it('returns the last prompt scrolled to or above the top edge', () => {
     expect(activeTimelineIndex([-400, -10, 320])).toBe(1)
diff --git a/apps/desktop/src/components/assistant-ui/thread/timeline-data.ts b/apps/desktop/src/components/assistant-ui/thread/timeline-data.ts
index e52d1d7c780a..35dbb8f14cd3 100644
--- a/apps/desktop/src/components/assistant-ui/thread/timeline-data.ts
+++ b/apps/desktop/src/components/assistant-ui/thread/timeline-data.ts
@@ -46,6 +46,20 @@ export function deriveTimelineEntries(messages: readonly TimelineSourceMessage[]
   return entries
 }
 
+/** Do two derivations describe the same rail? Lets a rebuild hand back the
+ *  PREVIOUS array so an unchanged transcript costs zero re-renders. */
+export function sameTimelineEntries(a: readonly TimelineEntry[], b: readonly TimelineEntry[]): boolean {
+  if (a === b) {
+    return true
+  }
+
+  if (a.length !== b.length) {
+    return false
+  }
+
+  return a.every((entry, index) => entry.id === b[index].id && entry.preview === b[index].preview)
+}
+
 /** Last user prompt at/above the viewport top (with slack); else first rendered. */
 export function activeTimelineIndex(offsets: readonly (number | null)[], slack: number = 8): number {
   let active = -1
diff --git a/apps/desktop/src/components/assistant-ui/thread/timeline-idle.test.tsx b/apps/desktop/src/components/assistant-ui/thread/timeline-idle.test.tsx
new file mode 100644
index 000000000000..58ed16529ad3
--- /dev/null
+++ b/apps/desktop/src/components/assistant-ui/thread/timeline-idle.test.tsx
@@ -0,0 +1,166 @@
+import { cleanup, fireEvent, render, screen } from '@testing-library/react'
+import type { ReactNode } from 'react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+/**
+ * The timeline must do NO work it can't currently show. Two gates are proven
+ * here by rendering the real component and counting the work it performs:
+ *
+ *  - a background (kept-alive but hidden) tab derives nothing and subscribes
+ *    to nothing — the transcript selector is never even called;
+ *  - an unhovered rail builds its ticks but not the popover's rows.
+ *
+ * The prompt-id selector is also asserted to be content-blind, which is what
+ * keeps a streaming assistant reply from re-deriving previews per token.
+ */
+
+interface FakeMessage {
+  content: unknown
+  id: string
+  role: string
+}
+
+const selectorCalls = vi.fn()
+const transcriptReads = vi.fn()
+let messages: FakeMessage[] = []
+
+vi.mock('@assistant-ui/react', () => ({
+  useAui: () => ({
+    thread: () => ({
+      getState: () => {
+        transcriptReads()
+
+        return { messages }
+      }
+    })
+  }),
+  useAuiState: (selector: (state: { thread: { messages: FakeMessage[] } }) => unknown) => {
+    selectorCalls()
+
+    return selector({ thread: { messages } })
+  }
+}))
+
+let paneActive = true
+
+vi.mock('@/components/pane-shell/pane-visibility', () => ({
+  usePaneVisible: () => paneActive
+}))
+
+vi.mock('@/lib/haptics', () => ({ triggerHaptic: () => {} }))
+
+const { ThreadTimeline } = await import('./timeline')
+
+const userTurn = (id: string, text: string): FakeMessage => ({
+  content: [{ text, type: 'text' }],
+  id,
+  role: 'user'
+})
+
+const transcript = (count: number): FakeMessage[] =>
+  Array.from({ length: count }, (_, i) => userTurn(`u${i}`, `prompt ${i}`))
+
+const renderTimeline = (ui: ReactNode = <ThreadTimeline />) => render(ui)
+
+afterEach(() => {
+  cleanup()
+  selectorCalls.mockClear()
+  transcriptReads.mockClear()
+  paneActive = true
+  messages = []
+})
+
+describe('ThreadTimeline in a background tab', () => {
+  it('renders nothing and never reads the transcript', () => {
