From 4e998861e10eec40d1f36b3006ea898d7c8aa661 Mon Sep 17 00:00:00 2001
From: Jinwoo Hong <73622457+Jinwoo-H@users.noreply.github.com>
Date: Tue, 20 Oct 2026 06:50:11 -0400
Subject: [PATCH] test: shared switch-on fixtures so 71 layout tests pass with
 main owning the layout (#31076)

* test: shared switch-on fixtures so 71 layout-owner test files run with the switch on

- layoutApplyTestTransport answers layout.apply (accept, apply to main's model with its frame, pending, refuse)
- main's loaded layout as snapshot frames, shared from the frame-sync test
- tab-layout maps for the editor store harnesses; missing store methods in the main harnesses
- workspaceRepos/registerWorkspaceRepos so main's load keeps seeded layouts
- ORCA_LAYOUT_OWNER=main runs vitest with the switch on
- the 4 rollback-build tests stay switch-off, labelled

* test: keep switch-off assertions exact, settle only after layout commands, reset the transport per test

* test: parse the transport's command without Reflect.get; name the staged-session type

* test: pin the test transport in the layout.apply census; route the autosave fixture as unit support

---------

Co-authored-by: E2E Test <e2e@test.local>
---
 .../scripts/pr-unit-support-routing.test.mjs  |   4 +-
 config/vitest.config.ts                       |   5 +-
 .../ipc/renderer-shutdown-checkpoint.test.ts  |  17 +-
 src/main/ipc/session.test.ts                  |   3 +
 .../worktrees-canonical-ssh-listing.test.ts   |   7 +-
 src/main/ipc/worktrees-test-harness.ts        |   5 +
 src/main/ipc/worktrees-test-ipc-surface.ts    |   5 +
 ...ersistence-pane-identity-migration.test.ts |  21 +-
 ...istence-pty-binding-reconciliation.test.ts |  11 +-
 ...persistence-split-pane-incarnation.test.ts |   6 +-
 src/main/persistence-test-harness.ts          |  18 ++
 ...tence-workspace-session-scrollback.test.ts |  13 +-
 ...sistence-worktree-deletion-fencing.test.ts |   6 +-
 .../pty-binding-placement-inertness.test.ts   |   3 +
 .../terminal-late-bind-layout-rules.test.ts   |   3 +
 .../runtime/agent-session-tab-layout.test.ts  |   5 +-
 ...orca-runtime-serve-browser-restore.test.ts |   9 +-
 ...a-runtime-terminal-split-authority.test.ts |  10 +
 .../orca-runtime-test-fixtures.spec.ts        |  13 +
 ...ructured-agent-session-rpc.test-fixture.ts |   7 +-
 ...editor-autosave-controller-test-fixture.ts |   4 +-
 .../editor/editor-autosave-controller.test.ts |   4 +-
 .../editor/editor-save-user-named-tab.test.ts |   8 +-
 .../floating-workspace-tab-reorder.test.ts    |  20 +-
 .../tab-bar/TabBar.context-menu.test.ts       |   9 +
 .../TabGroupSplitLayout.drag.test.tsx         |  20 +-
 .../launch-parity-window-tab.test.ts          |   7 +
 .../pty-connection-pty-exit-teardown.spec.ts  |   7 +
 .../pty-connection-restore-and-resume.test.ts |   9 +-
 .../pty-connection-session-liveness.spec.ts   |   3 +
 .../pty-connection-test-pane-fixtures.ts      |   4 +
 ...pty-connection-visibility-and-exit.test.ts |   9 +-
 ...nt-launch-pane-verdict-application.test.ts |   7 +
 ...t-background-session-launch-parity.test.ts |   7 +
 ...unch-agent-in-new-tab-main-surface.test.ts |   7 +
 ...launch-agent-structured-chat-guard.test.ts |   2 +
 .../lib/launch-parity-plus-planner.test.ts    |   7 +
 .../src/lib/pi-session-resume-wake.test.ts    |   9 +-
 ...ping-agent-session-live-ssh-dedupe.test.ts |   7 +
 ...eping-agent-session-provider-claim.test.ts |   9 +-
 ...-agent-session-suppress-navigation.test.ts |   7 +
 .../lib/resume-sleeping-agent-session.test.ts |   7 +
 .../lib/retracted-host-pane-resume.test.ts    |   9 +-
 .../lib/settled-worker-wake-policy.test.ts    |   7 +
 ...etup-split-activation-single-owner.test.ts |  55 ++--
 ...-agent-session-new-chat-own-launch.test.ts |  11 +-
 ...leeping-agents-live-unmounted-pane.test.ts |   7 +
 .../workspace-terminal-host-authority.test.ts |   7 +
 ...ktree-activation-automation-filter.test.ts |   7 +
 .../worktree-activation-created-agent.test.ts |   7 +
 ...tree-activation-default-agent-chat.test.ts |   7 +
 ...ctivation-emptied-workspace-reseed.test.ts |   7 +
 ...activation-structured-chat-surface.test.ts |   7 +
 .../worktree-agent-activation-seam.test.ts    |  47 ++-
 ...worktree-creation-background-mount.test.ts |   7 +
 ...on-structured-session-host-decline.test.ts |   7 +
 ...ivation-preserved-pane-replacement.test.ts |   7 +
 ...tion-runtime-owned-resume-deferral.test.ts |   7 +
 .../layout-apply-paired-server-guard.test.ts  |   2 +
 ...ayout-apply-test-transport.test-fixture.ts | 304 ++++++++++++++++++
 .../main-loaded-layout-frames.test-fixture.ts |  53 +++
 ...tabs-sync-remote-status-title-flap.test.ts |  10 +-
 .../workspace-layout-frame-sync.test.ts       |  44 +--
 .../workspace-layout-frame-tab-props.test.ts  |  10 +
 .../slices/agent-generated-tab-title.test.ts  |  49 ++-
 ...rowser-page-close-intent-recording.test.ts |   6 +
 .../store/slices/cmd-j-create-actions.test.ts |   7 +
 .../src/store/slices/editor-open-diff.test.ts |  22 +-
 .../store/slices/editor-slice-test-harness.ts |  11 +-
 .../store/slices/editor-tab-placement.test.ts |  30 +-
 .../src/store/slices/new-markdown.test.ts     |   4 +-
 .../store/slices/recently-closed-tabs.test.ts |  64 +++-
 .../src/store/slices/tab-view-mode.test.ts    |  26 +-
 .../structured-chat-tab-rename.test.ts        |  18 +-
 .../workspace-layout-first-group-off.test.ts  |   5 +-
 75 files changed, 1045 insertions(+), 170 deletions(-)
 create mode 100644 src/renderer/src/runtime/layout-apply-test-transport.test-fixture.ts
 create mode 100644 src/renderer/src/runtime/main-loaded-layout-frames.test-fixture.ts

diff --git a/config/scripts/pr-unit-support-routing.test.mjs b/config/scripts/pr-unit-support-routing.test.mjs
index bdef0ec1579..238a55114e5 100644
--- a/config/scripts/pr-unit-support-routing.test.mjs
+++ b/config/scripts/pr-unit-support-routing.test.mjs
@@ -23,7 +23,9 @@ const unitHarnesses = [
 ]
 const supportBridges = new Set([
   'src/renderer/src/runtime/web-runtime-browser-creation-placement-test-rig.ts',
-  'src/renderer/src/components/editor/editor-model-lifetime-fixture.ts'
+  'src/renderer/src/components/editor/editor-model-lifetime-fixture.ts',
+  // Autosave suites' store rig; it takes the editor harness's tab-layout maps.
+  'src/renderer/src/components/editor/editor-autosave-controller-test-fixture.ts'
 ])
 
 function productConsumers(file, graph, unitFiles) {
diff --git a/config/vitest.config.ts b/config/vitest.config.ts
index 6af92217a6e..49ef5ffcfca 100644
--- a/config/vitest.config.ts
+++ b/config/vitest.config.ts
@@ -1,3 +1,4 @@
+import { createRequire } from 'node:module'
 import { resolve } from 'node:path'
 import { defaultExclude, defineConfig } from 'vitest/config'
 import { UNIT_INCLUDE, UNIT_EXCLUDE } from './scripts/ci-unit-files.mjs'
@@ -16,7 +17,9 @@ const transforms = {
   define: {
     ORCA_FEATURE_WALL_ENABLED: 'true',
     // Why: tests only interpolate it; they must not depend on the checkout's HEAD.
-    ORCA_BUILD_COMMIT: JSON.stringify('0'.repeat(40))
+    ORCA_BUILD_COMMIT: JSON.stringify('0'.repeat(40)),
+    // A dark switch set to 'main' in the env (`ORCA_LAYOUT_OWNER=main pnpm vitest`) is on in tests.
+    ...createRequire(import.meta.url)('./dark-build-switches.cjs').turnedOnDarkBuildDefines()
   },
   resolve: {
     alias: [
diff --git a/src/main/ipc/renderer-shutdown-checkpoint.test.ts b/src/main/ipc/renderer-shutdown-checkpoint.test.ts
index 43491ba31af..14b0819ded4 100644
--- a/src/main/ipc/renderer-shutdown-checkpoint.test.ts
+++ b/src/main/ipc/renderer-shutdown-checkpoint.test.ts
@@ -24,6 +24,9 @@ vi.mock('electron', () => ({
   }
 }))
 
+import { getDefaultWorkspaceSession } from '../../shared/constants'
+import type { WorkspaceSessionState } from '../../shared/workspace-session-state-types'
+import { ORCA_LAYOUT_OWNER_IS_MAIN } from '../../shared/layout-owner-switch'
 import {
   registerRendererShutdownCheckpointHandler,
   SHUTDOWN_CHECKPOINT_FLUSH_DEADLINE_MS
@@ -31,6 +34,11 @@ import {
 
 const AWAIT_CHANNEL = 'app:await-before-unload-checkpoint'
 
+/** What is staged for a window's save: the save itself, or main's merge of it when main owns the
+ *  layout (its content is covered by renderer-shutdown-checkpoint-sleep-records.test.ts). */
+const windowSave = (state: Partial<WorkspaceSessionState>): unknown =>
+  ORCA_LAYOUT_OWNER_IS_MAIN ? expect.any(Object) : state
+
 describe('registerRendererShutdownCheckpointHandler', () => {
   beforeEach(() => {
     syncHandlers.clear()
@@ -41,6 +49,8 @@ describe('registerRendererShutdownCheckpointHandler', () => {
   it('stages every shutdown mutation before queueing persistence', () => {
     const callOrder: string[] = []
     const store = {
+      // Read by the switch-on staging, which keeps main's own layout.
+      getWorkspaceSession: vi.fn(() => getDefaultWorkspaceSession()),
       stageWorkspaceSessionBeforeUnload: vi.fn((_state, hostId?: string) => {
         callOrder.push(`session:${hostId ?? 'local'}`)
       }),
@@ -64,12 +74,12 @@ describe('registerRendererShutdownCheckpointHandler', () => {
 
     expect(store.stageWorkspaceSessionBeforeUnload).toHaveBeenNthCalledWith(
       1,
-      localSession,
+      windowSave(localSession),
       undefined
     )
     expect(store.stageWorkspaceSessionBeforeUnload).toHaveBeenNthCalledWith(
       2,
-      remoteSession,
+      windowSave(remoteSession),
       'runtime:host-1'
     )
     expect(store.updateUI).toHaveBeenCalledWith({ activeView: 'settings' })
@@ -84,6 +94,7 @@ describe('registerRendererShutdownCheckpointHandler', () => {
 
   it('never stages the source partition of a fenced host', () => {
     const store = {
+      getWorkspaceSession: vi.fn(() => getDefaultWorkspaceSession()),
       stageWorkspaceSessionBeforeUnload: vi.fn(),
       getSshTarget: vi.fn(() => ({ orcadFence: { environmentId: 'env-1' } })),
       updateUI: vi.fn(),
@@ -98,7 +109,7 @@ describe('registerRendererShutdownCheckpointHandler', () => {
     })
 
     expect(store.stageWorkspaceSessionBeforeUnload).toHaveBeenCalledTimes(1)
-    expect(store.stageWorkspaceSessionBeforeUnload).toHaveBeenCalledWith({}, undefined)
+    expect(store.stageWorkspaceSessionBeforeUnload).toHaveBeenCalledWith(windowSave({}), undefined)
     expect(event.returnValue).toEqual({ ok: true })
   })
 
diff --git a/src/main/ipc/session.test.ts b/src/main/ipc/session.test.ts
index f481b356193..5387c13b66f 100644
--- a/src/main/ipc/session.test.ts
+++ b/src/main/ipc/session.test.ts
@@ -25,6 +25,7 @@ vi.mock('electron', () => ({
   }
 }))
 
+import { getDefaultWorkspaceSession } from '../../shared/constants'
 import { registerSessionHandlers } from './session'
 
 describe('registerSessionHandlers', () => {
@@ -33,6 +34,8 @@ describe('registerSessionHandlers', () => {
       getSshTarget: vi.fn((id: string) =>
         id === 'fenced' ? { orcadFence: { environmentId: 'env-1' } } : {}
       ),
+      // Read by the switch-on save, which keeps main's own layout.
+      getWorkspaceSession: vi.fn(() => getDefaultWorkspaceSession()),
       setWorkspaceSession: vi.fn(),
       patchWorkspaceSession: vi.fn(),
       flushPendingOrThrowAsync: vi.fn(() => Promise.resolve())
diff --git a/src/main/ipc/worktrees-canonical-ssh-listing.test.ts b/src/main/ipc/worktrees-canonical-ssh-listing.test.ts
index 0eccdba499b..59820525df6 100644
--- a/src/main/ipc/worktrees-canonical-ssh-listing.test.ts
+++ b/src/main/ipc/worktrees-canonical-ssh-listing.test.ts
@@ -47,6 +47,7 @@ import {
   hostWorktreeTarget,
   type HostWorktreeTarget
 } from '../../shared/worktree/host-qualified-identity'
+import { getDefaultWorkspaceSession } from '../../shared/constants'
 
 const path = '/remote/repository'
 const rows = [{ path, head: 'abc', branch: 'main', isBare: false, isMainWorktree: true }]
@@ -68,7 +69,11 @@ function fixture(fields: Pick<Repo, 'connectionId' | 'executionHostId'>) {
     getAllWorktreeLineage: () => ({}),
     getAllWorkspaceLineage: () => ({}),
     removeWorktreeLineage: vi.fn(),
-    captureNativeLocalWorktreeMetadataScanExpectation: () => undefined
+    captureNativeLocalWorktreeMetadataScanExpectation: () => undefined,
+    // Read by main's switch-on layout purge after a listing.
+    getWorkspaceSessionHostIds: () => ['local'],
+    getWorkspaceSession: () => getDefaultWorkspaceSession(),
+    removeWorkspaceSessionStateForWorktree: vi.fn()
   }
   const context = {
     store,
diff --git a/src/main/ipc/worktrees-test-harness.ts b/src/main/ipc/worktrees-test-harness.ts
index f583c275791..daaee96560d 100644
--- a/src/main/ipc/worktrees-test-harness.ts
+++ b/src/main/ipc/worktrees-test-harness.ts
@@ -10,6 +10,7 @@ import { resetSshProviderAuthorities } from '../ssh/ssh-provider-authority'
 import { createWorktreeRuntimeStub, type WorktreeRuntimeStub } from './worktrees-test-runtime-stub'
 import { handlers, mainWindow, store } from './worktrees-test-ipc-surface'
 import { configureMetadataPruningStoreMocks } from './worktrees-test-metadata-pruning-store'
+import { getDefaultWorkspaceSession } from '../../shared/constants'
 import {
   ORIGINAL_PLATFORM,
   setPlatform,
@@ -143,6 +144,8 @@ export function setupWorktreeHandlers(): WorktreeRuntimeStub {
     store.removeWorktreeMeta,
     store.pruneSessionlessMissingLocalWorktreeMetadataForRepo,
     store.removeWorkspaceSessionStateForWorktree,
+    store.getWorkspaceSessionHostIds,
+    store.getWorkspaceSession,
     store.getAllWorktreeLineage,
     store.removeWorktreeLineage,
     store.getAllWorkspaceLineage,
@@ -220,6 +223,8 @@ export function setupWorktreeHandlers(): WorktreeRuntimeStub {
       updatedAt: 0
     }
   ])
+  store.getWorkspaceSessionHostIds.mockReturnValue(['local'])
+  store.getWorkspaceSession.mockReturnValue(getDefaultWorkspaceSession())
   store.getAllWorktreeLineage.mockReturnValue({})
   store.getAllWorkspaceLineage.mockReturnValue({})
   store.getFolderWorkspaces.mockReturnValue([])
diff --git a/src/main/ipc/worktrees-test-ipc-surface.ts b/src/main/ipc/worktrees-test-ipc-surface.ts
index 4c23765fe15..5be261ad5d3 100644
--- a/src/main/ipc/worktrees-test-ipc-surface.ts
+++ b/src/main/ipc/worktrees-test-ipc-surface.ts
@@ -37,6 +37,9 @@ export type TestStore = {
   removeWorktreeMeta: TargetStoreMock
   pruneSessionlessMissingLocalWorktreeMetadataForRepo: StoreMock
   removeWorkspaceSessionStateForWorktree: KeyedStoreMock
+  /** Read by main's switch-on layout purge after a listing. */
+  getWorkspaceSessionHostIds: StoreMock
+  getWorkspaceSession: StoreMock
   getAllWorktreeLineage: StoreMock
   removeWorktreeLineage: TargetStoreMock
   getAllWorkspaceLineage: StoreMock
@@ -72,6 +75,8 @@ export const store: TestStore = {
   removeWorktreeMeta: vi.fn(),
   pruneSessionlessMissingLocalWorktreeMetadataForRepo: vi.fn(),
   removeWorkspaceSessionStateForWorktree: vi.fn(),
+  getWorkspaceSessionHostIds: vi.fn(),
+  getWorkspaceSession: vi.fn(),
   getAllWorktreeLineage: vi.fn(),
   removeWorktreeLineage: vi.fn(),
   getAllWorkspaceLineage: vi.fn(),
diff --git a/src/main/persistence-pane-identity-migration.test.ts b/src/main/persistence-pane-identity-migration.test.ts
index 882cd8fa2e8..11bbc49881c 100644
--- a/src/main/persistence-pane-identity-migration.test.ts
+++ b/src/main/persistence-pane-identity-migration.test.ts
@@ -1,9 +1,10 @@
 import {
   closeTestStores,
-  testState,
   createStore,
-  writeDataFile,
-  readDataFile
+  readDataFile,
+  testState,
+  workspaceRepos,
+  writeDataFile
 } from './persistence-test-harness'
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { writeFileSync, rmSync, mkdtempSync, mkdirSync } from 'node:fs'
@@ -77,7 +78,7 @@ describe('Store', () => {
     mkdirSync(agentHooksDir, { recursive: true })
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
@@ -189,7 +190,7 @@ describe('Store', () => {
     mkdirSync(agentHooksDir, { recursive: true })
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
@@ -300,7 +301,7 @@ describe('Store', () => {
     mkdirSync(agentHooksDir, { recursive: true })
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
@@ -396,7 +397,7 @@ describe('Store', () => {
     const ownerPaneKey = makePaneKey('tab-target', TEST_LEAF_2)
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
@@ -432,7 +433,7 @@ describe('Store', () => {
   it('persists fallback aliases when a legacy split layout has no PTY leaf bindings', async () => {
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
@@ -522,7 +523,7 @@ describe('Store', () => {
     const leafCount = 130_000
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
@@ -578,7 +579,7 @@ describe('Store', () => {
     const stablePaneKey = makePaneKey('tab1', TEST_LEAF_1)
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
diff --git a/src/main/persistence-pty-binding-reconciliation.test.ts b/src/main/persistence-pty-binding-reconciliation.test.ts
index 364960a1c5a..378b9263d68 100644
--- a/src/main/persistence-pty-binding-reconciliation.test.ts
+++ b/src/main/persistence-pty-binding-reconciliation.test.ts
@@ -1,9 +1,11 @@
 import {
   closeTestStores,
-  testState,
   createStore,
-  writeDataFile,
-  makeTerminalTab
+  makeTerminalTab,
+  registerWorkspaceRepos,
+  testState,
+  workspaceRepos,
+  writeDataFile
 } from './persistence-test-harness'
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { rmSync, mkdtempSync } from 'node:fs'
@@ -68,7 +70,7 @@ describe('Store', () => {
   it('remaps legacy SSH lease leaf ids by PTY when the layout is already normalized', async () => {
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
@@ -320,6 +322,7 @@ describe('Store', () => {
 
