diff --git a/apps/desktop/src/app/chat/index.test.tsx b/apps/desktop/src/app/chat/index.test.tsx
new file mode 100644
index 000000000000..8caace4cc98b
--- /dev/null
+++ b/apps/desktop/src/app/chat/index.test.tsx
@@ -0,0 +1,164 @@
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import { cleanup, fireEvent, render, screen } from '@testing-library/react'
+import { useState } from 'react'
+import { MemoryRouter } from 'react-router'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { assistantTextPart, type ChatMessage } from '@/lib/chat-messages'
+import {
+  $activeSessionId,
+  $awaitingResponse,
+  $busy,
+  $contextSuggestions,
+  $currentCwd,
+  $currentModel,
+  $currentProvider,
+  $freshDraftReady,
+  $gatewayState,
+  $messages,
+  $selectedStoredSessionId,
+  $sessions
+} from '@/store/session'
+
+const threadRenderCount = vi.hoisted(() => ({ current: 0 }))
+
+vi.mock('@/components/assistant-ui/thread', async () => {
+  const React = await import('react')
+
+  return {
+    Thread: () => {
+      threadRenderCount.current += 1
+
+      return React.createElement('div', { 'data-testid': 'thread' })
+    }
+  }
+})
+
+vi.mock('@/components/Backdrop', async () => {
+  const React = await import('react')
+
+  return { Backdrop: () => React.createElement('div', { 'data-testid': 'backdrop' }) }
+})
+
+vi.mock('@/components/prompt-overlays', () => ({ PromptOverlays: () => null }))
+vi.mock('@/components/chat/vibe-hearts', () => ({ COMPOSER_HEART_CONFIG: {}, HeartField: () => null }))
+vi.mock('@/lib/model-options', () => ({
+  modelOptionsQueryKey: (...parts: unknown[]) => ['model-options', ...parts],
+  requestModelOptions: vi.fn(async () => ({ models: [] }))
+}))
+vi.mock('./chat-drop-overlay', () => ({ ChatDropOverlay: () => null }))
+vi.mock('./chat-swap-overlay', () => ({ ChatSwapOverlay: () => null }))
+vi.mock('./composer', () => ({ ChatBar: () => null, ChatBarFallback: () => null }))
+vi.mock('./hooks/use-file-drop-zone', () => ({
+  useFileDropZone: () => ({ dragKind: null, dropHandlers: {} })
+}))
+vi.mock('./sidebar/session-actions-menu', async () => {
+  const React = await import('react')
+
+  return {
+    SessionActionsMenu: ({ children }: { children: React.ReactNode }) =>
+      React.createElement('div', { 'data-testid': 'session-actions-menu' }, children)
+  }
+})
+
+const { ChatView } = await import('./index')
+
+function assistantMessage(id: string, text: string): ChatMessage {
+  return {
+    id,
+    parts: [assistantTextPart(text)],
+    role: 'assistant'
+  }
+}
+
+describe('ChatView render isolation', () => {
+  beforeEach(() => {
+    threadRenderCount.current = 0
+    $activeSessionId.set('runtime-1')
+    $awaitingResponse.set(false)
+    $busy.set(false)
+    $contextSuggestions.set([])
+    $currentCwd.set('/work')
+    $currentModel.set('test-model')
+    $currentProvider.set('test-provider')
+    $freshDraftReady.set(false)
+    $gatewayState.set('closed')
+    $messages.set([assistantMessage('assistant-1', 'Stable historical answer')])
+    $selectedStoredSessionId.set('stored-1')
+    $sessions.set([{ id: 'stored-1', message_count: 1, title: 'Stable chat' } as never])
+  })
+
+  afterEach(() => {
+    cleanup()
+    vi.restoreAllMocks()
+    $activeSessionId.set(null)
+    $awaitingResponse.set(false)
+    $busy.set(false)
+    $contextSuggestions.set([])
+    $currentCwd.set('')
+    $currentModel.set('')
+    $currentProvider.set('')
+    $freshDraftReady.set(false)
+    $gatewayState.set('idle')
+    $messages.set([])
+    $selectedStoredSessionId.set(null)
+    $sessions.set([])
+  })
+
+  it('does not re-render chat history when an unrelated parent idle tick updates', () => {
+    const props = {
+      gateway: null,
+      maxVoiceRecordingSeconds: 120,
+      onAddContextRef: vi.fn(),
+      onAddUrl: vi.fn(),
+      onAttachDroppedItems: vi.fn(),
+      onAttachImageBlob: vi.fn(),
+      onBranchInNewChat: vi.fn(),
+      onCancel: vi.fn(),
+      onDeleteSelectedSession: vi.fn(),
+      onEdit: vi.fn(),
+      onPasteClipboardImage: vi.fn(),
+      onPickFiles: vi.fn(),
+      onPickFolders: vi.fn(),
+      onPickImages: vi.fn(),
+      onReload: vi.fn(),
+      onRemoveAttachment: vi.fn(),
+      onRetryResume: vi.fn(),
+      onSteer: vi.fn(),
+      onSubmit: vi.fn(),
+      onThreadMessagesChange: vi.fn(),
+      onToggleSelectedPin: vi.fn(),
+      onTranscribeAudio: vi.fn()
+    }
+
+    const queryClient = new QueryClient({
+      defaultOptions: { queries: { retry: false } }
+    })
+
+    function ParentTickHarness() {
+      const [tick, setTick] = useState(0)
+
+      return (
+        <QueryClientProvider client={queryClient}>
+          <MemoryRouter initialEntries={['/stored-1']}>
+            <button onClick={() => setTick(value => value + 1)} type="button">
+              parent tick {tick}
+            </button>
+            <ChatView {...props} />
+          </MemoryRouter>
+        </QueryClientProvider>
+      )
+    }
+
+    render(<ParentTickHarness />)
+
+    expect(screen.getByTestId('thread')).toBeTruthy()
+    expect(threadRenderCount.current).toBe(1)
+
+    fireEvent.click(screen.getByRole('button', { name: /parent tick/i }))
+
+    // memo(ChatView) with stable props must absorb the parent's idle tick —
+    // the transcript (Thread) must not re-render. This is PR #38470's contract.
+    expect(threadRenderCount.current).toBe(1)
+  })
+})
diff --git a/apps/desktop/src/app/chat/index.tsx b/apps/desktop/src/app/chat/index.tsx
index 3d72bf341088..7a0c0a2674c3 100644
--- a/apps/desktop/src/app/chat/index.tsx
+++ b/apps/desktop/src/app/chat/index.tsx
@@ -3,7 +3,7 @@ import { useStore } from '@nanostores/react'
 import { useQuery } from '@tanstack/react-query'
 import type { ReadableAtom } from 'nanostores'
 import type * as React from 'react'
-import { Suspense, useCallback, useEffect, useMemo, useState } from 'react'
+import { memo, Suspense, useCallback, useEffect, useMemo, useState } from 'react'
 import { useLocation } from 'react-router'
 