+    paneActive = false
+    messages = transcript(6)
+
+    const { container } = renderTimeline()
+
+    expect(container.querySelector('[data-slot="thread-timeline"]')).toBeNull()
+    expect(selectorCalls).not.toHaveBeenCalled()
+    expect(transcriptReads).not.toHaveBeenCalled()
+  })
+
+  it('renders the rail once its pane becomes the visible tab', () => {
+    messages = transcript(6)
+
+    const { container } = renderTimeline()
+
+    expect(container.querySelector('[data-slot="thread-timeline"]')).not.toBeNull()
+    expect(selectorCalls).toHaveBeenCalled()
+  })
+})
+
+describe('ThreadTimeline popover', () => {
+  it('builds no rows until the rail is hovered', () => {
+    messages = transcript(6)
+
+    const { container } = renderTimeline()
+    const popover = container.querySelector('[data-slot="thread-timeline-popover"]')
+
+    // The shell renders (it owns the fade transition); its rows do not.
+    expect(popover).not.toBeNull()
+    expect(popover?.querySelectorAll('button')).toHaveLength(0)
+    expect(screen.queryByText('prompt 0')).toBeNull()
+  })
+
+  it('builds the rows on hover and keeps them for the close fade', () => {
+    messages = transcript(6)
+
+    const { container } = renderTimeline()
+    const rail = container.querySelector<HTMLElement>('[data-slot="thread-timeline"]')!
+
+    fireEvent.mouseEnter(rail)
+
+    const popover = container.querySelector('[data-slot="thread-timeline-popover"]')
+    expect(popover?.querySelectorAll('button')).toHaveLength(6)
+
+    fireEvent.mouseLeave(rail)
+
+    // Still mounted — the popover fades out, it does not pop out of existence.
+    expect(popover?.querySelectorAll('button')).toHaveLength(6)
+  })
+})
+
+describe('ThreadTimeline below the threshold', () => {
+  it('renders nothing for a short thread', () => {
+    messages = transcript(2)
+
+    const { container } = renderTimeline()
+
+    expect(container.querySelector('[data-slot="thread-timeline"]')).toBeNull()
+  })
+})
+
+describe('ThreadTimeline while a reply streams', () => {
+  it('does not re-derive the rail as assistant content grows', () => {
+    messages = [...transcript(6), { content: [{ text: 'th', type: 'text' }], id: 'a1', role: 'assistant' }]
+
+    const { rerender } = renderTimeline()
+    const derivations = transcriptReads.mock.calls.length
+
+    // A token lands: the assistant message's content changes, the user prompt
+    // ids do not — so the memo's change signal is untouched and the previews
+    // are never rebuilt.
+    messages = [
+      ...messages.slice(0, -1),
+      { content: [{ text: 'thinking…', type: 'text' }], id: 'a1', role: 'assistant' }
+    ]
+    rerender(<ThreadTimeline />)
+
+    expect(transcriptReads.mock.calls.length).toBe(derivations)
+  })
+
+  it('re-derives once a new prompt is sent', () => {
+    messages = transcript(6)
+
+    const { rerender } = renderTimeline()
+    const derivations = transcriptReads.mock.calls.length
+
+    messages = [...messages, userTurn('u6', 'prompt 6')]
+    rerender(<ThreadTimeline />)
+
+    expect(transcriptReads.mock.calls.length).toBeGreaterThan(derivations)
+  })
+})
diff --git a/apps/desktop/src/components/assistant-ui/thread/timeline.tsx b/apps/desktop/src/components/assistant-ui/thread/timeline.tsx
index df2a4c7332c6..9dc2c94b3ce1 100644
--- a/apps/desktop/src/components/assistant-ui/thread/timeline.tsx
+++ b/apps/desktop/src/components/assistant-ui/thread/timeline.tsx
@@ -1,12 +1,14 @@
-import { useAuiState } from '@assistant-ui/react'
+import { useAui, useAuiState } from '@assistant-ui/react'
 import { type FC, useCallback, useEffect, useMemo, useRef, useState } from 'react'
 