   it('reconciles only the incarnation of an unchanged durable PTY binding', async () => {
     const store = await createStore()
+    registerWorkspaceRepos(store, 'wt1')
     const paneKey = `tab1:${TEST_LEAF_1}`
     store.setWorkspaceSession({
       ...getDefaultWorkspaceSession(),
diff --git a/src/main/persistence-split-pane-incarnation.test.ts b/src/main/persistence-split-pane-incarnation.test.ts
index 9be1133cb66..c34cf951ca1 100644
--- a/src/main/persistence-split-pane-incarnation.test.ts
+++ b/src/main/persistence-split-pane-incarnation.test.ts
@@ -1,8 +1,9 @@
 import {
   closeTestStores,
-  testState,
   createStore,
-  makeTerminalTab
+  makeTerminalTab,
+  registerWorkspaceRepos,
+  testState
 } from './persistence-test-harness'
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 
@@ -290,6 +291,7 @@ describe('Store', () => {
     'preserves a reconciled incarnation across a renderer snapshot ($label)',
     async ({ hostId }) => {
       const store = await createStore()
+      registerWorkspaceRepos(store, 'wt1')
       const paneKey = `tab1:${TEST_LEAF_1}`
       store.setWorkspaceSession(
         {
diff --git a/src/main/persistence-test-harness.ts b/src/main/persistence-test-harness.ts
index 5a9d54a8348..75980f46788 100644
--- a/src/main/persistence-test-harness.ts
+++ b/src/main/persistence-test-harness.ts
@@ -5,6 +5,7 @@ import type { Project, ProjectHostSetup } from '../shared/project-types'
 import type { Repo } from '../shared/repo-types'
 import type { WorkspaceLineage, WorktreeLineage } from '../shared/worktree/lineage-types'
 import { folderWorkspaceKey, worktreeWorkspaceKey } from '../shared/workspace-scope'
+import { getRepoIdFromWorktreeId } from '../shared/worktree/id'
 import type { PersistedState } from '../shared/persisted-state-types'
 import { getDefaultPersistedState } from '../shared/constants'
 import type { SshRemotePtyLease } from '../shared/ssh-types'
@@ -165,6 +166,23 @@ export const makeRepo = (overrides: Partial<Repo> = {}): Repo => ({
   ...overrides
 })
 
+/**
+ * The repos of the given workspaces, as a real profile registers them: main's load drops a stored
+ * layout whose repo is not registered.
+ */
+export function workspaceRepos(...worktreeIds: string[]): Repo[] {
+  return [...new Set(worktreeIds.map(getRepoIdFromWorktreeId))].map((id) => makeRepo({ id }))
+}
+
+/** Registers the repos of the given workspaces in a running store; see `workspaceRepos`. */
+export function registerWorkspaceRepos(store: Store, ...worktreeIds: string[]): void {
+  for (const repo of workspaceRepos(...worktreeIds)) {
+    if (!store.getRepo(repo.id)) {
+      store.addRepo(repo)
+    }
+  }
+}
+
 export const makeProject = (overrides: Partial<Project> = {}): Project => ({
   id: 'project-1',
   displayName: 'Project',
diff --git a/src/main/persistence-workspace-session-scrollback.test.ts b/src/main/persistence-workspace-session-scrollback.test.ts
index 873e82615bb..658e9a3094e 100644
--- a/src/main/persistence-workspace-session-scrollback.test.ts
+++ b/src/main/persistence-workspace-session-scrollback.test.ts
@@ -9,13 +9,14 @@ import { MAX_BROWSER_HISTORY_ENTRIES } from '../shared/workspace-session-browser
 import {
   closeTestStores,
   createSqliteTestStore,
-  readPersistedStateJson,
-  testState,
   createStore,
   dataFile,
-  writeDataFile,
   makeRepo,
-  makeTerminalTab
+  makeTerminalTab,
+  readPersistedStateJson,
+  testState,
+  workspaceRepos,
+  writeDataFile
 } from './persistence-test-harness'
 import {
   TEST_LEAF_1,
@@ -457,7 +458,7 @@ describe('Store', () => {
   it('remaps legacy SSH lease leaf ids when loading legacy workspace layouts', async () => {
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
@@ -519,7 +520,7 @@ describe('Store', () => {
     mkdirSync(agentHooksDir, { recursive: true })
     writeDataFile({
       schemaVersion: 1,
-      repos: [],
+      repos: workspaceRepos('wt1'),
       worktreeMeta: {},
       settings: {},
       ui: {},
diff --git a/src/main/persistence-worktree-deletion-fencing.test.ts b/src/main/persistence-worktree-deletion-fencing.test.ts
index 1dcf80578d8..81322ecbf17 100644
--- a/src/main/persistence-worktree-deletion-fencing.test.ts
+++ b/src/main/persistence-worktree-deletion-fencing.test.ts
@@ -1,8 +1,9 @@
 import {
   closeTestStores,
-  testState,
   createStore,
-  makeTerminalTab
+  makeTerminalTab,
+  registerWorkspaceRepos,
+  testState
 } from './persistence-test-harness'
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { rmSync, mkdtempSync } from 'node:fs'
@@ -68,6 +69,7 @@ describe('Store', () => {
   })
   it('adds a missing split leaf to the durable root when a new pane spawns before layout debounce', async () => {
     const store = await createStore()
+    registerWorkspaceRepos(store, 'wt1')
     store.setWorkspaceSession({
       activeRepoId: 'r1',
       activeWorktreeId: 'wt1',
diff --git a/src/main/persistence/loading-store/pty-binding-placement-inertness.test.ts b/src/main/persistence/loading-store/pty-binding-placement-inertness.test.ts
index 1ca2e60f7c9..3e50e41ae09 100644
--- a/src/main/persistence/loading-store/pty-binding-placement-inertness.test.ts
+++ b/src/main/persistence/loading-store/pty-binding-placement-inertness.test.ts
@@ -8,6 +8,9 @@ import { fixture } from './profile-state-delayed-authority-fixture'
 import type * as AgreementModule from '../terminal-topology/terminal-pane-placement-agreement'
 import type { PersistPtyBindingArgs } from './pty-binding-persistence'
 
+// rollback build (switch-off) — delete with the switch-off code post-flip
+vi.mock('../../../shared/layout-owner-switch', () => ({ ORCA_LAYOUT_OWNER_IS_MAIN: false }))
+
 const agreementCheck = vi.hoisted(() => ({ throws: false }))
 vi.mock('../terminal-topology/terminal-pane-placement-agreement', async (importOriginal) => {
   const actual = await importOriginal<typeof AgreementModule>()
diff --git a/src/main/persistence/terminal-topology/terminal-late-bind-layout-rules.test.ts b/src/main/persistence/terminal-topology/terminal-late-bind-layout-rules.test.ts
index 67372b9c161..640fcd81efe 100644
--- a/src/main/persistence/terminal-topology/terminal-late-bind-layout-rules.test.ts
+++ b/src/main/persistence/terminal-topology/terminal-late-bind-layout-rules.test.ts
@@ -14,6 +14,9 @@ import {
 import { WindowSession, withoutLeaf } from './terminal-topology-window-session-fixture'
 import { checkWorkspaceLayoutRules } from '../../../shared/workspace-layout/workspace-layout-rules'
 
+// rollback build (switch-off) — delete with the switch-off code post-flip
+vi.mock('../../../shared/layout-owner-switch', () => ({ ORCA_LAYOUT_OWNER_IS_MAIN: false }))
+
 vi.mock('../../telemetry/client', () => ({ track: vi.fn() }))
 vi.mock('../../ssh/config/ssh-config-parser', () => ({
   loadUserSshConfig: () => ({ hosts: [] }),
diff --git a/src/main/runtime/agent-session-tab-layout.test.ts b/src/main/runtime/agent-session-tab-layout.test.ts
index 5b6c4adc330..4609ac2b344 100644
--- a/src/main/runtime/agent-session-tab-layout.test.ts
+++ b/src/main/runtime/agent-session-tab-layout.test.ts
@@ -1,7 +1,10 @@
-import { describe, expect, it } from 'vitest'
+import { describe, expect, it, vi } from 'vitest'
 import type { WorkspaceSessionState } from '../../shared/workspace-session-state-types'
 import { withChatTabChanges, type ChatTabChange } from './agent-session-tab-layout'
 
+// rollback build (switch-off) — delete with the switch-off code post-flip
+vi.mock('../../shared/layout-owner-switch', () => ({ ORCA_LAYOUT_OWNER_IS_MAIN: false }))
+
 const WT = 'repo-1::/tmp/worktree-a'
 
 const EMPTY: WorkspaceSessionState = {
diff --git a/src/main/runtime/orca-runtime-serve-browser-restore.test.ts b/src/main/runtime/orca-runtime-serve-browser-restore.test.ts
index ba4cf19857a..afd7d8798b2 100644
--- a/src/main/runtime/orca-runtime-serve-browser-restore.test.ts
+++ b/src/main/runtime/orca-runtime-serve-browser-restore.test.ts
@@ -181,8 +181,13 @@ describe('a saved browser tab on a host with no window', () => {
       page: 'page-new'
     })
 
-    const saved = getSession().browserTabsByWorktree?.[TEST_WORKTREE_ID] ?? []
-    expect(saved.map((tab) => [tab.id, tab.url])).toEqual([['page-new', 'https://example.com/new']])
+    // A switch-on build saves the page once main's layout write lands.
+    await vi.waitFor(() => {
+      const saved = getSession().browserTabsByWorktree?.[TEST_WORKTREE_ID] ?? []
+      expect(saved.map((tab) => [tab.id, tab.url])).toEqual([
+        ['page-new', 'https://example.com/new']
+      ])
+    })
     // Which tab shows is a view's choice; opening one records none.
     expect(getSession().activeBrowserTabIdByWorktree?.[TEST_WORKTREE_ID]).toBeUndefined()
   })
diff --git a/src/main/runtime/orca-runtime-terminal-split-authority.test.ts b/src/main/runtime/orca-runtime-terminal-split-authority.test.ts
index 080327db04c..dbbd6331521 100644
--- a/src/main/runtime/orca-runtime-terminal-split-authority.test.ts
+++ b/src/main/runtime/orca-runtime-terminal-split-authority.test.ts
@@ -6,6 +6,7 @@ import type { TerminalLayoutSnapshot } from '../../shared/terminal-tab-types'
 import type { WorkspaceSessionState } from '../../shared/workspace-session-state-types'
 import { makePaneKey } from '../../shared/stable-pane-id'
 import { OrcaRuntimeService } from './orca-runtime'
+import { ORCA_LAYOUT_OWNER_IS_MAIN } from '../../shared/layout-owner-switch'
 
 const REPO_ID = 'repo-1'
 const WORKTREE_ID = `${REPO_ID}::/workspace`
@@ -222,6 +223,9 @@ function createHarness(
         }
       }
     },
+    /** The split pane's terminal starting, as its window reports it. */
+    registerSplitPty: (leafId: string) =>
+      runtime.registerPty(SPLIT_PTY_ID, WORKTREE_ID, connectionId, { tabId: TAB_ID, leafId }),
     resolveReveal: () => resolveReveal?.({ tabId: TAB_ID }),
     resolveSpawn: () => resolveSpawn?.({ id: SPLIT_PTY_ID, incarnationId: SPLIT_INCARNATION_ID })
   }
@@ -232,6 +236,8 @@ describe('remote runtime terminal split authority', () => {
     const harness = createHarness(true, { rendererMounted: true, graphOnlySource: true })
 
     const split = harness.runtime.splitTerminal(harness.handle, { direction: 'vertical' })
+    // A switch-on build tells the window once main has admitted the new pane.
+    await vi.waitFor(() => expect(harness.rendererSplitTerminal).toHaveBeenCalled())
 
     const newLeafId = harness.rendererSplitTerminal.mock.calls[0]?.[2]?.newLeafId
     expect(newLeafId).toEqual(expect.any(String))
@@ -246,6 +252,10 @@ describe('remote runtime terminal split authority', () => {
       telemetrySource: undefined,
       newLeafId
     })
+    // A switch-on build binds the new pane only once its terminal registered, before the graph.
+    if (ORCA_LAYOUT_OWNER_IS_MAIN) {
+      harness.registerSplitPty(newLeafId)
+    }
     harness.runtime.syncWindowGraph(1, {
       tabs: [
         {
diff --git a/src/main/runtime/orca-runtime-test-fixtures.spec.ts b/src/main/runtime/orca-runtime-test-fixtures.spec.ts
index 0f8c72449a3..f6511cb98e5 100644
--- a/src/main/runtime/orca-runtime-test-fixtures.spec.ts
+++ b/src/main/runtime/orca-runtime-test-fixtures.spec.ts
@@ -9,6 +9,9 @@ import {
 } from '../../shared/agent-prompt-injection'
 import { getDefaultWorkspaceSession } from '../../shared/constants'
 import { makePaneKey } from '../../shared/stable-pane-id'
+import type { LayoutCommand } from '../../shared/workspace-layout/workspace-layout-command-types'
+import { applyCommandToSession } from '../persistence/terminal-topology/terminal-pane-admission'
+import { resolveHostId } from '../persistence/loading-store/session-host-partitions'
 import { toComparableRelaySshPtyId } from '../../shared/ssh-pty-id'
 import { spawnedPtyEnv } from './spawned-pty-env-fixtures'
 import {
@@ -589,6 +592,16 @@ function makeRuntimeStoreWithWorkspaceSession(
       getWorkspaceSession: (hostId?: string) =>
         hostId === undefined || hostId === ownerHostId ? session : getDefaultWorkspaceSession(),
       setWorkspaceSession: vi.fn(setSession),
+      // Main's `layout.apply` on this partition, as the Store applies it (switch-on writes).
+      applyLayoutCommand: vi.fn(async (command: LayoutCommand, hostIds?: readonly string[]) => {
+        const change = applyCommandToSession(
+          resolveHostId(hostIds?.[0] ?? ownerHostId),
+          session,
+          command
+        )
+        session = change.session ?? session
+        return change.value
+      }),
       // Headless close is a durable transaction; keep the in-memory fixture's
       // persistence contract equivalent to the production store.
       flushOrThrow: vi.fn(),
diff --git a/src/main/runtime/rpc/methods/structured-agent-session-rpc.test-fixture.ts b/src/main/runtime/rpc/methods/structured-agent-session-rpc.test-fixture.ts
index 739d470cc04..09cf9ceeb46 100644
--- a/src/main/runtime/rpc/methods/structured-agent-session-rpc.test-fixture.ts
+++ b/src/main/runtime/rpc/methods/structured-agent-session-rpc.test-fixture.ts
@@ -21,6 +21,7 @@ import { RpcDispatcher } from '../dispatcher'
 import { STRUCTURED_AGENT_SESSION_METHODS } from './structured-agent-session'
 import { createStructuredAgentSessionLogger } from '../../../native-chat/agent-session-wire/structured-agent-session-logger'
 import { recordingStructuredAgentSessionLogger } from '../../../native-chat/agent-session-wire/structured-agent-session-logger-test-support'
+import { resetStructuredAgentSessionClosesBeforeCreateForTests } from './structured-agent-session-close-before-create'
 
 export const SESSION = 'session-alpha'
 export const FINGERPRINT = 'f'.repeat(64)
@@ -210,9 +211,9 @@ export function hostStub(): StructuredAgentSessionHost {
     subscribeTurnCompletions: vi.fn(() => () => undefined),
     unsubscribe: vi.fn()
   })
+  const store = { getOperationRow: vi.fn(() => null), getRecord: vi.fn(() => null) }
   // Not a call: the logger the host hands a runtime caller that reports for it.
-  const logger = recordingStructuredAgentSessionLogger().logger
-  Reflect.set(hostCalls, 'deps', { logger, store: { getOperationRow: vi.fn(() => null) } })
+  Reflect.set(hostCalls, 'deps', { logger: recordingStructuredAgentSessionLogger().logger, store })
   return hostCalls as unknown as StructuredAgentSessionHost
 }
 