 import type { SubmitTextOptions } from '@/app/session/hooks/use-prompt-actions/utils'
@@ -240,7 +240,10 @@ function ChatRuntimeBoundary({
   return <AssistantRuntimeProvider runtime={runtime}>{children}</AssistantRuntimeProvider>
 }
 
-export function ChatView({
+// Memoized: the tile caller (session-tile.tsx) and the contrib surface re-render
+// on idle ticks unrelated to the chat; with stable callback props (hoisted to
+// useCallback at the call sites) memo() lets the whole chat shell skip those.
+export const ChatView = memo(function ChatView({
   className,
   gateway,
   modelMenuContent,
@@ -596,4 +599,4 @@ export function ChatView({
       </ChatRuntimeBoundary>
     </div>
   )
-}
+})
diff --git a/apps/desktop/src/app/chat/session-tile.tsx b/apps/desktop/src/app/chat/session-tile.tsx
index 27954abc2a56..d279aea439e7 100644
--- a/apps/desktop/src/app/chat/session-tile.tsx
+++ b/apps/desktop/src/app/chat/session-tile.tsx
@@ -104,6 +104,13 @@ function buildTileView(storedSessionId: string): SessionView {
   }
 }
 
+// Module-level constants so these ChatView props are referentially stable —
+// tiles have no pin/delete affordance, and transcription needs no per-tile state.
+const noop = () => undefined
+
+const tileTranscribeAudio = async (audio: Blob) =>
+  (await transcribeAudio(await blobToDataUrl(audio), audio.type)).transcript
+
 function TileChat({
   runtimeId,
   storedSessionId,
@@ -144,6 +151,27 @@ function TileChat({
     scope: { add: attachments.add, remove: attachments.remove, target: scope.target }
   })
 