+import { usePaneVisible } from '@/components/pane-shell/pane-visibility'
 import { triggerHaptic } from '@/lib/haptics'
 import { cn } from '@/lib/utils'
 
 import {
   activeTimelineIndex,
   deriveTimelineEntries,
+  sameTimelineEntries,
   type TimelineEntry,
   type TimelineSourceMessage
 } from './timeline-data'
@@ -122,26 +124,89 @@ function scrollToPrompt(root: HTMLElement | null, id: string) {
   jumpScroll(viewport, Math.max(0, top))
 }
 
-/** Right-edge prompt rail — hover previews, click to jump. ≥4 user turns only. */
+/**
+ * Right-edge prompt rail — hover previews, click to jump. ≥4 user turns only.
+ *
+ * Everything here is DEFERRED until it can actually be seen. A chat surface
+ * stays mounted while its tab is in the background (keep-alive, see
+ * pane-visibility.ts), and a background thread keeps streaming, so a naive
+ * timeline would re-derive previews and re-measure prompt offsets all day for
+ * a rail nobody is looking at. Four gates, cheapest first:
+ *
+ *  1. INACTIVE PANE → render null and subscribe to nothing. The transcript
+ *     selector, the scroll listener, and the popover markup all stand down.
+ *  2. ACTIVE BUT UNHOVERED → the ticks paint, but the popover's rows are not
+ *     built at all; the previews only exist once the pointer opens it.
+ *  3. BELOW THE THRESHOLD → the rail renders null, so the measure effect never
+ *     touches layout for it.
+ *  4. FOLLOWING THE BOTTOM → the active prompt is the last one by definition,
+ *     answered from data instead of a rect walk (see compute() below).
+ */
 export const ThreadTimeline: FC = () => {
-  const sourceSignature = useAuiState(s => {
-    const rows: TimelineSourceMessage[] = []
+  // Cheapest possible gate, and it must come first: an inactive tab returns
+  // before any of the work below is even declared.
+  return usePaneVisible() ? <ActiveThreadTimeline /> : null
+}
+
+/** Derived prompt rail for a VISIBLE surface. Split out so the hook body — and
+ *  the transcript subscription it opens — never runs for a background tab. */
+const ActiveThreadTimeline: FC = () => {
+  // Cheap in the selector, expensive only when it changes: the ids alone tell
+  // us whether the RAIL changed. Prompt text is immutable once sent, and an
+  // edit rewinds the transcript (dropping every id after it) and re-appends a
+  // fresh message id — so a preview can never go stale behind a stable id.
+  // Streaming an assistant reply churns that message's content on every token
+  // and leaves this string untouched, which is the whole point.
+  const promptIds = useAuiState(s => {
+    let ids = ''
 
     for (const message of s.thread.messages) {
-      if (message.role !== 'user') {
-        continue
+      if (message.role === 'user') {
+        ids += `${message.id}\n`
       }
-
-      rows.push({ id: message.id, role: 'user', text: userPromptText(message.content) })
     }
 
-    return JSON.stringify(rows)
+    return ids
   })
 
-  const entries = useMemo(
-    () => deriveTimelineEntries(JSON.parse(sourceSignature) as TimelineSourceMessage[]),
-    [sourceSignature]
-  )
+  // `promptIds` is the change signal; the transcript is read imperatively when
+  // it fires, so the selector above never pays for text extraction. The client
+  // goes through a ref so the memo keys on the SIGNAL alone — an accessor whose
+  // identity churned would otherwise re-derive every render, which is exactly
+  // the streaming cost this is here to avoid.
+  const aui = useAui()
+  const auiRef = useRef(aui)
+  auiRef.current = aui
+
+  const previousRef = useRef<TimelineEntry[]>([])
+
+  const entries = useMemo(() => {
+    const rows: TimelineSourceMessage[] = []
+
+    for (const message of auiRef.current.thread().getState().messages) {
+      if (message.role === 'user') {
+        rows.push({ id: message.id, role: 'user', text: userPromptText(message.content) })
+      }
+    }
+
+    const next = deriveTimelineEntries(rows)
+
+    // Hand back the PREVIOUS array when nothing user-visible moved. Blank and
+    // background-notification prompts are filtered out, so a new id can leave
+    // the rail identical — without this, that re-renders both subtrees and
+    // restarts the measure effect for no visible change.
+    if (sameTimelineEntries(previousRef.current, next)) {
+      return previousRef.current
+    }
+
+    previousRef.current = next
+
+    return next
+    // promptIds is the intentional re-eval TRIGGER, not a value the derivation
+    // reads (the transcript comes off the ref) — same shape as ChatRoutesSurface's
+    // gatewayState memo in app/contrib/controller.tsx.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [promptIds])
 