@@ -322,6 +323,8 @@ export const STRUCTURED_MOBILE_CLIENT = {
 
 /** Every suite wants the same lifecycle: a fresh stub per test, no host left installed. */
 export function installStructuredHostStub(): void {
+  // A switch-on close remembers the session, refusing its later create; each test starts clean.
+  resetStructuredAgentSessionClosesBeforeCreateForTests()
   setStructuredAgentSessionHost(hostStub())
 }
 
diff --git a/src/renderer/src/components/editor/editor-autosave-controller-test-fixture.ts b/src/renderer/src/components/editor/editor-autosave-controller-test-fixture.ts
index 4bb93ede790..4e91f57a874 100644
--- a/src/renderer/src/components/editor/editor-autosave-controller-test-fixture.ts
+++ b/src/renderer/src/components/editor/editor-autosave-controller-test-fixture.ts
@@ -6,6 +6,7 @@ import { createStore, type StoreApi } from 'zustand/vanilla'
 import { createEditorSlice } from '@/store/slices/editor'
 import type { AppState } from '@/store'
 import { makeWorktree } from '@/store/slices/worktrees-slice-test-fixtures'
+import { emptyTabLayoutState } from '@/store/slices/editor-slice-test-harness'
 
 export type FakeEditorDisk = {
   files: Map<string, string>
@@ -71,8 +72,9 @@ export function stubEditorWindow(): ReturnType<typeof vi.fn> {
 }
 
 export function createEditorStore(): StoreApi<AppState> {
-  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  // oxlint-disable-next-line typescript/no-explicit-any, typescript/consistent-type-assertions -- SAFETY: a partial store; tests read only the slices it builds.
   return createStore<any>()((...args: any[]) => ({
+    ...emptyTabLayoutState(),
     activeWorktreeId: 'wt-1',
     repos: [],
     worktreesByRepo: {
diff --git a/src/renderer/src/components/editor/editor-autosave-controller.test.ts b/src/renderer/src/components/editor/editor-autosave-controller.test.ts
index 5ee2f60e78b..bddb243e8af 100644
--- a/src/renderer/src/components/editor/editor-autosave-controller.test.ts
+++ b/src/renderer/src/components/editor/editor-autosave-controller.test.ts
@@ -16,6 +16,7 @@ import {
 } from '@/runtime/runtime-compatibility-test-fixture'
 import { clearRuntimeCompatibilityCacheForTests } from '@/runtime/runtime-rpc-client'
 import { folderWorkspaceKey } from '../../../../shared/workspace-scope'
+import { emptyTabLayoutState } from '../../store/slices/editor-slice-test-harness'
 
 const mocks = vi.hoisted(() => ({
   getConnectionIdForFile: vi.fn()
@@ -42,8 +43,9 @@ type WindowStub = {
 }
 
 function createEditorStore(): StoreApi<AppState> {
-  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  // oxlint-disable-next-line typescript/no-explicit-any, typescript/consistent-type-assertions -- SAFETY: a partial store; tests read only the slices it builds.
   return createStore<any>()((...args: any[]) => ({
+    ...emptyTabLayoutState(),
     activeWorktreeId: 'wt-1',
     settings: {
       editorAutoSave: true,
diff --git a/src/renderer/src/components/editor/editor-save-user-named-tab.test.ts b/src/renderer/src/components/editor/editor-save-user-named-tab.test.ts
index 36818ef2d24..b123f3afe96 100644
--- a/src/renderer/src/components/editor/editor-save-user-named-tab.test.ts
+++ b/src/renderer/src/components/editor/editor-save-user-named-tab.test.ts
@@ -6,12 +6,14 @@ import { FLOATING_TERMINAL_WORKTREE_ID } from '../../../../shared/constants'
 import { ORCA_EDITOR_SAVE_DIRTY_FILES_EVENT } from '../../../../shared/editor-save-events'
 import { attachEditorAutosaveController } from './editor-autosave-controller'
 import { __clearSelfWriteRegistryForTests } from './editor-self-write-registry'
+import { emptyTabLayoutState } from '../../store/slices/editor-slice-test-harness'
 
 vi.mock('@/lib/connection-context', () => ({ getConnectionIdForFile: () => null }))
 
 function createEditorStore(): StoreApi<AppState> {
   // oxlint-disable-next-line typescript/consistent-type-assertions, typescript/no-explicit-any -- SAFETY: the editor save path reads only the slice and fields built here.
   return createStore<any>()((...args: any[]) => ({
+    ...emptyTabLayoutState(),
     settings: { editorAutoSave: false },
     repos: [],
     worktreesByRepo: {
@@ -71,8 +73,10 @@ describe('saving a restored tab', () => {
     store
       .getState()
       .openFile({ filePath, relativePath, worktreeId, language: 'markdown', mode: 'edit' })
-    store.getState().setEditorDraft(filePath, 'edited')
-    store.getState().markFileDirty(filePath, true)
+    // The file's own id: its path, or an owned id when main owns the layout.
+    const fileId = store.getState().openFiles.find((file) => file.filePath === filePath)!.id
+    store.getState().setEditorDraft(fileId, 'edited')
+    store.getState().markFileDirty(fileId, true)
     const detach = attachEditorAutosaveController(store)
     try {
       await saveDirtyFiles()
diff --git a/src/renderer/src/components/floating-terminal/floating-workspace-tab-reorder.test.ts b/src/renderer/src/components/floating-terminal/floating-workspace-tab-reorder.test.ts
index 6c1d0930af9..a5215217552 100644
--- a/src/renderer/src/components/floating-terminal/floating-workspace-tab-reorder.test.ts
+++ b/src/renderer/src/components/floating-terminal/floating-workspace-tab-reorder.test.ts
@@ -9,6 +9,14 @@ import type { Tab, TabGroup } from '../../../../shared/tab-types'
 import { useAppStore } from '../../store'
 import { useTabDragSplit, type TabDragItemData } from '../tab-group/useTabDragSplit'
 import { isFloatingTerminalDragTarget } from './floating-terminal-titlebar-drag-target'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
+
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
 
 vi.mock('../browser-pane/host-guest/webview-registry', () => ({
   acquireWebviewsDragPassthrough: vi.fn(() => vi.fn())
@@ -145,23 +153,27 @@ afterEach(() => {
 })
 
 describe('floating workspace tab reorder', () => {
-  it('moves a tab after the hovered tab when released on its right half', () => {
+  it('moves a tab after the hovered tab when released on its right half', async () => {
     dropTab('tab-a', 'tab-c', 290)
+    await layoutApplyTestTransport.settle()
     expect(floatingTabOrder()).toEqual(['tab-b', 'tab-c', 'tab-a'])
   })
 
-  it('moves a tab before the hovered tab when released on its left half', () => {
+  it('moves a tab before the hovered tab when released on its left half', async () => {
     dropTab('tab-c', 'tab-a', 210)
+    await layoutApplyTestTransport.settle()
     expect(floatingTabOrder()).toEqual(['tab-c', 'tab-a', 'tab-b'])
   })
 
-  it('leaves the order alone when a tab is dropped on itself', () => {
+  it('leaves the order alone when a tab is dropped on itself', async () => {
     dropTab('tab-b', 'tab-b', 290)
+    await layoutApplyTestTransport.settle()
     expect(floatingTabOrder()).toEqual(['tab-a', 'tab-b', 'tab-c'])
   })
 
-  it('republishes sortOrder so the panel renders the committed order', () => {
+  it('republishes sortOrder so the panel renders the committed order', async () => {
     dropTab('tab-a', 'tab-c', 290)
+    await layoutApplyTestTransport.settle()
     const tabs = useAppStore.getState().unifiedTabsByWorktree[FLOATING_TERMINAL_WORKTREE_ID] ?? []
     expect(tabs.map((tab) => [tab.id, tab.sortOrder])).toEqual([
       ['tab-a', 2],
diff --git a/src/renderer/src/components/tab-bar/TabBar.context-menu.test.ts b/src/renderer/src/components/tab-bar/TabBar.context-menu.test.ts
index b6cf6641781..d0be80e55d7 100644
--- a/src/renderer/src/components/tab-bar/TabBar.context-menu.test.ts
+++ b/src/renderer/src/components/tab-bar/TabBar.context-menu.test.ts
@@ -5,6 +5,14 @@ import {
   stubShallowSelector,
   stubTooltip
 } from './tab-bar-windows-shell-launch-render-stubs'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
+
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
 
 const appStoreSnapshot: {
   activeTabId: string | null
@@ -452,6 +460,7 @@ describe('TabBar context menu wiring', () => {
     expect(sortable[0].props.isPinned).toBe(true)
 
     ;(sortable[0].props.onTogglePin as () => void)()
+    await layoutApplyTestTransport.settle()
 
     expect(unpinTabMock).toHaveBeenCalledWith('unified-term-1')
     expect(pinTabMock).not.toHaveBeenCalled()
diff --git a/src/renderer/src/components/tab-group/TabGroupSplitLayout.drag.test.tsx b/src/renderer/src/components/tab-group/TabGroupSplitLayout.drag.test.tsx
index 8cd7374b86a..aa50fe497f6 100644
--- a/src/renderer/src/components/tab-group/TabGroupSplitLayout.drag.test.tsx
+++ b/src/renderer/src/components/tab-group/TabGroupSplitLayout.drag.test.tsx
@@ -13,13 +13,19 @@ globalThis.IS_REACT_ACT_ENVIRONMENT = true
 const setTabGroupSplitRatioMock = vi.fn()
 const recordFeatureInteractionMock = vi.fn()
 
-vi.mock('../../store', () => ({
-  useAppStore: (selector: (state: Record<string, unknown>) => unknown) =>
-    selector({
-      recordFeatureInteraction: recordFeatureInteractionMock,
-      setTabGroupSplitRatio: setTabGroupSplitRatioMock
-    })
-}))
+vi.mock('../../store', () => {
+  const state = (): Record<string, unknown> => ({
+    recordFeatureInteraction: recordFeatureInteractionMock,
+    setTabGroupSplitRatio: setTabGroupSplitRatioMock,
+    layoutByWorktree: {}
+  })
+  // The switch-on ratio commit reads the store directly.
+  const useAppStore = Object.assign(
+    (selector: (state: Record<string, unknown>) => unknown) => selector(state()),
+    { getState: state }
+  )
+  return { useAppStore }
+})
 
 vi.mock('./TabGroupPanel', () => ({
   default: ({ groupId }: { groupId: string }) => <div data-testid={`panel-${groupId}`} />
diff --git a/src/renderer/src/components/terminal-pane/launch-parity-window-tab.test.ts b/src/renderer/src/components/terminal-pane/launch-parity-window-tab.test.ts
index 6ba97e10cd8..9c4dc519574 100644
--- a/src/renderer/src/components/terminal-pane/launch-parity-window-tab.test.ts
+++ b/src/renderer/src/components/terminal-pane/launch-parity-window-tab.test.ts
@@ -29,6 +29,13 @@ import {
   type WindowLaunchCase
 } from '../../../../shared/launch-parity-window-request.test-fixture'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const holder = vi.hoisted((): LaunchStoreHolder => ({ store: null }))
 vi.mock('@/store', async () =>
   (await import('@/lib/launch-parity-renderer.test-fixture')).launchStoreModuleMock(holder)
diff --git a/src/renderer/src/components/terminal-pane/pty-connection-pty-exit-teardown.spec.ts b/src/renderer/src/components/terminal-pane/pty-connection-pty-exit-teardown.spec.ts
index 9d67ce14d49..8ffea7c279e 100644
--- a/src/renderer/src/components/terminal-pane/pty-connection-pty-exit-teardown.spec.ts
+++ b/src/renderer/src/components/terminal-pane/pty-connection-pty-exit-teardown.spec.ts
@@ -3,6 +3,7 @@ import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { makePaneKey } from '../../../../shared/stable-pane-id'
 import { sendTerminalInputThroughPane } from './pty-connection-test-dom'
 import { flushAsyncTicks } from './pty-connection-test-async'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
 import {
   LEAF_1,
   LEAF_2,
@@ -259,6 +260,8 @@ export function registerPtyConnectionPtyExitTeardownTests(): void {
       expect(onPtyExit).toBeTypeOf('function')
 
       onPtyExit?.('pty-pane-2')
+      // A switch-on build closes it on main's answer.
+      await layoutApplyTestTransport.settle()
 
       expect(deps.clearExitedPanePtyLayoutBinding).toHaveBeenCalledWith(2, 'pty-pane-2')
       expect(deps.onPtyExitRef.current).not.toHaveBeenCalled()
@@ -760,6 +763,8 @@ export function registerPtyConnectionPtyExitTeardownTests(): void {
 
       capturedDataCallback.current?.('shell prompt')
       onPtyExit?.('pty-pane-2')
+      // A switch-on build closes it on main's answer.
+      await layoutApplyTestTransport.settle()
 
       expect(manager.closePane).toHaveBeenCalledWith(2)
     })
@@ -809,6 +814,8 @@ export function registerPtyConnectionPtyExitTeardownTests(): void {
 
       terminalInputHandler?.('exit\r')
       onPtyExit?.('pty-pane-2')
+      // A switch-on build closes it on main's answer.
+      await layoutApplyTestTransport.settle()
 
       expect(manager.closePane).toHaveBeenCalledWith(2)
     })
diff --git a/src/renderer/src/components/terminal-pane/pty-connection-restore-and-resume.test.ts b/src/renderer/src/components/terminal-pane/pty-connection-restore-and-resume.test.ts
index b7fba75ab8b..884de20db45 100644
--- a/src/renderer/src/components/terminal-pane/pty-connection-restore-and-resume.test.ts
+++ b/src/renderer/src/components/terminal-pane/pty-connection-restore-and-resume.test.ts
@@ -1,4 +1,4 @@
-import { describe } from 'vitest'
+import { describe, vi } from 'vitest'
 import { registerPtyConnectionAgentSessionResumeTests } from './pty-connection-agent-session-resume.spec'
 import { registerPtyConnectionColdRestoreAgentResumeTests } from './pty-connection-cold-restore-agent-resume.spec'
 import { registerPtyConnectionColdRestoreRepaintTests } from './pty-connection-cold-restore-repaint.spec'
@@ -9,6 +9,13 @@ import { registerPtyConnectionRestartReplacementTests } from './pty-connection-r
 import { registerPtyConnectionSessionLivenessTests } from './pty-connection-session-liveness.spec'
 import { registerPtyConnectionSleepingResumeBannerTests } from './pty-connection-sleeping-resume-banner.spec'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 describe('pty-connection-agent-session-resume', registerPtyConnectionAgentSessionResumeTests)
 describe(
   'pty-connection-cold-restore-agent-resume',
diff --git a/src/renderer/src/components/terminal-pane/pty-connection-session-liveness.spec.ts b/src/renderer/src/components/terminal-pane/pty-connection-session-liveness.spec.ts
index c58c559bd18..82571cc00c7 100644
--- a/src/renderer/src/components/terminal-pane/pty-connection-session-liveness.spec.ts
+++ b/src/renderer/src/components/terminal-pane/pty-connection-session-liveness.spec.ts
@@ -1,4 +1,5 @@
 import { bindPtyConnectionTestMocks } from './pty-connection-mocks.spec'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { flushAsyncTicks } from './pty-connection-test-async'
 import { sendTerminalInputThroughPane } from './pty-connection-test-dom'
@@ -348,6 +349,8 @@ export function registerPtyConnectionSessionLivenessTests(): void {
         // Restart rebinds the pane to a new live PTY; its later real exit must not be ignored by a stale guard.
         transport.getPtyId.mockReturnValue('pty-pane-2-restarted')
         onPtyExit?.('pty-pane-2-restarted')
+        // A switch-on build closes it on main's answer.
+        await layoutApplyTestTransport.settle()
 
         expect(manager.closePane).toHaveBeenCalledTimes(1)
         expect(manager.closePane).toHaveBeenCalledWith(2)
diff --git a/src/renderer/src/components/terminal-pane/pty-connection-test-pane-fixtures.ts b/src/renderer/src/components/terminal-pane/pty-connection-test-pane-fixtures.ts
index 9a6ec1be734..1e868695d03 100644
--- a/src/renderer/src/components/terminal-pane/pty-connection-test-pane-fixtures.ts
+++ b/src/renderer/src/components/terminal-pane/pty-connection-test-pane-fixtures.ts
@@ -238,6 +238,7 @@ export type MockPaneManager = {
   closePane: Mock
   getActivePane: Mock<() => { id: number; leafId?: string } | null>
   getNumericIdForLeaf: Mock<(leafId: string) => number | null>
+  getLeafId: Mock<(paneId: number) => string | null>
   setActivePane: Mock<(paneId: number) => void>
 }
 