+  // ChatView is memo()d — every callback prop must be referentially stable or
+  // the memo never holds and each tile-level render (idle ticks, unrelated
+  // store updates) re-renders the whole chat shell. The individual composer
+  // functions are useCallback'd inside useComposerActions, so hoisting these
+  // wrappers onto them keeps identity stable across renders.
+  const { addContextRefAttachment, pasteClipboardImage, pickContextPaths, pickImages, removeAttachment } = composer
+
+  const onAddUrl = useCallback(
+    (url: string) => addContextRefAttachment(`@url:${formatRefValue(url)}`, url),
+    [addContextRefAttachment]
+  )
+  const onPasteClipboardImage = useCallback(
+    (opts?: { silent?: boolean }) => pasteClipboardImage(opts),
+    [pasteClipboardImage]
+  )
+  const onPickFiles = useCallback(() => void pickContextPaths('file'), [pickContextPaths])
+  const onPickFolders = useCallback(() => void pickContextPaths('folder'), [pickContextPaths])
+  const onPickImages = useCallback(() => void pickImages(), [pickImages])
+  const onRemoveAttachment = useCallback((id: string) => void removeAttachment(id), [removeAttachment])
+  const onRetryResume = useCallback(() => patchSessionTile(storedSessionId, { error: undefined }), [storedSessionId])
+
   // Per-tile model menu — rendered under this tile's SessionView so the pill
   // + switch target THIS runtime, not the primary (which may be mid-turn).
   const modelMenuContent = useMemo(
@@ -165,27 +193,27 @@ function TileChat({
         <ChatView
           gateway={gateway}
           modelMenuContent={modelMenuContent}
-          onAddContextRef={composer.addContextRefAttachment}
-          onAddUrl={url => composer.addContextRefAttachment(`@url:${formatRefValue(url)}`, url)}
+          onAddContextRef={addContextRefAttachment}
+          onAddUrl={onAddUrl}
           onAttachDroppedItems={composer.attachDroppedItems}
           onAttachImageBlob={composer.attachImageBlob}
           onCancel={actions.cancelRun}
-          onDeleteSelectedSession={() => undefined}
+          onDeleteSelectedSession={noop}
           onDismissError={actions.dismissError}
           onEdit={actions.editMessage}
-          onPasteClipboardImage={opts => composer.pasteClipboardImage(opts)}
-          onPickFiles={() => void composer.pickContextPaths('file')}
-          onPickFolders={() => void composer.pickContextPaths('folder')}
-          onPickImages={() => void composer.pickImages()}
+          onPasteClipboardImage={onPasteClipboardImage}
+          onPickFiles={onPickFiles}
+          onPickFolders={onPickFolders}
+          onPickImages={onPickImages}
           onReload={actions.reloadFromMessage}
-          onRemoveAttachment={id => void composer.removeAttachment(id)}
+          onRemoveAttachment={onRemoveAttachment}
           onRestoreToMessage={actions.restoreToMessage}
-          onRetryResume={() => patchSessionTile(storedSessionId, { error: undefined })}
+          onRetryResume={onRetryResume}
           onSteer={actions.steerPrompt}
           onSubmit={actions.submitText}
           onThreadMessagesChange={actions.handleThreadMessagesChange}
-          onToggleSelectedPin={() => undefined}
-          onTranscribeAudio={async audio => (await transcribeAudio(await blobToDataUrl(audio), audio.type)).transcript}
+          onToggleSelectedPin={noop}
+          onTranscribeAudio={tileTranscribeAudio}
         />
       </ComposerScopeProvider>
     </SessionViewProvider>
diff --git a/apps/desktop/src/lib/incremental-external-store-runtime.ts b/apps/desktop/src/lib/incremental-external-store-runtime.ts
index 0df3ed9b2e09..1ccc5121d978 100644
--- a/apps/desktop/src/lib/incremental-external-store-runtime.ts
+++ b/apps/desktop/src/lib/incremental-external-store-runtime.ts
@@ -243,9 +243,13 @@ export function useIncrementalExternalStoreRuntime<T extends ThreadMessage>(
 ): AssistantRuntime {
   const [runtime] = useState(() => new IncrementalExternalStoreRuntimeCore(store as ExternalStoreAdapter))
 
+  // Re-sync the adapter only when it actually changes — a dep-less effect ran
+  // on EVERY render of the chat surface. `__internal_setAdapter` early-exits
+  // when the store is unchanged, so gating on [runtime, store] is behavior-
+  // preserving while skipping the per-render call entirely.
   useEffect(() => {
     runtime.setAdapter(store as ExternalStoreAdapter)
-  })
+  }, [runtime, store])
 
   const { modelContext } = useRuntimeAdapters() ?? {}