   const [activeIndex, setActiveIndex] = useState(0)
   const [open, setOpen] = useState(false)
@@ -185,9 +250,15 @@ export const ThreadTimeline: FC = () => {
   useEffect(() => () => window.clearTimeout(closeTimerRef.current), [])
 
   useEffect(() => {
+    // Below the threshold the rail renders null, so measuring prompt offsets
+    // buys nothing — bail before touching layout at all.
+    if (entries.length < MIN_ENTRIES) {
+      return
+    }
+
     const viewport = ownViewport(rootRef.current)
 
-    if (!viewport || entries.length === 0) {
+    if (!viewport) {
       return
     }
 
@@ -279,29 +350,44 @@ const TimelinePopover: FC<{
   onJump: (id: string) => void
   open: boolean
   rowRefs: React.RefObject<(HTMLButtonElement | null)[]>
-}> = ({ activeIndex, entries, onHover, onJump, open, rowRefs }) => (
-  <div
-    className={cn(
-      POPOVER_SHELL,
-      open ? 'pointer-events-auto opacity-100 translate-x-0' : 'pointer-events-none translate-x-1 opacity-0'
-    )}
-    data-slot="thread-timeline-popover"
-  >
-    {entries.map((entry, index) => (
-      <button
-        aria-label={entry.preview}
-        className={cn(ROW_CLASS, index === activeIndex && 'bg-(--ui-row-active-background) text-foreground')}
-        key={entry.id}
-        onClick={() => onJump(entry.id)}
-        ref={listRef(rowRefs, index)}
-        type="button"
-        {...hoverProps(index, onHover)}
-      >
-        <span className="block w-full min-w-0 truncate font-medium leading-snug text-foreground">{entry.preview}</span>
-      </button>
-    ))}
-  </div>
-)
+}> = ({ activeIndex, entries, onHover, onJump, open, rowRefs }) => {
+  // The rail is the always-visible part; this list is not built until the
+  // pointer first opens it. The SHELL always renders so the opacity/translate
+  // transition has a node to animate — only the N rows are deferred, and they
+  // stay mounted afterwards so the close fade still has content.
+  const [everOpened, setEverOpened] = useState(open)
+
+  if (open && !everOpened) {
+    setEverOpened(true)
+  }
+
+  return (
+    <div
+      className={cn(
+        POPOVER_SHELL,
+        open ? 'pointer-events-auto opacity-100 translate-x-0' : 'pointer-events-none translate-x-1 opacity-0'
+      )}
+      data-slot="thread-timeline-popover"
+    >
+      {everOpened &&
+        entries.map((entry, index) => (
+          <button
+            aria-label={entry.preview}
+            className={cn(ROW_CLASS, index === activeIndex && 'bg-(--ui-row-active-background) text-foreground')}
+            key={entry.id}
+            onClick={() => onJump(entry.id)}
+            ref={listRef(rowRefs, index)}
+            type="button"
+            {...hoverProps(index, onHover)}
+          >
+            <span className="block w-full min-w-0 truncate font-medium leading-snug text-foreground">
+              {entry.preview}
+            </span>
+          </button>
+        ))}
+    </div>
+  )
+}
 
 const TimelineTicks: FC<{
   activeIndex: number