@@ -265,6 +266,9 @@ export function createManager(
     getNumericIdForLeaf: vi.fn((leafId: string) => {
       return panes.find((candidate) => candidate.leafId === leafId)?.id ?? null
     }),
+    getLeafId: vi.fn((paneId: number) => {
+      return panes.find((candidate) => candidate.id === paneId)?.leafId ?? null
+    }),
     setActivePane: vi.fn((paneId: number) => {
       activePaneId = paneId
     })
diff --git a/src/renderer/src/components/terminal-pane/pty-connection-visibility-and-exit.test.ts b/src/renderer/src/components/terminal-pane/pty-connection-visibility-and-exit.test.ts
index adb2e1e0171..5e2944f2bec 100644
--- a/src/renderer/src/components/terminal-pane/pty-connection-visibility-and-exit.test.ts
+++ b/src/renderer/src/components/terminal-pane/pty-connection-visibility-and-exit.test.ts
@@ -1,4 +1,4 @@
-import { describe } from 'vitest'
+import { describe, vi } from 'vitest'
 import { registerPtyConnectionVisibilityResumeSizeTests } from './pty-connection-visibility-resume-size.spec'
 import { registerPtyConnectionHiddenCodexQueriesTests } from './pty-connection-hidden-codex-queries.spec'
 import { registerPtyConnectionHiddenDeliveryGateTests } from './pty-connection-hidden-delivery-gate.spec'
@@ -6,6 +6,13 @@ import { registerPtyConnectionParkRevealModelSnapshotTests } from './pty-connect
 import { registerPtyConnectionReplayPayloadHandlingTests } from './pty-connection-replay-payload-handling.spec'
 import { registerPtyConnectionPtyExitTeardownTests } from './pty-connection-pty-exit-teardown.spec'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 describe('pty-connection-visibility-resume-size', registerPtyConnectionVisibilityResumeSizeTests)
 describe('pty-connection-hidden-codex-queries', registerPtyConnectionHiddenCodexQueriesTests)
 describe('pty-connection-hidden-delivery-gate', registerPtyConnectionHiddenDeliveryGateTests)
diff --git a/src/renderer/src/lib/agent-launch-pane-verdict-application.test.ts b/src/renderer/src/lib/agent-launch-pane-verdict-application.test.ts
index e49cb2edf04..57267c70c8b 100644
--- a/src/renderer/src/lib/agent-launch-pane-verdict-application.test.ts
+++ b/src/renderer/src/lib/agent-launch-pane-verdict-application.test.ts
@@ -4,6 +4,13 @@ import type { AgentLaunchPaneVerdict } from '../../../shared/agent-launch-pane-v
 import { createTabsSliceMockApi } from '../store/slices/tabs-slice-test-harness'
 import { createTestStore } from '../store/slices/store-test-helpers'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 vi.mock('sonner', () => ({ toast: { info: vi.fn(), success: vi.fn(), error: vi.fn() } }))
 vi.mock('@/lib/agent-status', async (importOriginal) => ({
   ...(await importOriginal<typeof AgentStatusModule>()),
diff --git a/src/renderer/src/lib/launch-agent-background-session-launch-parity.test.ts b/src/renderer/src/lib/launch-agent-background-session-launch-parity.test.ts
index 22c561117de..bfa11b38040 100644
--- a/src/renderer/src/lib/launch-agent-background-session-launch-parity.test.ts
+++ b/src/renderer/src/lib/launch-agent-background-session-launch-parity.test.ts
@@ -19,6 +19,13 @@ import {
   launchWorkspaceId
 } from '../../../shared/launch-parity-window-request.test-fixture'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const holder = vi.hoisted((): LaunchStoreHolder => ({ store: null }))
 vi.mock('@/store', async () =>
   (await import('@/lib/launch-parity-renderer.test-fixture')).launchStoreModuleMock(holder)
diff --git a/src/renderer/src/lib/launch-agent-in-new-tab-main-surface.test.ts b/src/renderer/src/lib/launch-agent-in-new-tab-main-surface.test.ts
index c1c311d2731..0403b5a8c33 100644
--- a/src/renderer/src/lib/launch-agent-in-new-tab-main-surface.test.ts
+++ b/src/renderer/src/lib/launch-agent-in-new-tab-main-surface.test.ts
@@ -5,6 +5,13 @@ import { FLOATING_TERMINAL_WORKTREE_ID, getDefaultSettings } from '../../../shar
 import { createTestStore, makeWorktree, seedStore } from '../store/slices/store-test-helpers'
 import { createStoreCascadesMockApi } from '../store/slices/store-cascades-test-harness'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const storeBox = vi.hoisted(() => {
   const box: { store: unknown } = { store: null }
   return box
diff --git a/src/renderer/src/lib/launch-agent-structured-chat-guard.test.ts b/src/renderer/src/lib/launch-agent-structured-chat-guard.test.ts
index ee3898257dd..840744fa15a 100644
--- a/src/renderer/src/lib/launch-agent-structured-chat-guard.test.ts
+++ b/src/renderer/src/lib/launch-agent-structured-chat-guard.test.ts
@@ -105,6 +105,8 @@ const store = {
   allWorktrees: vi.fn(() => store.worktreesByRepo['repo-1']),
   tabsByWorktree: { 'wt-1': [{ id: 'tab-1' }] },
   unifiedTabsByWorktree: emptyUnifiedTabsByWorktree,
+  groupsByWorktree: {},
+  layoutByWorktree: {},
   activeGroupIdByWorktree: {},
   openFiles: [] as { id: string; worktreeId: string }[],
   browserTabsByWorktree: {} as Record<string, { id: string }[]>,
diff --git a/src/renderer/src/lib/launch-parity-plus-planner.test.ts b/src/renderer/src/lib/launch-parity-plus-planner.test.ts
index c3e37690f9a..c7c1b0f40ba 100644
--- a/src/renderer/src/lib/launch-parity-plus-planner.test.ts
+++ b/src/renderer/src/lib/launch-parity-plus-planner.test.ts
@@ -8,6 +8,13 @@ import { getDefaultSettings } from '../../../shared/constants'
 import { WIN_PATH, WSL_PATH } from '../../../shared/launch-parity-window-cases.test-fixture'
 import { launchWorkspaceId } from '../../../shared/launch-parity-window-request.test-fixture'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const holder = vi.hoisted((): LaunchStoreHolder => ({ store: null }))
 vi.mock('@/store', async () =>
   (await import('@/lib/launch-parity-renderer.test-fixture')).launchStoreModuleMock(holder)
diff --git a/src/renderer/src/lib/pi-session-resume-wake.test.ts b/src/renderer/src/lib/pi-session-resume-wake.test.ts
index 984a4fd9639..5a4e2951bde 100644
--- a/src/renderer/src/lib/pi-session-resume-wake.test.ts
+++ b/src/renderer/src/lib/pi-session-resume-wake.test.ts
@@ -1,11 +1,18 @@
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
-import { afterEach, describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import type { SleepingAgentSessionRecord } from '../../../shared/agent-session-resume'
 import type { TerminalTab } from '../../../shared/terminal-tab-types'
 import { useAppStore } from '@/store'
 import { resumeSleepingAgentSessionsForWorktree } from './resume-sleeping-agent-session'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 const PI_TRANSCRIPT_PATH = join(tmpdir(), 'pi-session-1.jsonl')
 
diff --git a/src/renderer/src/lib/resume-sleeping-agent-session-live-ssh-dedupe.test.ts b/src/renderer/src/lib/resume-sleeping-agent-session-live-ssh-dedupe.test.ts
index 117329b1e19..03b356f7f52 100644
--- a/src/renderer/src/lib/resume-sleeping-agent-session-live-ssh-dedupe.test.ts
+++ b/src/renderer/src/lib/resume-sleeping-agent-session-live-ssh-dedupe.test.ts
@@ -5,6 +5,13 @@ import { useAppStore } from '@/store'
 import { makeWorktree, TEST_REPO } from '../store/slices/worktrees-slice-test-fixtures'
 import { resumeSleepingAgentSessionsForWorktree } from './resume-sleeping-agent-session'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 const LEAF_ID = '11111111-1111-4111-8111-111111111111'
 const OTHER_LEAF_ID = '22222222-2222-4222-8222-222222222222'
diff --git a/src/renderer/src/lib/resume-sleeping-agent-session-provider-claim.test.ts b/src/renderer/src/lib/resume-sleeping-agent-session-provider-claim.test.ts
index b3c5075a4c8..80650f8dd63 100644
--- a/src/renderer/src/lib/resume-sleeping-agent-session-provider-claim.test.ts
+++ b/src/renderer/src/lib/resume-sleeping-agent-session-provider-claim.test.ts
@@ -1,10 +1,17 @@
-import { afterEach, describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import type { SleepingAgentSessionRecord } from '../../../shared/agent-session-resume'
 import { makePaneKey } from '../../../shared/stable-pane-id'
 import { getProviderSessionClaimKey } from './sleeping-agent-pane-ownership'
 import { useAppStore } from '@/store'
 import { resumeSleepingAgentSessionsForWorktree } from './resume-sleeping-agent-session'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 const LEAF_ID = '11111111-1111-4111-8111-111111111111'
 const OTHER_LEAF_ID = '22222222-2222-4222-8222-222222222222'
diff --git a/src/renderer/src/lib/resume-sleeping-agent-session-suppress-navigation.test.ts b/src/renderer/src/lib/resume-sleeping-agent-session-suppress-navigation.test.ts
index 6f03d9544f4..e195e4c6a98 100644
--- a/src/renderer/src/lib/resume-sleeping-agent-session-suppress-navigation.test.ts
+++ b/src/renderer/src/lib/resume-sleeping-agent-session-suppress-navigation.test.ts
@@ -5,6 +5,13 @@ import { useAppStore } from '@/store'
 import { resumeSleepingAgentSessionsForWorktree } from './resume-sleeping-agent-session'
 import { getProviderSessionClaimKey } from './sleeping-agent-pane-ownership'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 
 afterEach(() => {
diff --git a/src/renderer/src/lib/resume-sleeping-agent-session.test.ts b/src/renderer/src/lib/resume-sleeping-agent-session.test.ts
index c90aff730c4..9cc8923f70b 100644
--- a/src/renderer/src/lib/resume-sleeping-agent-session.test.ts
+++ b/src/renderer/src/lib/resume-sleeping-agent-session.test.ts
@@ -5,6 +5,13 @@ import { parseWorkspaceSession } from '../../../shared/workspace-session-schema'
 import { useAppStore } from '@/store'
 import { resumeSleepingAgentSessionsForWorktree } from './resume-sleeping-agent-session'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 const LEAF_ID = '11111111-1111-4111-8111-111111111111'
 const OTHER_LEAF_ID = '22222222-2222-4222-8222-222222222222'
diff --git a/src/renderer/src/lib/retracted-host-pane-resume.test.ts b/src/renderer/src/lib/retracted-host-pane-resume.test.ts
index 52245ba5519..669cfa0a3a7 100644
--- a/src/renderer/src/lib/retracted-host-pane-resume.test.ts
+++ b/src/renderer/src/lib/retracted-host-pane-resume.test.ts
@@ -1,9 +1,16 @@
-import { afterEach, describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import { useAppStore } from '@/store'
 import type { SleepingAgentSessionRecord } from '../../../shared/agent-session-resume'
 import { makePaneKey } from '../../../shared/stable-pane-id'
 import { resumeSleepingAgentSessionsForWorktree } from './resume-sleeping-agent-session'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialState = useAppStore.getState()
 const LEAF_ID = '11111111-1111-4111-8111-111111111111'
 const HOST_TAB_ID = 'web-terminal-host-tab'
diff --git a/src/renderer/src/lib/settled-worker-wake-policy.test.ts b/src/renderer/src/lib/settled-worker-wake-policy.test.ts
index d227568cb00..a61471096ac 100644
--- a/src/renderer/src/lib/settled-worker-wake-policy.test.ts
+++ b/src/renderer/src/lib/settled-worker-wake-policy.test.ts
@@ -2,6 +2,13 @@ import { afterEach, expect, it, vi } from 'vitest'
 import { useAppStore } from '@/store'
 import { resumeSleepingAgentSessionsForWorktree } from './resume-sleeping-agent-session'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialState = useAppStore.getState()
 
 afterEach(() => {
diff --git a/src/renderer/src/lib/setup-split-activation-single-owner.test.ts b/src/renderer/src/lib/setup-split-activation-single-owner.test.ts
index 2ddd15ef67c..4dfd68bb7bc 100644
--- a/src/renderer/src/lib/setup-split-activation-single-owner.test.ts
+++ b/src/renderer/src/lib/setup-split-activation-single-owner.test.ts
@@ -4,12 +4,20 @@ import type { AppState } from '@/store'
 import { useAppStore } from '@/store'
 import { registerTerminalPresentationIpcBridge } from '@/hooks/ipc-events/terminal-presentation-ipc-bridge'
 import { getDefaultSettings } from '../../../shared/constants'
+import { ORCA_LAYOUT_OWNER_IS_MAIN } from '../../../shared/layout-owner-switch'
 import type { RuntimeTerminalSummary } from '../../../shared/runtime-types'
 import type { TerminalPaneLayoutNode } from '../../../shared/terminal-tab-types'
 import { activateAndRevealWorktree } from './worktree-activation'
 import { gateWorktreeAgentActivation } from './worktree-agent-activation-gate'
 import { makeCreatedAgentWorktree as makeWorktree } from './worktree-activation-created-agent.test-fixture'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 /**
  * Conditional old-host reconstruction, not proof of STA-9417: a host-created worktree with a
  * split of the (unmounted) primary tab, then the first activation of that worktree. When main's
@@ -257,30 +265,33 @@ describe('old-host split of an unmounted tab, then first activation', () => {
     await runScenario(false)
   })
 
-  // The window alone, as above; the odd runs mint the duplicate.
-  it.fails('stays duplicate-free across 8 create-then-activate cycles, with or without the reveal', async () => {
-    let duplicates = 0
-    for (let run = 0; run < 8; run++) {
-      const worktree = makeWorktree()
-      const primaryPtyId = `${worktree.id}@@primary-${run}`
-      const setupPtyId = `${worktree.id}@@setup-${run}`
-      useAppStore.setState(initialState, true)
-      useAppStore.setState(baseState())
-      const host = stubHost(primaryPtyId, setupPtyId)
-      registerTerminalPresentationIpcBridge([])
-      seedRevealedPrimaryTab(worktree.id, primaryPtyId)
-      // Odd runs lose the reveal, as the natural repro did.
-      if (run % 2 === 0) {
-        host.reveal()(setupSplitReveal(`reveal-${run}`, worktree.id, setupPtyId))
+  // The window alone, as above; the odd runs mint the duplicate. Main owning the create fixes it.
+  ;(ORCA_LAYOUT_OWNER_IS_MAIN ? it : it.fails)(
+    'stays duplicate-free across 8 create-then-activate cycles, with or without the reveal',
+    async () => {
+      let duplicates = 0
+      for (let run = 0; run < 8; run++) {
+        const worktree = makeWorktree()
+        const primaryPtyId = `${worktree.id}@@primary-${run}`
+        const setupPtyId = `${worktree.id}@@setup-${run}`
+        useAppStore.setState(initialState, true)
+        useAppStore.setState(baseState())
+        const host = stubHost(primaryPtyId, setupPtyId)
+        registerTerminalPresentationIpcBridge([])
+        seedRevealedPrimaryTab(worktree.id, primaryPtyId)
+        // Odd runs lose the reveal, as the natural repro did.
+        if (run % 2 === 0) {
+          host.reveal()(setupSplitReveal(`reveal-${run}`, worktree.id, setupPtyId))
+        }
+        await activateFirstTime(worktree.id)
+        duplicates += [...ownersByPty(worktree.id).values()].filter(
+          (owners) => owners.length > 1
+        ).length
+        duplicates += (useAppStore.getState().tabsByWorktree[worktree.id] ?? []).length - 1
       }
-      await activateFirstTime(worktree.id)
-      duplicates += [...ownersByPty(worktree.id).values()].filter(
-        (owners) => owners.length > 1
-      ).length
-      duplicates += (useAppStore.getState().tabsByWorktree[worktree.id] ?? []).length - 1
+      expect(duplicates).toBe(0)
     }
-    expect(duplicates).toBe(0)
-  })
+  )
 
   it('rejects a split into a tab it lacks without switching to that workspace', () => {
     const worktree = makeWorktree()
diff --git a/src/renderer/src/lib/structured-agent-session-new-chat-own-launch.test.ts b/src/renderer/src/lib/structured-agent-session-new-chat-own-launch.test.ts
index 5a6c8690fb0..9de47ccd66d 100644
--- a/src/renderer/src/lib/structured-agent-session-new-chat-own-launch.test.ts
+++ b/src/renderer/src/lib/structured-agent-session-new-chat-own-launch.test.ts
@@ -2,7 +2,7 @@
 
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 import type { RuntimeMobileSessionTabsResult } from '../../../shared/runtime-session-contracts'
-import type { Tab, TabGroup } from '../../../shared/tab-types'
+import type { Tab, TabGroup, TabGroupLayoutNode } from '../../../shared/tab-types'
 import type { AgentSessionStatusSummary } from '../../../shared/agent-session-wire'
 import type { StructuredAgentSessionLaunchIntent } from '@/lib/launch-structured-agent-session'
 import type * as LaunchAdmissionModule from './structured-agent-session-launch-admission'
@@ -21,6 +21,7 @@ const mocks = vi.hoisted(() => ({
 type StoreState = {
   unifiedTabsByWorktree: Record<string, Tab[]>
   groupsByWorktree: Record<string, TabGroup[]>
+  layoutByWorktree: Record<string, TabGroupLayoutNode>
   activeGroupIdByWorktree: Record<string, string>
   nativeChatLaunchDraftByTabId: Record<string, { text: string; adopted?: boolean }>
 }
@@ -35,6 +36,14 @@ function emptyStoreState(): StoreState {
   return {
     unifiedTabsByWorktree: {},
     groupsByWorktree: { 'wt-new-chat': [group('group-left'), group('group-right')] },
+    layoutByWorktree: {
+      'wt-new-chat': {
+        type: 'split',
+        direction: 'horizontal',
+        first: { type: 'leaf', groupId: 'group-left' },
+        second: { type: 'leaf', groupId: 'group-right' }
+      }
+    },
     activeGroupIdByWorktree: { 'wt-new-chat': 'group-left' },
     nativeChatLaunchDraftByTabId: {}
   }
diff --git a/src/renderer/src/lib/wake-sleeping-agents-live-unmounted-pane.test.ts b/src/renderer/src/lib/wake-sleeping-agents-live-unmounted-pane.test.ts
index 06e86500992..f505993f740 100644
--- a/src/renderer/src/lib/wake-sleeping-agents-live-unmounted-pane.test.ts
+++ b/src/renderer/src/lib/wake-sleeping-agents-live-unmounted-pane.test.ts
@@ -9,6 +9,13 @@ import type { AppState } from '@/store/types'
 import { wakeSleepingAgentsForWorktreeInBackground } from './wake-sleeping-agents-in-background'
 import { waitForWorktreeAgentActivationGateForTests } from './worktree-agent-activation-gate'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 const WT = 'repo-1::/repo'
 const LEAF_ID = '11111111-1111-4111-8111-111111111111'
diff --git a/src/renderer/src/lib/workspace-terminal-host-authority.test.ts b/src/renderer/src/lib/workspace-terminal-host-authority.test.ts
index 89979637bfb..9afb2e662e7 100644
--- a/src/renderer/src/lib/workspace-terminal-host-authority.test.ts
+++ b/src/renderer/src/lib/workspace-terminal-host-authority.test.ts
@@ -12,6 +12,13 @@ import {
 } from '@/lib/workspace-terminal-host-authority'
 import { makeFolderWorkspace } from '@/store/slices/worktrees-slice-test-fixtures'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 vi.mock('sonner', () => ({ toast: { info: vi.fn(), success: vi.fn(), error: vi.fn() } }))
 vi.mock('@/lib/agent-status', async (importOriginal) => {
   const actual = await importOriginal<typeof AgentStatusModule>()
diff --git a/src/renderer/src/lib/worktree-activation-automation-filter.test.ts b/src/renderer/src/lib/worktree-activation-automation-filter.test.ts
index 8ddc5ff4225..eb9858bc4cd 100644
--- a/src/renderer/src/lib/worktree-activation-automation-filter.test.ts
+++ b/src/renderer/src/lib/worktree-activation-automation-filter.test.ts
@@ -3,6 +3,13 @@ import type { Worktree } from '../../../shared/worktree/types'
 import { useAppStore } from '@/store'
 import { activateAndRevealWorkspace, activateAndRevealWorktree } from './worktree-activation'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 
 afterEach(() => {
diff --git a/src/renderer/src/lib/worktree-activation-created-agent.test.ts b/src/renderer/src/lib/worktree-activation-created-agent.test.ts
index 9ce08a1b768..d4017ee4901 100644
--- a/src/renderer/src/lib/worktree-activation-created-agent.test.ts
+++ b/src/renderer/src/lib/worktree-activation-created-agent.test.ts
@@ -13,6 +13,13 @@ import {
 } from '@/lib/worktree-activation-created-agent.test-fixture'
 import { stubRuntimeEnvironmentBridge } from '../runtime/runtime-environment-bridge-fixture'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 
 function makeWebRuntimeWorktree() {
diff --git a/src/renderer/src/lib/worktree-activation-default-agent-chat.test.ts b/src/renderer/src/lib/worktree-activation-default-agent-chat.test.ts
index c4d86793f69..739d5930683 100644
--- a/src/renderer/src/lib/worktree-activation-default-agent-chat.test.ts
+++ b/src/renderer/src/lib/worktree-activation-default-agent-chat.test.ts
@@ -20,6 +20,13 @@ import {
   registerWorktreeActivationReset
 } from './worktree-activation.test-fixture'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const defaultChat = vi.hoisted(() => ({
   open: vi.fn(),
   awaitsDetection: vi.fn(),
diff --git a/src/renderer/src/lib/worktree-activation-emptied-workspace-reseed.test.ts b/src/renderer/src/lib/worktree-activation-emptied-workspace-reseed.test.ts
index 6425c9681b2..47dd851e5d8 100644
--- a/src/renderer/src/lib/worktree-activation-emptied-workspace-reseed.test.ts
+++ b/src/renderer/src/lib/worktree-activation-emptied-workspace-reseed.test.ts
@@ -15,6 +15,13 @@ import {
 } from '@/lib/worktree-activation-created-agent.test-fixture'
 import { waitForWorktreeAgentActivationGateForTests } from './worktree-agent-activation-gate'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 
 afterEach(() => {
diff --git a/src/renderer/src/lib/worktree-activation-structured-chat-surface.test.ts b/src/renderer/src/lib/worktree-activation-structured-chat-surface.test.ts
index dae397b554b..ec2d92f8f3f 100644
--- a/src/renderer/src/lib/worktree-activation-structured-chat-surface.test.ts
+++ b/src/renderer/src/lib/worktree-activation-structured-chat-surface.test.ts
@@ -13,6 +13,13 @@ import {
   createdTabStartups
 } from './worktree-activation.test-fixture'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 
 registerWorktreeActivationReset()
diff --git a/src/renderer/src/lib/worktree-agent-activation-seam.test.ts b/src/renderer/src/lib/worktree-agent-activation-seam.test.ts
index 99ae44a6fc8..4a70c090290 100644
--- a/src/renderer/src/lib/worktree-agent-activation-seam.test.ts
+++ b/src/renderer/src/lib/worktree-agent-activation-seam.test.ts
@@ -1,4 +1,5 @@
 import { getDefaultSettings } from '../../../shared/constants'
+import { ORCA_LAYOUT_OWNER_IS_MAIN } from '../../../shared/layout-owner-switch'
 import path from 'node:path'
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import type { AppState } from '@/store'
@@ -11,6 +12,23 @@ import * as sleepingResume from './resume-sleeping-agent-session'
 import { activateAndRevealWorktree } from './worktree-activation'
 import { waitForWorktreeAgentActivationGateForTests } from './worktree-agent-activation-gate'
 import { makeCreatedAgentWorktree as makeWorktree } from './worktree-activation-created-agent.test-fixture'
+import { emptySession } from '../../../shared/workspace-layout/workspace-layout-session.test-fixture'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
+import { loadMainLayoutModel } from '@/runtime/main-loaded-layout-frames.test-fixture'
+import { openWorkspaceLayoutFrameSync } from '@/runtime/workspace-layout-frame-sync'
+
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
+vi.mock('@/runtime/runtime-layout-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeLayoutClientWithTestFrames(await importOriginal())
+)
 
 const initialState = useAppStore.getState()
 
@@ -163,6 +181,7 @@ afterEach(() => {
   vi.restoreAllMocks()
   vi.unstubAllGlobals()
   useAppStore.setState(initialState, true)
+  layoutApplyTestTransport.reset()
 })
 
 describe('worktree agent activation seam', () => {
@@ -212,15 +231,29 @@ describe('worktree agent activation seam', () => {
   it('adopts a live terminal without spawning a fallback', async () => {
     const worktree = makeWorktree()
     const livePtyId = `${worktree.id}@@live-codex`
-    useAppStore.setState(baseState())
+    // Restored with no tabs: frames draw this workspace.
+    useAppStore.setState({ ...baseState(), unifiedTabsByWorktree: { [worktree.id]: [] } })
     stubInventory({ livePtyId })
+    // A switch-on build shows the adopted terminal from main's frame.
+    layoutApplyTestTransport.answerWith('apply')
+    layoutApplyTestTransport.useMainModel(loadMainLayoutModel('local', emptySession()))
+    const frames = openWorkspaceLayoutFrameSync(useAppStore)
+    frames.start()
+    try {
+      expect(activateAndRevealWorktree(worktree.id)).toEqual({ primaryTabId: null })
+      await waitForWorktreeAgentActivationGateForTests(worktree.id)
+      await layoutApplyTestTransport.settle()
 
-    expect(activateAndRevealWorktree(worktree.id)).toEqual({ primaryTabId: null })
-    await waitForWorktreeAgentActivationGateForTests(worktree.id)
-
-    const tabs = useAppStore.getState().tabsByWorktree[worktree.id] ?? []
-    expect(tabs).toHaveLength(1)
-    expect(tabs[0]?.ptyId).toBe(livePtyId)
+      const tabs = useAppStore.getState().tabsByWorktree[worktree.id] ?? []
+      expect(tabs).toHaveLength(1)
+      // Switch-on, the pane binds to the terminal main's frame names; the row keeps no ptyId.
+      const shownPtyId = ORCA_LAYOUT_OWNER_IS_MAIN
+        ? frames.terminalTab(worktree.id, tabs[0]!.id)?.terminal?.panes[0]?.ptyId
+        : tabs[0]?.ptyId
+      expect(shownPtyId).toBe(livePtyId)
+    } finally {
+      frames.close()
+    }
   })
 
   it('spawns a fallback when the workspace has no agent', async () => {
diff --git a/src/renderer/src/lib/worktree-creation-background-mount.test.ts b/src/renderer/src/lib/worktree-creation-background-mount.test.ts
index a39bd6dcac2..41c81455d76 100644
--- a/src/renderer/src/lib/worktree-creation-background-mount.test.ts
+++ b/src/renderer/src/lib/worktree-creation-background-mount.test.ts
@@ -25,6 +25,13 @@ import {
   seedEmptyActivatableWorktree
 } from './worktree-activation-created-agent.test-fixture'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initialAppStoreState = useAppStore.getState()
 const worktree = makeCreatedAgentWorktree()
 
diff --git a/src/renderer/src/lib/worktree-creation-structured-session-host-decline.test.ts b/src/renderer/src/lib/worktree-creation-structured-session-host-decline.test.ts
index 82426c322b3..fec03660786 100644
--- a/src/renderer/src/lib/worktree-creation-structured-session-host-decline.test.ts
+++ b/src/renderer/src/lib/worktree-creation-structured-session-host-decline.test.ts
@@ -18,6 +18,13 @@ vi.mock('@/runtime/structured-agent-session-client', () => ({
 import { useAppStore } from '@/store'
 import { launchStructuredWorktreeSession } from './worktree-creation-structured-session'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const initial = useAppStore.getState()
 
 function worktree(name: string, host: 'paired' | 'local' = 'paired'): Worktree {
diff --git a/src/renderer/src/lib/worktree-reactivation-preserved-pane-replacement.test.ts b/src/renderer/src/lib/worktree-reactivation-preserved-pane-replacement.test.ts
index 6332bd39b68..bbc135af0d1 100644
--- a/src/renderer/src/lib/worktree-reactivation-preserved-pane-replacement.test.ts
+++ b/src/renderer/src/lib/worktree-reactivation-preserved-pane-replacement.test.ts
@@ -13,6 +13,13 @@ import {
   resetHostSessionMirrorHydrationForTests
 } from '@/runtime/host-session-mirror-hydration'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 // Pins the activation contract behind the run6-review-pr-11959 incident shape:
 // a persisted (husk) tab whose pane cannot resume in place gets ONE appended
 // replacement tab per provider session — the husk is retained for scrollback —
diff --git a/src/renderer/src/lib/worktree-reactivation-runtime-owned-resume-deferral.test.ts b/src/renderer/src/lib/worktree-reactivation-runtime-owned-resume-deferral.test.ts
index 33fa2b55f8d..332ab96a132 100644
--- a/src/renderer/src/lib/worktree-reactivation-runtime-owned-resume-deferral.test.ts
+++ b/src/renderer/src/lib/worktree-reactivation-runtime-owned-resume-deferral.test.ts
@@ -7,6 +7,13 @@ import { makeCreatedAgentWorktree } from '@/lib/worktree-activation-created-agen
 import { makePaneKey } from '../../../shared/stable-pane-id'
 import { waitForWorktreeAgentActivationGateForTests } from './worktree-agent-activation-gate'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 // Red repro for the aug20 "windows 2" incident (restart-reattach/resume-relaunch):
 // a runtime-owned (paired remote) worktree's web-mirror tab holds a sleeping
 // record for an agent whose host PTY is STILL ALIVE, but at activation time the
diff --git a/src/renderer/src/runtime/layout-apply-paired-server-guard.test.ts b/src/renderer/src/runtime/layout-apply-paired-server-guard.test.ts
index 385e5425899..12c4bdaff0f 100644
--- a/src/renderer/src/runtime/layout-apply-paired-server-guard.test.ts
+++ b/src/renderer/src/runtime/layout-apply-paired-server-guard.test.ts
@@ -149,6 +149,8 @@ describe('layout.apply senders', () => {
     expect(layoutApplySenders(path.resolve(__dirname, '..'))).toEqual({
       'runtime/generated-tab-titles-send.ts': 1,
       'runtime/layout-apply-send.ts': 1,
+      // Tests' stand-in for main: it answers layout.apply, never sends it.
+      'runtime/layout-apply-test-transport.test-fixture.ts': 1,
       'store/terminals/terminal-tab-create-through-layout-owner.ts': 2
     })
   })
diff --git a/src/renderer/src/runtime/layout-apply-test-transport.test-fixture.ts b/src/renderer/src/runtime/layout-apply-test-transport.test-fixture.ts
new file mode 100644
index 00000000000..3413f6f3a73
--- /dev/null
+++ b/src/renderer/src/runtime/layout-apply-test-transport.test-fixture.ts
@@ -0,0 +1,304 @@
+// A stand-in for main's `layout.apply` and `layout.subscribe` in switch-on renderer tests. It answers
+// each command the window sends (accept, apply to main's model, leave pending, or refuse) and, when
+// applying, emits main's frame at the reply's stream position, as main does.
+//
+// Install it in a test file, before any import that reaches the RPC client:
+//   vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+//     (await import('@/runtime/layout-apply-test-transport.test-fixture'))
+//       .runtimeRpcClientWithLayoutApply(await importOriginal()))
+// and, for frames, `@/runtime/runtime-layout-client` with `runtimeLayoutClientWithTestFrames`.
+// Imports stay clear of the RPC client at runtime: a mock factory must not reach what it replaces.
+
+import { createBrowserUuid } from '@/lib/browser-uuid'
+import { applyLayoutCommand } from '../../../shared/workspace-layout/workspace-layout-commands'
+import type {
+  LayoutApplyReply,
+  LayoutCommand,
+  LayoutCommandResult
+} from '../../../shared/workspace-layout/workspace-layout-command-types'
+import type { WorkspaceLayoutModel } from '../../../shared/workspace-layout/workspace-layout-model'
+import { publishWorkspaceLayout } from '../../../shared/workspace-layout/workspace-layout-published'
+import type { WorkspaceLayoutChangeFrame } from '../../../shared/workspace-layout/workspace-layout-stream-frames'
+import type { RuntimeClientTarget } from '../../../shared/runtime-client-target'
+import { layoutSnapshotFrame, publishedLayoutsOf } from './main-loaded-layout-frames.test-fixture'
+import type * as LayoutClient from './runtime-layout-client'
+import type * as RpcClient from './runtime-rpc-client'
+
+type Refusal = Extract<LayoutApplyReply, { ok: false }>['code']
+
+/**
+ * `accept`: ok, with a result naming the ids the command did; no frame (main's copy is not
+ * modelled). `apply`: main's model (`useMainModel`) applies it; the reply carries its real result
+ * and the position of the frame emitted with it. `pending`: no answer until `answerPending`.
+ */
+export type LayoutApplyTestAnswer =
+  | 'accept'
+  | 'apply'
+  | 'pending'
+  | { refuse: Refusal }
+  | ((command: LayoutCommand) => LayoutApplyReply | 'pending')
+
+type FrameSink = {
+  onFrame: (frame: WorkspaceLayoutChangeFrame) => void
+  onPosition?: (seq: number) => void
+}
+
+type PendingApply = { command: LayoutCommand; resolve: (reply: LayoutApplyReply) => void }
+
+type TransportState = {
+  answer: LayoutApplyTestAnswer
+  sent: LayoutCommand[]
+  pending: PendingApply[]
+  model: WorkspaceLayoutModel | null
+  seq: number
+  sinks: Set<FrameSink>
+  inFlight: Promise<LayoutApplyReply>[]
+}
+
+const fresh = (): TransportState => ({
+  answer: 'accept',
+  sent: [],
+  pending: [],
+  model: null,
+  seq: 0,
+  sinks: new Set(),
+  inFlight: []
+})
+
+let state = fresh()
+
+/** What main's result names for a command it accepted, read off the command. */
+function acceptedResult(command: LayoutCommand): LayoutCommandResult {
+  const result: LayoutCommandResult = {}
+  if ('tabId' in command && typeof command.tabId === 'string') {
+    result.tabId = command.tabId
+  }
+  if ('leafId' in command && typeof command.leafId === 'string') {
+    result.leafId = command.leafId
+  }
+  if ('groupId' in command && typeof command.groupId === 'string') {
+    result.groupId = command.groupId
+  }
+  if ('pageId' in command && typeof command.pageId === 'string') {
+    result.pageId = command.pageId
+  }
+  if (command.type === 'closeTabs') {
+    return { ...result, closed: [...command.tabIds], refused: [] }
+  }
+  if (command.type === 'sleep') {
+    return { ...result, slept: [...(command.paneKeys ?? [])] }
+  }
+  if (command.type === 'wake') {
+    return { ...result, woken: [...(command.paneKeys ?? [])] }
+  }
+  if (command.type === 'splitPane' && command.newLeafId) {
+    return { ...result, leafId: command.newLeafId }
+  }
+  return result
+}
+
+// Shape check only: the window's own sender built it.
+function isLayoutCommand(value: unknown): value is LayoutCommand {
+  return (
+    typeof value === 'object' &&
+    value !== null &&
+    'type' in value &&
+    typeof value.type === 'string' &&
+    'workspace' in value &&
+    typeof value.workspace === 'string'
+  )
+}
+
+/** The command of a `layout.apply` request's params. */
+function commandOf(params: unknown): unknown {
+  return typeof params === 'object' && params !== null && 'command' in params
+    ? params.command
+    : undefined
+}
+
+function emit(frame: WorkspaceLayoutChangeFrame): void {
+  for (const sink of state.sinks) {
+    sink.onFrame(frame)
+    if (frame.seq !== undefined) {
+      sink.onPosition?.(frame.seq)
+    }
+  }
+}
+
+/** Main's apply without its routing: no host partitions, `host_not_owned`, forced closes, or a
+ *  frame position on refusals. */
+function applyToMainModel(command: LayoutCommand): LayoutApplyReply {
+  const model = state.model
+  if (!model) {
+    throw new Error('layoutApplyTestTransport: answer "apply" needs useMainModel first')
+  }
+  const applied = applyLayoutCommand(model, command, {
+    mintId: createBrowserUuid,
+    mintLeafId: createBrowserUuid,
+    now: () => Date.now()
+  })
+  if (!applied.ok) {
+    return applied
+  }
+  state.model = applied.model
+  state.seq += 1
+  const seq = state.seq
+  const key = command.workspace
+  const workspace = applied.model.workspaces[key]
+  // Main publishes before it replies.
+  emit(
+    workspace
+      ? { type: 'workspace', key, layout: publishWorkspaceLayout(workspace, model.hostId), seq }
+      : { type: 'removed', key, seq }
+  )
+  return { ok: true, result: applied.result, frameSeq: seq }
+}
+
+/** The current answer to one command; `pending` when it waits for `answerPending`. */
+function replyTo(command: LayoutCommand): LayoutApplyReply | 'pending' {
+  const answer = state.answer
+  if (typeof answer === 'function') {
+    return answer(command)
+  }
+  if (answer === 'accept') {
+    return { ok: true, result: acceptedResult(command) }
+  }
+  if (answer === 'apply') {
+    return applyToMainModel(command)
+  }
+  return answer === 'pending' ? 'pending' : { ok: false, code: answer.refuse }
+}
+
+function answerCommand(command: LayoutCommand): Promise<LayoutApplyReply> {
+  state.sent.push(command)
+  const reply = replyTo(command)
+  if (reply === 'pending') {
+    return new Promise((resolve) => state.pending.push({ command, resolve }))
+  }
+  const answered = Promise.resolve(reply)
+  state.inFlight.push(answered)
+  return answered
+}
+
+// layout-apply-send.ts's chain (reply, frame wait, handler) runs a few microtasks after the answer.
+const REPLY_CHAIN_TURNS = 5
+
+export const layoutApplyTestTransport = {
+  /** How later commands are answered; `accept` after each `reset`. */
+  answerWith(answer: LayoutApplyTestAnswer): void {
+    state.answer = answer
+  },
+  /** Main's model, which `apply` edits and frames publish. */
+  useMainModel(model: WorkspaceLayoutModel): void {
+    state.model = model
+  },
+  get mainModel(): WorkspaceLayoutModel | null {
+    return state.model
+  },
+  /** Every command sent, in order, with its workspace. */
+  get sent(): readonly LayoutCommand[] {
+    return state.sent
+  },
+  sentOfType<T extends LayoutCommand['type']>(type: T): Extract<LayoutCommand, { type: T }>[] {
+    return state.sent.filter(
+      (command): command is Extract<LayoutCommand, { type: T }> => command.type === type
+    )
+  },
+  get pendingCount(): number {
+    return state.pending.length
+  },
+  /** Answers every pending command: with `reply`, else as the current answer (`accept` when that
+   *  is still `pending`). */
+  answerPending(reply?: LayoutApplyReply): void {
+    const pending = state.pending
+    state.pending = []
+    for (const { command, resolve } of pending) {
+      const next = replyTo(command)
+      const answer =
+        reply ?? (next === 'pending' ? { ok: true, result: acceptedResult(command) } : next)
+      state.inFlight.push(Promise.resolve(answer))
+      resolve(answer)
+    }
+  },
+  /** Resolves once every answered command's handler ran; real or fake timers alike. */
+  async settle(): Promise<void> {
+    while (state.inFlight.length > 0) {
+      const answered = state.inFlight
+      state.inFlight = []
+      await Promise.all(answered)
+      for (let turn = 0; turn < REPLY_CHAIN_TURNS; turn += 1) {
+        await Promise.resolve()
+      }
+    }
+  },
+  /** Sends a frame main made on its own (a CLI create, a load repair). */
+  emitFrame(frame: WorkspaceLayoutChangeFrame): void {
+    emit(frame)
+  },
+  /** Answers one `layout.apply` request's params; the RPC mock below routes here. */
+  call(params: unknown): Promise<LayoutApplyReply> {
+    const command = commandOf(params)
+    if (!isLayoutCommand(command)) {
+      return Promise.reject(new Error('layout.apply without a command'))
+    }
+    return answerCommand(command)
+  },
+  reset(): void {
+    state = fresh()
+  }
+}
+
+type RpcClientModule = typeof RpcClient
+
+/** The RPC client module with local `layout.apply` answered by the transport. */
+export function runtimeRpcClientWithLayoutApply(actual: RpcClientModule): RpcClientModule {
+  function callRuntimeRpc<TResult>(
+    target: RuntimeClientTarget,
+    method: string,
+    params?: unknown,
+    options?: Parameters<RpcClientModule['callRuntimeRpc']>[3]
+  ): Promise<TResult> {
+    if (target.kind === 'local' && method === 'layout.apply') {
+      // oxlint-disable-next-line typescript/consistent-type-assertions -- SAFETY: callers of layout.apply type TResult as LayoutApplyReply.
+      return layoutApplyTestTransport.call(params) as Promise<TResult>
+    }
+    return actual.callRuntimeRpc<TResult>(target, method, params, options)
+  }
+  return { ...actual, callRuntimeRpc }
+}
+
+type LayoutClientModule = typeof LayoutClient
+
+/** The layout client module with the local stream fed by the transport: main's model's snapshot
+ *  first, then the frames `apply` and `emitFrame` send. */
+export function runtimeLayoutClientWithTestFrames(actual: LayoutClientModule): LayoutClientModule {
+  const subscribeRuntimeLayout: LayoutClientModule['subscribeRuntimeLayout'] = (
+    target,
+    params,
+    onFrame,
+    onStatus,
+    onPosition
+  ) => {
+    if (target.kind !== 'local') {
+      return actual.subscribeRuntimeLayout(target, params, onFrame, onStatus, onPosition)
+    }
+    const sink: FrameSink = { onFrame, onPosition }
+    state.sinks.add(sink)
+    onStatus({ state: 'connecting' })
+    queueMicrotask(() => {
+      if (!state.sinks.has(sink)) {
+        return
+      }
+      onStatus({ state: 'live' })
+      onFrame(layoutSnapshotFrame(state.model ? publishedLayoutsOf(state.model) : {}, state.seq))
+      onPosition?.(state.seq)
+    })
+    return {
+      close: () => {
+        state.sinks.delete(sink)
+        onStatus({ state: 'closed' })
+      }
+    }
+  }
+  return { ...actual, subscribeRuntimeLayout }
+}
diff --git a/src/renderer/src/runtime/main-loaded-layout-frames.test-fixture.ts b/src/renderer/src/runtime/main-loaded-layout-frames.test-fixture.ts
new file mode 100644
index 00000000000..c25b8c345fa
--- /dev/null
+++ b/src/renderer/src/runtime/main-loaded-layout-frames.test-fixture.ts
@@ -0,0 +1,53 @@
+// Main's loaded layout as the frames its stream sends: what main publishes for a stored partition,
+// so a switch-on test feeds the window the layout main holds instead of the window's own restore.
+
+import type { ExecutionHostId } from '../../../shared/execution-host'
+import type { WorkspaceSessionState } from '../../../shared/workspace-session-state-types'
+import { loadWorkspaceLayoutWithoutReport } from '../../../shared/workspace-layout/workspace-layout-load'
+import { nameBasedLoadContext } from '../../../shared/workspace-layout/workspace-layout-minted-ids'
+import type { WorkspaceLayoutModel } from '../../../shared/workspace-layout/workspace-layout-model'
+import {
+  publishWorkspaceLayout,
+  type PublishedWorkspaceLayout
+} from '../../../shared/workspace-layout/workspace-layout-published'
+import type { WorkspaceLayoutChangeFrame } from '../../../shared/workspace-layout/workspace-layout-stream-frames'
+
+/** The model main loads from one stored partition, with the ids it mints at load. */
+export function loadMainLayoutModel(
+  hostId: ExecutionHostId,
+  session: WorkspaceSessionState
+): WorkspaceLayoutModel {
+  return loadWorkspaceLayoutWithoutReport(hostId, session, nameBasedLoadContext()).layout
+}
+
+/** What main publishes for each workspace of a model, by workspace key. */
+export function publishedLayoutsOf(
+  model: WorkspaceLayoutModel
+): Record<string, PublishedWorkspaceLayout> {
+  return Object.fromEntries(
+    Object.entries(model.workspaces).map(([key, workspace]) => [
+      key,
+      publishWorkspaceLayout(workspace, model.hostId)
+    ])
+  )
+}
+
+/** What main publishes for one stored partition, by workspace key. */
+export function mainLoadedLayouts(
+  hostId: ExecutionHostId,
+  session: WorkspaceSessionState
+): Record<string, PublishedWorkspaceLayout> {
+  return publishedLayoutsOf(loadMainLayoutModel(hostId, session))
+}
+
+/** The snapshot frame a stream opens with. */
+export function layoutSnapshotFrame(
+  layouts: Record<string, PublishedWorkspaceLayout>,
+  seq?: number
+): WorkspaceLayoutChangeFrame {
+  return {
+    type: 'snapshot',
+    workspaces: Object.entries(layouts).map(([key, layout]) => ({ key, layout })),
+    ...(seq === undefined ? {} : { seq })
+  }
+}
diff --git a/src/renderer/src/runtime/web-session-tabs-sync-remote-status-title-flap.test.ts b/src/renderer/src/runtime/web-session-tabs-sync-remote-status-title-flap.test.ts
index 5402e3747ff..7692d3257cf 100644
--- a/src/renderer/src/runtime/web-session-tabs-sync-remote-status-title-flap.test.ts
+++ b/src/renderer/src/runtime/web-session-tabs-sync-remote-status-title-flap.test.ts
@@ -224,12 +224,18 @@ function seedPairedClientStore(): TestStore {
     },
     worktreesByRepo: {
       repo1: [
-        makeWorktree({ id: WT, repoId: 'repo1', path: '/path/wt1' }),
+        // The paired server owns this workspace, so its titles stay the window's to apply.
+        makeWorktree({
+          id: WT,
+          repoId: 'repo1',
+          path: '/path/wt1',
+          runtimeOwnerEnvironmentId: ENV
+        }),
         makeWorktree({ id: LOCAL_WT, repoId: 'repo1', path: '/path/local-wt' })
       ]
     },
     activeWorktreeId: WT
-  } as Partial<AppState>)
+  })
   return store
 }
 
diff --git a/src/renderer/src/runtime/workspace-layout-frame-sync.test.ts b/src/renderer/src/runtime/workspace-layout-frame-sync.test.ts
index e276b1f37fe..1f4780e8291 100644
--- a/src/renderer/src/runtime/workspace-layout-frame-sync.test.ts
+++ b/src/renderer/src/runtime/workspace-layout-frame-sync.test.ts
@@ -3,16 +3,10 @@ import type * as AgentStatusModule from '@/lib/agent-status'
 import type { ExecutionHostId } from '../../../shared/execution-host'
 import type { WorkspaceSessionState } from '../../../shared/workspace-session-state-types'
 import { LEGACY_WORKSPACE_LAYOUT_PROFILES } from '../../../shared/workspace-layout/workspace-layout-legacy-profile.test-fixture'
-import { loadWorkspaceLayoutWithoutReport } from '../../../shared/workspace-layout/workspace-layout-load'
-import { nameBasedLoadContext } from '../../../shared/workspace-layout/workspace-layout-minted-ids'
 import {
   localDesktopSession,
   WORKSPACE_LAYOUT_PROFILES
 } from '../../../shared/workspace-layout/workspace-layout-profile.test-fixture'
-import {
-  publishWorkspaceLayout,
-  type PublishedWorkspaceLayout
-} from '../../../shared/workspace-layout/workspace-layout-published'
 import type { WorkspaceLayoutChangeFrame } from '../../../shared/workspace-layout/workspace-layout-stream-frames'
 import {
   emptySession,
@@ -30,6 +24,7 @@ import {
 } from './layout-frame-position'
 import { holdTabPaneTree, requestTabPaneTreeRedraw } from './layout-owner-pane-intents'
 import { openWorkspaceLayoutFrameSync } from './workspace-layout-frame-sync'
+import { layoutSnapshotFrame, mainLoadedLayouts } from './main-loaded-layout-frames.test-fixture'
 
 vi.mock('sonner', () => ({ toast: { info: vi.fn(), success: vi.fn(), error: vi.fn() } }))
 vi.mock('@/lib/agent-status', async (importOriginal) => {
@@ -85,35 +80,14 @@ const SLICES: (keyof LayoutFrameSlices)[] = [
   'activeTabTypeByWorktree'
 ]
 
-/** What main publishes for one stored partition. */
-function published(
-  hostId: ExecutionHostId,
-  session: WorkspaceSessionState
-): Record<string, PublishedWorkspaceLayout> {
-  const { layout } = loadWorkspaceLayoutWithoutReport(hostId, session, nameBasedLoadContext())
-  return Object.fromEntries(
-    Object.entries(layout.workspaces).map(([key, workspace]) => [
-      key,
-      publishWorkspaceLayout(workspace, hostId)
-    ])
-  )
-}
-
-function snapshot(layouts: Record<string, PublishedWorkspaceLayout>): WorkspaceLayoutChangeFrame {
-  return {
-    type: 'snapshot',
-    workspaces: Object.entries(layouts).map(([key, layout]) => ({ key, layout }))
-  }
-}
-
 const flushMicrotasks = () => new Promise<void>((resolve) => setTimeout(resolve, 0))
 
 async function bootWithFrames(hostId: ExecutionHostId, session: WorkspaceSessionState) {
   const store = createTestStore()
   await bootWindowFromPartitions(store, [{ hostId, session }])
   const sync = openWorkspaceLayoutFrameSync(store)
-  const layouts = published(hostId, session)
-  stream.onFrame!(snapshot(layouts))
+  const layouts = mainLoadedLayouts(hostId, session)
+  stream.onFrame!(layoutSnapshotFrame(layouts))
   return { store, sync, layouts }
 }
 
@@ -145,7 +119,7 @@ describe('the window drawn from layout frames', () => {
     await bootWindowFromPartitions(store, [{ hostId: profile.hostId, session: profile.build() }])
     const before = store.getState()
     const sync = openWorkspaceLayoutFrameSync(store)
-    stream.onFrame!(snapshot(published(profile.hostId, profile.build())))
+    stream.onFrame!(layoutSnapshotFrame(mainLoadedLayouts(profile.hostId, profile.build())))
     sync.start()
     const after = store.getState()
     expect(SLICES.filter((slice) => after[slice] !== before[slice])).toEqual([])
@@ -333,7 +307,7 @@ describe('the window drawn from layout frames', () => {
     expect(layoutFramesFlowing()).toBe(true)
     const renamed = structuredClone(layouts)
     renamed[GIT_KEY]!.tabs.find((tab) => tab.id === 'tab-unbound')!.customTitle = 'from main'
-    stream.onFrame!(snapshot(renamed))
+    stream.onFrame!(layoutSnapshotFrame(renamed))
     stream.onStatus!({ state: 'live' })
     await flushMicrotasks()
 
@@ -412,8 +386,8 @@ describe('the window drawn from layout frames', () => {
     const store = createTestStore()
     await bootWindowFromPartitions(store, [{ hostId: 'local', session: emptySession() }])
     const sync = openWorkspaceLayoutFrameSync(store)
-    const layouts = published('local', localDesktopSession())
-    stream.onFrame!(snapshot(layouts))
+    const layouts = mainLoadedLayouts('local', localDesktopSession())
+    stream.onFrame!(layoutSnapshotFrame(layouts))
     sync.start()
     await flushMicrotasks()
     expect(store.getState().tabsByWorktree[GIT_KEY]).toBeUndefined()
@@ -434,9 +408,9 @@ describe('the window drawn from layout frames', () => {
     await bootWindowFromPartitions(store, [{ hostId: 'local', session: localDesktopSession() }])
     const sync = openWorkspaceLayoutFrameSync(store)
     sync.start()
-    const layouts = published('local', localDesktopSession())
+    const layouts = mainLoadedLayouts('local', localDesktopSession())
     layouts[GIT_KEY]!.tabs.find((tab) => tab.id === 'tab-shell')!.customTitle = 'from main'
-    stream.onFrame!(snapshot(layouts))
+    stream.onFrame!(layoutSnapshotFrame(layouts))
     await flushMicrotasks()
     expect(
       store.getState().tabsByWorktree[GIT_KEY]!.find((tab) => tab.id === 'tab-shell')!.customTitle
diff --git a/src/renderer/src/runtime/workspace-layout-frame-tab-props.test.ts b/src/renderer/src/runtime/workspace-layout-frame-tab-props.test.ts
index f80994e8c18..a3a8f582660 100644
--- a/src/renderer/src/runtime/workspace-layout-frame-tab-props.test.ts
+++ b/src/renderer/src/runtime/workspace-layout-frame-tab-props.test.ts
@@ -22,8 +22,16 @@ import {
 import { createStoreSessionMockApi } from '../store/slices/store-session-test-harness'
 import { createTestStore } from '../store/slices/store-test-helpers'
 import type { AppState } from '../store/types'
+import { layoutApplyTestTransport } from './layout-apply-test-transport.test-fixture'
 import { openWorkspaceLayoutFrameSync } from './workspace-layout-frame-sync'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 vi.mock('sonner', () => ({ toast: { info: vi.fn(), success: vi.fn(), error: vi.fn() } }))
 vi.mock('@/lib/agent-status', async (importOriginal) => {
   const actual = await importOriginal<typeof AgentStatusModule>()
@@ -228,6 +236,7 @@ describe('tab properties through main', () => {
       // Today: the window acts on its own copy.
       const today = await boot(stored)
       entry.windowAction(today.getState())
+      await layoutApplyTestTransport.settle()
       const todayTabs = shownTabs(today.getState())
       expect(todayTabs).not.toEqual(shownTabs((await boot(stored)).getState()))
       const expected = { ...todayTabs, ...entry.mainShows }
@@ -245,6 +254,7 @@ describe('tab properties through main', () => {
 
       // Main's answer then runs the window's own action, which leaves what today's action shows.
       entry.windowAction(drawn.getState())
+      await layoutApplyTestTransport.settle()
       expect(shownTabs(drawn.getState())).toEqual(expected)
       sync.close()
 
diff --git a/src/renderer/src/store/slices/agent-generated-tab-title.test.ts b/src/renderer/src/store/slices/agent-generated-tab-title.test.ts
index 16a5efe0d6a..994cae08520 100644
--- a/src/renderer/src/store/slices/agent-generated-tab-title.test.ts
+++ b/src/renderer/src/store/slices/agent-generated-tab-title.test.ts
@@ -4,6 +4,14 @@ import { getDefaultSettings } from '../../../../shared/constants'
 import { makePaneKey } from '../../../../shared/stable-pane-id'
 import { resolveTerminalTabTitle } from '../../../../shared/tab-title-resolution'
 import { createTestStore, makeWorktree, seedStore } from './store-test-helpers'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
+
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
 
 const WORKTREE_ID = 'repo1::/path/wt1'
 const LEAF_ID = '11111111-1111-4111-8111-111111111111'
@@ -26,7 +34,7 @@ describe('generated agent tab titles', () => {
     vi.useRealTimers()
   })
 
-  it('stays disabled by default when agent prompts arrive', () => {
+  it('stays disabled by default when agent prompts arrive', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     const tabId = seedWorktree(store, false)
@@ -36,12 +44,13 @@ describe('generated agent tab titles', () => {
       prompt: 'Refactor the auth middleware',
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBeUndefined()
     expect(store.getState().unifiedTabsByWorktree[WORKTREE_ID][0].generatedLabel).toBeUndefined()
   })
 
-  it('generates one stable title from the first known agent prompt when enabled', () => {
+  it('generates one stable title from the first known agent prompt when enabled', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     const tabId = seedWorktree(store, true)
@@ -51,11 +60,13 @@ describe('generated agent tab titles', () => {
       prompt: 'Can you please refactor the auth middleware to use JWT tokens?',
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
     store.getState().setAgentStatus(makePaneKey(tabId, LEAF_ID), {
       state: 'working',
       prompt: 'Replace this with a later task name',
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Refactor the auth middleware to use JWT'
@@ -65,7 +76,7 @@ describe('generated agent tab titles', () => {
     )
   })
 
-  it('replaces a raw dispatch preamble title when orchestration display metadata arrives', () => {
+  it('replaces a raw dispatch preamble title when orchestration display metadata arrives', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     const tabId = seedWorktree(store, true)
@@ -83,6 +94,7 @@ orca orchestration send --to term_parent
 Implement the detailed worker instructions that should not be the short label`,
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Implement the detailed worker'
@@ -96,6 +108,7 @@ Implement the detailed worker instructions that should not be the short label`,
         displayName: 'Better worker label'
       }
     })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Better worker label'
@@ -105,7 +118,7 @@ Implement the detailed worker instructions that should not be the short label`,
     )
   })
 
-  it('does not replace with sticky orchestration when a new non-dispatch prompt arrives', () => {
+  it('does not replace with sticky orchestration when a new non-dispatch prompt arrives', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     const tabId = seedWorktree(store, true)
@@ -124,6 +137,7 @@ Implement the detailed worker instructions that should not be the short label`
       prompt: dispatchPrompt,
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
     store.getState().setRuntimeAgentOrchestrationByPaneKey({
       [paneKey]: {
         taskId: 'task-1',
@@ -132,6 +146,7 @@ Implement the detailed worker instructions that should not be the short label`
         displayName: 'Better worker label'
       }
     })
+    await layoutApplyTestTransport.settle()
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Better worker label'
     )
@@ -143,13 +158,14 @@ Implement the detailed worker instructions that should not be the short label`
       prompt: 'Refactor the auth middleware to use JWT tokens for session recovery',
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Better worker label'
     )
   })
 
-  it('generates from a new non-dispatch prompt when sticky orchestration remains but no title exists', () => {
+  it('generates from a new non-dispatch prompt when sticky orchestration remains but no title exists', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     const tabId = seedWorktree(store, true)
@@ -165,6 +181,7 @@ Your task ID is: task-1
 Old dispatch task that already finished`,
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
     store.getState().setRuntimeAgentOrchestrationByPaneKey({
       [paneKey]: {
         taskId: 'task-1',
@@ -173,6 +190,7 @@ Old dispatch task that already finished`,
         displayName: 'Stale worker label'
       }
     })
+    await layoutApplyTestTransport.settle()
     // Clear any title set during the dispatch turn so the next non-dispatch
     // prompt is a pure first-write with sticky orchestration still present.
     const tabs = store.getState().tabsByWorktree[WORKTREE_ID]
@@ -196,6 +214,7 @@ Old dispatch task that already finished`,
       prompt: 'Can you please refactor the auth middleware to use JWT tokens?',
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Refactor the auth middleware to use JWT'
@@ -205,7 +224,7 @@ Old dispatch task that already finished`,
     )
   })
 
-  it('does not re-pin sticky task A labels onto a later dispatch task B preamble', () => {
+  it('does not re-pin sticky task A labels onto a later dispatch task B preamble', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     const tabId = seedWorktree(store, true)
@@ -220,6 +239,7 @@ Your task ID is: task-a
 Implement task A worker instructions that should not stick`,
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
     store.getState().setRuntimeAgentOrchestrationByPaneKey({
       [paneKey]: {
         taskId: 'task-a',
@@ -228,6 +248,7 @@ Implement task A worker instructions that should not stick`,
         displayName: 'Worker A label'
       }
     })
+    await layoutApplyTestTransport.settle()
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe('Worker A label')
 
     store.getState().setAgentStatus(paneKey, {
@@ -239,13 +260,14 @@ Your task ID is: task-b
 Implement task B worker instructions for the next dispatch`,
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Implement task B worker instructions'
     )
   })
 
-  it('does not force-replace titles when sticky orchestration updates after a non-dispatch prompt', () => {
+  it('does not force-replace titles when sticky orchestration updates after a non-dispatch prompt', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     const tabId = seedWorktree(store, true)
@@ -256,6 +278,7 @@ Implement task B worker instructions for the next dispatch`,
       prompt: 'Can you please refactor the auth middleware to use JWT tokens?',
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Refactor the auth middleware to use JWT'
     )
@@ -268,13 +291,14 @@ Implement task B worker instructions for the next dispatch`,
         displayName: 'Stale orchestration label'
       }
     })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().tabsByWorktree[WORKTREE_ID][0].generatedTitle).toBe(
       'Refactor the auth middleware to use JWT'
     )
   })
 
-  it('does not trim the full paste-sized prompt before generating an optional title', () => {
+  it('does not trim the full paste-sized prompt before generating an optional title', async () => {
     vi.useFakeTimers()
     const trimSpy = vi.spyOn(String.prototype, 'trim')
     const store = createTestStore()
@@ -286,6 +310,7 @@ Implement task B worker instructions for the next dispatch`,
       prompt,
       agentType: 'codex'
     })
+    await layoutApplyTestTransport.settle()
 
     const tab = store.getState().tabsByWorktree[WORKTREE_ID][0]
     expect(tab.generatedTitle).toBe('Fix the flaky status tests large pasted')
@@ -296,7 +321,7 @@ Implement task B worker instructions for the next dispatch`,
     ).toBe(false)
   })
 
-  it('keeps manual rename precedence over generated and live titles', () => {
+  it('keeps manual rename precedence over generated and live titles', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     const tabId = seedWorktree(store, true)
@@ -306,15 +331,18 @@ Implement task B worker instructions for the next dispatch`,
       prompt: 'Fix the flaky status tests',
       agentType: 'claude'
     })
+    await layoutApplyTestTransport.settle()
     store.getState().updateTabTitle(tabId, 'Claude working')
+    await layoutApplyTestTransport.settle()
     store.getState().setTabCustomTitle(tabId, 'Status tests')
+    await layoutApplyTestTransport.settle()
 
     const tab = store.getState().tabsByWorktree[WORKTREE_ID][0]
     expect(resolveTerminalTabTitle(tab, true)).toBe('Status tests')
     expect(tab.generatedTitle).toBe('Fix the flaky status tests')
   })
 
-  it('does not generate a title for quick command labeled tabs', () => {
+  it('does not generate a title for quick command labeled tabs', async () => {
     vi.useFakeTimers()
     const store = createTestStore()
     seedStore(store, {
@@ -335,6 +363,7 @@ Implement task B worker instructions for the next dispatch`,
       prompt: 'Fix the flaky status tests',
       agentType: 'claude'
     })
+    await layoutApplyTestTransport.settle()
 
     const tab = store.getState().tabsByWorktree[WORKTREE_ID][0]
     expect(tab.generatedTitle).toBeUndefined()
diff --git a/src/renderer/src/store/slices/browser-page-close-intent-recording.test.ts b/src/renderer/src/store/slices/browser-page-close-intent-recording.test.ts
index a315a561b05..3e91acab288 100644
--- a/src/renderer/src/store/slices/browser-page-close-intent-recording.test.ts
+++ b/src/renderer/src/store/slices/browser-page-close-intent-recording.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it, vi } from 'vitest'
 import type * as AgentStatusModule from '@/lib/agent-status'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
 import { createTestStore, makeWorktree } from './store-test-helpers'
 
 const mocks = vi.hoisted(() => ({ callRuntimeRpc: vi.fn() }))
@@ -18,6 +19,11 @@ vi.mock('@/lib/agent-status', async (importOriginal) => {
   return { ...actual, detectAgentStatusFromTitle: vi.fn().mockReturnValue(null) }
 })
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+mocks.callRuntimeRpc.mockImplementation((_target: unknown, method: string, params: unknown) =>
+  method === 'layout.apply' ? layoutApplyTestTransport.call(params) : undefined
+)
+
 const WORKTREE_ID = 'repo1::/path/wt1'
 const DOC_LOCATION = {
   kind: 'workspace-doc' as const,
diff --git a/src/renderer/src/store/slices/cmd-j-create-actions.test.ts b/src/renderer/src/store/slices/cmd-j-create-actions.test.ts
index 8b187215d56..6c9840a4a6b 100644
--- a/src/renderer/src/store/slices/cmd-j-create-actions.test.ts
+++ b/src/renderer/src/store/slices/cmd-j-create-actions.test.ts
@@ -8,6 +8,13 @@ import {
   MANAGED_BROWSER_UNAVAILABLE_MESSAGE
 } from '@/lib/client-creation-action-policy'
 
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
+
 const createWebRuntimeSessionBrowserTabMock = vi.hoisted(() => vi.fn())
 const createWebRuntimeSessionTerminalMock = vi.hoisted(() => vi.fn())
 
diff --git a/src/renderer/src/store/slices/editor-open-diff.test.ts b/src/renderer/src/store/slices/editor-open-diff.test.ts
index 901def57fd6..ffe98f5a6a9 100644
--- a/src/renderer/src/store/slices/editor-open-diff.test.ts
+++ b/src/renderer/src/store/slices/editor-open-diff.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it, vi } from 'vitest'
 import { createEditorStore, createEditorTabsStore } from './editor-slice-test-harness'
 import type { AppState } from '../types'
+import { ORCA_LAYOUT_OWNER_IS_MAIN } from '../../../../shared/layout-owner-switch'
 
 const { toastErrorMock } = vi.hoisted(() => ({
   toastErrorMock: vi.fn()
@@ -644,14 +645,17 @@ describe('createEditorSlice openDiff', () => {
     expect(store.getState().unifiedTabsByWorktree['wt-1']).toEqual([
       expect.objectContaining({
         entityId: '/repo/a.ts',
-        isPinned: true,
-        isPreview: false
+        isPinned: true
       }),
       expect.objectContaining({
         entityId: 'wt-1::diff::unstaged::b.ts',
         isPreview: true
       })
     ])
+    // Not a preview: a switch-on build omits the flag rather than writing false.
+    expect(store.getState().unifiedTabsByWorktree['wt-1']?.[0]?.isPreview).toBe(
+      ORCA_LAYOUT_OWNER_IS_MAIN ? undefined : false
+    )
   })
 
   it('makes a preview file permanent without pinning the tab', () => {
@@ -678,10 +682,13 @@ describe('createEditorSlice openDiff', () => {
     expect(store.getState().unifiedTabsByWorktree['wt-1']).toEqual([
       expect.objectContaining({
         entityId: '/repo/a.ts',
-        isPinned: undefined,
-        isPreview: false
+        isPinned: undefined
       })
     ])
+    // Not a preview: a switch-on build omits the flag rather than writing false.
+    expect(store.getState().unifiedTabsByWorktree['wt-1']?.[0]?.isPreview).toBe(
+      ORCA_LAYOUT_OWNER_IS_MAIN ? undefined : false
+    )
   })
 
   it('does not replace a dirty file that was opened as a preview', () => {
@@ -714,13 +721,16 @@ describe('createEditorSlice openDiff', () => {
     ])
     expect(store.getState().unifiedTabsByWorktree['wt-1']).toEqual([
       expect.objectContaining({
-        entityId: '/repo/a.ts',
-        isPreview: false
+        entityId: '/repo/a.ts'
       }),
       expect.objectContaining({
         entityId: 'wt-1::diff::unstaged::b.ts',
         isPreview: true
       })
     ])
+    // Not a preview: a switch-on build omits the flag rather than writing false.
+    expect(store.getState().unifiedTabsByWorktree['wt-1']?.[0]?.isPreview).toBe(
+      ORCA_LAYOUT_OWNER_IS_MAIN ? undefined : false
+    )
   })
 })
diff --git a/src/renderer/src/store/slices/editor-slice-test-harness.ts b/src/renderer/src/store/slices/editor-slice-test-harness.ts
index 6be3aa657e0..2adac4f2263 100644
--- a/src/renderer/src/store/slices/editor-slice-test-harness.ts
+++ b/src/renderer/src/store/slices/editor-slice-test-harness.ts
@@ -4,12 +4,21 @@ import { createEditorSlice } from './editor'
 import { createTabsSlice } from './tabs'
 import type { AppState } from '../types'
 
+/** The tab-bar maps switch-on editor paths read; a store without the tabs slice needs them too. */
+export function emptyTabLayoutState(): Pick<
+  AppState,
+  'unifiedTabsByWorktree' | 'groupsByWorktree' | 'layoutByWorktree'
+> {
+  return { unifiedTabsByWorktree: {}, groupsByWorktree: {}, layoutByWorktree: {} }
+}
+
 export function createEditorStore(): StoreApi<AppState> {
   // Only the editor slice + activeWorktreeId are needed for these tests.
-  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  // oxlint-disable-next-line typescript/no-explicit-any, typescript/consistent-type-assertions -- SAFETY: a partial store; tests read only the slices it builds.
   return createStore<any>()((...args: any[]) => ({
     activeWorktreeId: 'wt-1',
     tabsByWorktree: {},
+    ...emptyTabLayoutState(),
     browserTabsByWorktree: {},
     activeBrowserTabId: null,
     activeBrowserTabIdByWorktree: {},
diff --git a/src/renderer/src/store/slices/editor-tab-placement.test.ts b/src/renderer/src/store/slices/editor-tab-placement.test.ts
index 39ee3d9a902..5362aec92b5 100644
--- a/src/renderer/src/store/slices/editor-tab-placement.test.ts
+++ b/src/renderer/src/store/slices/editor-tab-placement.test.ts
@@ -1,8 +1,13 @@
 import type { StoreApi } from 'zustand/vanilla'
 import { describe, expect, it, vi } from 'vitest'
-import { createEditorStore, createEditorTabsStore } from './editor-slice-test-harness'
+import {
+  createEditorStore,
+  createEditorTabsStore,
+  ownedEditorFileId
+} from './editor-slice-test-harness'
 import type { AppState } from '../types'
 import { FLOATING_TERMINAL_WORKTREE_ID } from '../../../../shared/constants'
+import { ORCA_LAYOUT_OWNER_IS_MAIN } from '../../../../shared/layout-owner-switch'
 
 const { toastErrorMock } = vi.hoisted(() => ({
   toastErrorMock: vi.fn()
@@ -20,6 +25,17 @@ vi.mock('@/runtime/close-mirrored-editor-tab', () => ({
     notifyHostOfMirroredEditorCloseMock(...args)
 }))
 
+function fileIdOf(store: { getState: () => AppState }, filePath: string): string | undefined {
+  return store.getState().openFiles.find((file) => file.filePath === filePath)?.id
+}
+
+/** A floating file's id: its path, or an owned id when main owns the layout. */
+function expectedFileId(filePath: string): unknown {
+  return ORCA_LAYOUT_OWNER_IS_MAIN
+    ? ownedEditorFileId(filePath, FLOATING_TERMINAL_WORKTREE_ID, null)
+    : filePath
+}
+
 describe('createEditorSlice floating editor activation', () => {
   it('creates a visible floating editor tab when the floating workspace is empty', () => {
     const store = createEditorTabsStore()
@@ -36,10 +52,12 @@ describe('createEditorSlice floating editor activation', () => {
       { suppressActiveRuntimeFallback: true }
     )
 
+    const fileId = fileIdOf(store, '/tmp/orca/notes.md')
+    expect(fileId).toEqual(expectedFileId('/tmp/orca/notes.md'))
     const tab = store.getState().unifiedTabsByWorktree[FLOATING_TERMINAL_WORKTREE_ID]?.[0]
     expect(tab).toMatchObject({
       contentType: 'editor',
-      entityId: '/tmp/orca/notes.md',
+      entityId: fileId,
       label: 'notes.md',
       worktreeId: FLOATING_TERMINAL_WORKTREE_ID
     })
@@ -47,9 +65,7 @@ describe('createEditorSlice floating editor activation', () => {
       activeTabId: tab?.id,
       tabOrder: [tab?.id]
     })
-    expect(store.getState().activeFileIdByWorktree[FLOATING_TERMINAL_WORKTREE_ID]).toBe(
-      '/tmp/orca/notes.md'
-    )
+    expect(store.getState().activeFileIdByWorktree[FLOATING_TERMINAL_WORKTREE_ID]).toBe(fileId)
   })
 
   it('opens floating markdown tabs without changing the main active editor surface', () => {
@@ -72,8 +88,8 @@ describe('createEditorSlice floating editor activation', () => {
 
     expect(store.getState().activeFileId).toBe('/repo/main.md')
     expect(store.getState().activeTabType).toBe('editor')
-    expect(store.getState().activeFileIdByWorktree[FLOATING_TERMINAL_WORKTREE_ID]).toBe(
-      '/tmp/orca/untitled.md'
+    expect(store.getState().activeFileIdByWorktree[FLOATING_TERMINAL_WORKTREE_ID]).toEqual(
+      expectedFileId('/tmp/orca/untitled.md')
     )
     expect(store.getState().activeTabTypeByWorktree[FLOATING_TERMINAL_WORKTREE_ID]).toBe('editor')
   })
diff --git a/src/renderer/src/store/slices/new-markdown.test.ts b/src/renderer/src/store/slices/new-markdown.test.ts
index 982f804b6cb..09ce96c906d 100644
--- a/src/renderer/src/store/slices/new-markdown.test.ts
+++ b/src/renderer/src/store/slices/new-markdown.test.ts
@@ -5,10 +5,12 @@ import type { AppState } from '../types'
 import type { BrowserTab } from '../../../../shared/browser-workspace-types'
 import type { Tab, TabContentType, TabGroup } from '../../../../shared/tab-types'
 import { makeTab } from './store-session-test-harness'
+import { emptyTabLayoutState } from './editor-slice-test-harness'
 
 function createEditorStore(overrides?: Partial<AppState>): StoreApi<AppState> {
-  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  // oxlint-disable-next-line typescript/no-explicit-any, typescript/consistent-type-assertions -- SAFETY: a partial store; tests read only the slices it builds.
   return createStore<any>()((...args: any[]) => ({
+    ...emptyTabLayoutState(),
     activeWorktreeId: 'wt-1',
     tabsByWorktree: {},
     browserTabsByWorktree: {},
diff --git a/src/renderer/src/store/slices/recently-closed-tabs.test.ts b/src/renderer/src/store/slices/recently-closed-tabs.test.ts
index 8ca10f494e7..88cd5bef188 100644
--- a/src/renderer/src/store/slices/recently-closed-tabs.test.ts
+++ b/src/renderer/src/store/slices/recently-closed-tabs.test.ts
@@ -47,6 +47,15 @@ import {
   remapClosedTerminalTabSnapshotCwds,
   restoreRecentlyClosedTabPosition
 } from './recently-closed-tabs'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
+import { ORCA_LAYOUT_OWNER_IS_MAIN } from '../../../../shared/layout-owner-switch'
+
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
 
 const WT = 'repo1::/path/wt1'
 
@@ -66,15 +75,18 @@ beforeEach(() => {
 })
 
 describe('terminal recently-closed capture', () => {
-  it('captures a snapshot and kind entry on user close', () => {
+  it('captures a snapshot and kind entry on user close', async () => {
     const store = makeSeededStore()
     const tab = store
       .getState()
       .createTab(WT, undefined, undefined, { startupCwd: '/path/wt1/packages/app' })
     store.getState().setTabCustomTitle(tab.id, 'build shell')
+    await layoutApplyTestTransport.settle()
     store.getState().setTabColor(tab.id, '#ff0000')
+    await layoutApplyTestTransport.settle()
 
     store.getState().closeTab(tab.id)
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().recentlyClosedTerminalTabsByWorktree[WT]).toEqual([
       expect.objectContaining({
@@ -97,23 +109,25 @@ describe('terminal recently-closed capture', () => {
     expect(store.getState().recentlyClosedTabKindsByWorktree[WT]).toBeUndefined()
   })
 
-  it('skips capture for a confirmed parked PTY exit', () => {
+  it('skips capture for a confirmed parked PTY exit', async () => {
     const store = makeSeededStore()
     const tab = store.getState().createTab(WT)
 
     store.getState().closeTab(tab.id, { captureRecentlyClosed: false })
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().recentlyClosedTerminalTabsByWorktree[WT]).toBeUndefined()
     expect(store.getState().recentlyClosedTabKindsByWorktree[WT]).toBeUndefined()
   })
 
-  it('caps the terminal stack at 10 snapshots, newest first', () => {
+  it('caps the terminal stack at 10 snapshots, newest first', async () => {
     const store = makeSeededStore()
     for (let i = 0; i < 12; i++) {
       const tab = store
         .getState()
         .createTab(WT, undefined, undefined, { startupCwd: `/path/wt1/dir-${i}` })
       store.getState().closeTab(tab.id)
+      await layoutApplyTestTransport.settle()
     }
 
     const stack = store.getState().recentlyClosedTerminalTabsByWorktree[WT]
@@ -153,7 +167,7 @@ describe('terminal snapshot cwd remapping', () => {
 })
 
 describe('reopenClosedTerminalTab', () => {
-  it('restores the tab bar and group position of a closed middle tab', () => {
+  it('restores the tab bar and group position of a closed middle tab', async () => {
     const store = makeSeededStore()
     const first = store.getState().createTab(WT)
     const middle = store.getState().createTab(WT)
@@ -161,7 +175,9 @@ describe('reopenClosedTerminalTab', () => {
     store.getState().setTabBarOrder(WT, [first.id, middle.id, last.id])
 
     store.getState().closeTab(middle.id)
+    await layoutApplyTestTransport.settle()
     expect(store.getState().reopenClosedTerminalTab(WT)).toBe(true)
+    await layoutApplyTestTransport.settle()
 
     const restored = store
       .getState()
@@ -177,17 +193,21 @@ describe('reopenClosedTerminalTab', () => {
     ).toEqual([first.id, restored!.id, last.id])
   })
 
-  it('recreates a fresh terminal with the snapshot cwd, shell, title, and color', () => {
+  it('recreates a fresh terminal with the snapshot cwd, shell, title, and color', async () => {
     const store = makeSeededStore()
     const tab = store
       .getState()
       .createTab(WT, undefined, 'zsh', { startupCwd: '/path/wt1/packages/app' })
     store.getState().setTabCustomTitle(tab.id, 'build shell')
+    await layoutApplyTestTransport.settle()
     store.getState().setTabColor(tab.id, '#ff0000')
+    await layoutApplyTestTransport.settle()
     store.getState().closeTab(tab.id)
+    await layoutApplyTestTransport.settle()
     expect(store.getState().tabsByWorktree[WT]).toHaveLength(0)
 
     expect(store.getState().reopenClosedTerminalTab(WT)).toBe(true)
+    await layoutApplyTestTransport.settle()
 
     const restored = store.getState().tabsByWorktree[WT]
     expect(restored).toHaveLength(1)
@@ -203,17 +223,20 @@ describe('reopenClosedTerminalTab', () => {
     expect(store.getState().recentlyClosedTerminalTabsByWorktree[WT]).toHaveLength(0)
   })
 
-  it('returns false when the stack is empty (no double-restore)', () => {
+  it('returns false when the stack is empty (no double-restore)', async () => {
     const store = makeSeededStore()
     const tab = store.getState().createTab(WT)
     store.getState().closeTab(tab.id)
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().reopenClosedTerminalTab(WT)).toBe(true)
+    await layoutApplyTestTransport.settle()
     expect(store.getState().reopenClosedTerminalTab(WT)).toBe(false)
+    await layoutApplyTestTransport.settle()
     expect(store.getState().tabsByWorktree[WT]).toHaveLength(1)
   })
 
-  it('skips local reopen on a remote-runtime worktree (host owns the terminal)', () => {
+  it('skips local reopen on a remote-runtime worktree (host owns the terminal)', async () => {
     // Why: a `runtime:` host id makes the worktree host-owned; a local createTab
     // would leave an unbacked phantom tab, so reopen must bail and let the
     // cross-type dispatcher fall through to browser/editor.
@@ -232,24 +255,27 @@ describe('reopenClosedTerminalTab', () => {
     })
 
     expect(store.getState().reopenClosedTerminalTab(WT)).toBe(false)
+    await layoutApplyTestTransport.settle()
     // No local tab spawned, and the snapshot is left intact (not consumed).
     expect(store.getState().tabsByWorktree[WT] ?? []).toHaveLength(0)
     expect(store.getState().recentlyClosedTerminalTabsByWorktree[WT]).toHaveLength(1)
   })
 
-  it('reopens a legacy local worktree while another runtime is merely focused', () => {
+  it('reopens a legacy local worktree while another runtime is merely focused', async () => {
     const store = makeSeededStore()
     store.setState({
       settings: { ...getDefaultSettings('/tmp'), activeRuntimeEnvironmentId: 'env-1' }
     })
     const tab = store.getState().createTab(WT)
     store.getState().closeTab(tab.id)
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().reopenClosedTerminalTab(WT)).toBe(true)
+    await layoutApplyTestTransport.settle()
     expect(store.getState().tabsByWorktree[WT]).toHaveLength(1)
   })
 
-  it('reopens SSH terminals through the renderer-owned create path', () => {
+  it('reopens SSH terminals through the renderer-owned create path', async () => {
     const store = createTestStore()
     seedStore(store, {
       worktreesByRepo: {
@@ -263,8 +289,10 @@ describe('reopenClosedTerminalTab', () => {
       .getState()
       .createTab(WT, undefined, undefined, { startupCwd: '/remote/wt1/packages/app' })
     store.getState().closeTab(tab.id)
+    await layoutApplyTestTransport.settle()
 
     expect(store.getState().reopenClosedTerminalTab(WT)).toBe(true)
+    await layoutApplyTestTransport.settle()
     expect(store.getState().tabsByWorktree[WT]?.[0]?.startupCwd).toBe('/remote/wt1/packages/app')
   })
 })
@@ -325,12 +353,17 @@ describe('restoreRecentlyClosedTabPosition', () => {
       { groupId: 'captured-group', groupIndex: 0 }
     )
 
-    expect(reordered).toHaveBeenCalledWith('captured-group', ['captured-tab', 'captured-sibling'], {
-      recordInteraction: false
-    })
+    // A switch-on build also marks the reorder as one main already made.
+    expect(reordered).toHaveBeenCalledWith(
+      'captured-group',
+      ['captured-tab', 'captured-sibling'],
+      ORCA_LAYOUT_OWNER_IS_MAIN
+        ? { recordInteraction: false, layoutOwnerHandled: true }
+        : { recordInteraction: false }
+    )
   })
 
-  it('does not reuse a stale flat index after group drag reordering', () => {
+  it('does not reuse a stale flat index after group drag reordering', async () => {
     const store = makeSeededStore()
     const first = store.getState().createTab(WT)
     const middle = store.getState().createTab(WT)
@@ -343,12 +376,14 @@ describe('restoreRecentlyClosedTabPosition', () => {
     store.getState().reorderUnifiedTabs(groupId, [first.id, last.id, middle.id])
 
     store.getState().closeTab(middle.id)
+    await layoutApplyTestTransport.settle()
     expect(store.getState().recentlyClosedTerminalTabsByWorktree[WT]?.[0]?.position).toEqual({
       groupId,
       groupIndex: 2
     })
 
     expect(store.getState().reopenClosedTerminalTab(WT)).toBe(true)
+    await layoutApplyTestTransport.settle()
     const restored = store
       .getState()
       .tabsByWorktree[WT]?.find((tab) => tab.id !== first.id && tab.id !== last.id)
@@ -362,7 +397,7 @@ describe('restoreRecentlyClosedTabPosition', () => {
 })
 
 describe('reopenClosedTab cross-type MRU', () => {
-  it('reopens closed tabs of mixed kinds in most-recent-first order', () => {
+  it('reopens closed tabs of mixed kinds in most-recent-first order', async () => {
     const store = makeSeededStore()
 
     // Close order: editor → terminal → browser.
@@ -372,6 +407,7 @@ describe('reopenClosedTab cross-type MRU', () => {
       .getState()
       .createTab(WT, undefined, undefined, { startupCwd: '/path/wt1/api' })
     store.getState().closeTab(terminal.id)
+    await layoutApplyTestTransport.settle()
     const browser = store.getState().createBrowserTab(WT, 'https://example.com', { title: 'Ex' })
     store.getState().closeBrowserTab(browser.id)
 
diff --git a/src/renderer/src/store/slices/tab-view-mode.test.ts b/src/renderer/src/store/slices/tab-view-mode.test.ts
index 56b99c13ba4..94f33b6e451 100644
--- a/src/renderer/src/store/slices/tab-view-mode.test.ts
+++ b/src/renderer/src/store/slices/tab-view-mode.test.ts
@@ -1,6 +1,14 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest'
 import type { AppState } from '../types'
 import { createTestStore, makeUnifiedTab, makeTabGroup } from './store-test-helpers'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
+
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
 
 // Mock sonner (imported transitively by repos.ts via the full store).
 vi.mock('sonner', () => ({ toast: { info: vi.fn(), success: vi.fn(), error: vi.fn() } }))
@@ -51,25 +59,30 @@ describe('tab view mode', () => {
     seedTwoLeafTabs(store)
   })
 
-  it('toggles a tab to chat and back to terminal', () => {
+  it('toggles a tab to chat and back to terminal', async () => {
     expect(findTab(store, 'left-tab')?.viewMode).toBeUndefined()
 
     store.getState().toggleTabViewMode('left-tab')
+    await layoutApplyTestTransport.settle()
     expect(findTab(store, 'left-tab')?.viewMode).toBe('chat')
 
     store.getState().toggleTabViewMode('left-tab')
+    await layoutApplyTestTransport.settle()
     expect(findTab(store, 'left-tab')?.viewMode).toBe('terminal')
   })
 
-  it('setTabViewMode sets an explicit mode', () => {
+  it('setTabViewMode sets an explicit mode', async () => {
     store.getState().setTabViewMode('left-tab', 'chat')
+    await layoutApplyTestTransport.settle()
     expect(findTab(store, 'left-tab')?.viewMode).toBe('chat')
     store.getState().setTabViewMode('left-tab', 'terminal')
+    await layoutApplyTestTransport.settle()
     expect(findTab(store, 'left-tab')?.viewMode).toBe('terminal')
   })
 
-  it('mutates only the targeted tab, leaving a split sibling unchanged', () => {
+  it('mutates only the targeted tab, leaving a split sibling unchanged', async () => {
     store.getState().toggleTabViewMode('left-tab')
+    await layoutApplyTestTransport.settle()
 
     expect(findTab(store, 'left-tab')?.viewMode).toBe('chat')
     // The sibling leaf's tab keeps its (default) view mode.
@@ -105,16 +118,19 @@ describe('tab view mode', () => {
 
     let rowTabId = ''
 
-    it('toggleTabViewMode patches the row in the same write', () => {
+    it('toggleTabViewMode patches the row in the same write', async () => {
       store.getState().toggleTabViewMode(rowTabId)
+      await layoutApplyTestTransport.settle()
       expect(terminalRow(rowTabId)?.viewMode).toBe('chat')
 
       store.getState().toggleTabViewMode(rowTabId)
+      await layoutApplyTestTransport.settle()
       expect(terminalRow(rowTabId)?.viewMode).toBe('terminal')
     })
 
-    it('setTabViewMode patches the row in the same write', () => {
+    it('setTabViewMode patches the row in the same write', async () => {
       store.getState().setTabViewMode(rowTabId, 'chat')
+      await layoutApplyTestTransport.settle()
       expect(terminalRow(rowTabId)?.viewMode).toBe('chat')
     })
   })
diff --git a/src/renderer/src/store/terminals/structured-chat-tab-rename.test.ts b/src/renderer/src/store/terminals/structured-chat-tab-rename.test.ts
index ade01274659..2c8975db0fa 100644
--- a/src/renderer/src/store/terminals/structured-chat-tab-rename.test.ts
+++ b/src/renderer/src/store/terminals/structured-chat-tab-rename.test.ts
@@ -1,6 +1,14 @@
 import { describe, expect, it, vi } from 'vitest'
 import type { Tab } from '../../../../shared/tab-types'
 import { createTestStore, makeWorktree, seedStore } from '../slices/store-test-helpers'
+import { layoutApplyTestTransport } from '@/runtime/layout-apply-test-transport.test-fixture'
+
+// Main's answer to the window's layout commands, as a switch-on build sends them.
+vi.mock('@/runtime/runtime-rpc-client', async (importOriginal) =>
+  (
+    await import('@/runtime/layout-apply-test-transport.test-fixture')
+  ).runtimeRpcClientWithLayoutApply(await importOriginal())
+)
 
 vi.mock('sonner', () => ({
   toast: { info: vi.fn(), success: vi.fn(), error: vi.fn(), warning: vi.fn() }
@@ -68,7 +76,7 @@ function colorOf(store: ReturnType<typeof createTestStore>): string | null | und
 }
 
 describe('renaming a terminal tab still resolves', () => {
-  it('routes a terminal rename through its entityId, not the unified id', () => {
+  it('routes a terminal rename through its entityId, not the unified id', async () => {
     const store = createTestStore()
     seedStore(store, {
       repos: [{ id: 'local-repo', path: '/tmp/app', name: 'app' }] as never,
@@ -80,6 +88,7 @@ describe('renaming a terminal tab still resolves', () => {
 
     // Keyed by the TERMINAL's entityId — the structured tab must not absorb it.
     store.getState().setTabCustomTitle(TERMINAL_TAB_ID, 'Build logs')
+    await layoutApplyTestTransport.settle()
 
     const tabs = store.getState().unifiedTabsByWorktree[WORKTREE] ?? []
     expect(tabs.find((t) => t.id === TERMINAL_UNIFIED_ID)?.customLabel).toBe('Build logs')
@@ -88,21 +97,24 @@ describe('renaming a terminal tab still resolves', () => {
 })
 
 describe('recoloring a structured chat tab', () => {
-  it('writes the color onto the agent-session tab', () => {
+  it('writes the color onto the agent-session tab', async () => {
     const store = storeWithStructuredTab()
     store.getState().setTabColor(STRUCTURED_TAB_ID, 'red')
+    await layoutApplyTestTransport.settle()
     expect(colorOf(store)).toBe('red')
   })
 })
 
 describe('renaming a structured chat tab', () => {
-  it('clears the custom label when the rename is emptied', () => {
+  it('clears the custom label when the rename is emptied', async () => {
     const store = storeWithStructuredTab()
     store.getState().setTabCustomTitle(STRUCTURED_TAB_ID, 'Flaky retry test')
+    await layoutApplyTestTransport.settle()
     // Guard: without the intermediate assertion this case passes on a rename
     // that never wrote anything, since the label starts out null too.
     expect(labelOf(store)).toBe('Flaky retry test')
     store.getState().setTabCustomTitle(STRUCTURED_TAB_ID, null)
+    await layoutApplyTestTransport.settle()
     expect(labelOf(store)).toBeNull()
   })
 })
diff --git a/src/shared/workspace-layout/workspace-layout-first-group-off.test.ts b/src/shared/workspace-layout/workspace-layout-first-group-off.test.ts
index dd57b9000c6..bc216aa2775 100644
--- a/src/shared/workspace-layout/workspace-layout-first-group-off.test.ts
+++ b/src/shared/workspace-layout/workspace-layout-first-group-off.test.ts
@@ -1,8 +1,11 @@
 // Switch-off builds (the switch reads off in tests): a workspace's first group keeps a minted id.
 
-import { describe, expect, it } from 'vitest'
+import { describe, expect, it, vi } from 'vitest'
 import { build, testContext, WS } from './workspace-layout-command.test-fixture'
 
+// rollback build (switch-off) — delete with the switch-off code post-flip
+vi.mock('../layout-owner-switch', () => ({ ORCA_LAYOUT_OWNER_IS_MAIN: false }))
+
 describe('a workspace first group with the layout-owner switch off', () => {
   it('is a minted id, whatever group the sender names', () => {
     const plain = build(testContext(), [{ type: 'createTerminalTab', workspace: WS }])