diff --git a/packages/client/ui-chat/tests/apply-inject.client.spec.tsx b/packages/client/ui-chat/tests/apply-inject.client.spec.tsx
index 91c78811e0..947eb739ed 100644
--- a/packages/client/ui-chat/tests/apply-inject.client.spec.tsx
+++ b/packages/client/ui-chat/tests/apply-inject.client.spec.tsx
@@ -16,6 +16,7 @@ import {
   apply as applyChat, inject as injectChat, type ChatViewInjected,
 } from '@deepseek-ai/dsh-client-ui-chat/client'
 import { SessionSeq, type SessionId } from '@deepseek-ai/dsh-session/types'
+import type { WorkspaceId } from '@deepseek-ai/dsh-workspace/types'
 import { createChatStore } from '../src/client/stores.ts'
 
 usePinnedBrowserLanguages('zh-CN')
@@ -57,7 +58,10 @@ async function bench() {
   )
   new TestRemote(runtime.ctx, { session: { openWorkspacePath } })
   runtime.ctx.provide('uiWorkspace', {
-    connectWorkspace: vi.fn(async () => ROOT),
+    openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+      beforeOpen(ROOT)
+      runtime.sessions.open(ROOT)
+    }),
     openSession: (id: SessionId) => { runtime.sessions.open(id) },
   } as never)
   const session = sessionFakeFor()
diff --git a/packages/client/ui-chat/tests/chat-apply.client.spec.tsx b/packages/client/ui-chat/tests/chat-apply.client.spec.tsx
index 6c1ab9c6a0..ac364e4cb7 100644
--- a/packages/client/ui-chat/tests/chat-apply.client.spec.tsx
+++ b/packages/client/ui-chat/tests/chat-apply.client.spec.tsx
@@ -9,6 +9,7 @@ import { resolveSlotLabel } from '@deepseek-ai/dsh-client-ui-slots'
 import type { ObservableSnapshot } from '@deepseek-ai/dsh-client-store'
 import type { SessionBinding } from '@deepseek-ai/dsh-api-session-controller/client'
 import type { SessionId } from '@deepseek-ai/dsh-session/types'
+import type { WorkspaceId } from '@deepseek-ai/dsh-workspace/types'
 import {
   apply as applyConversation, inject as injectConversation,
 } from '@deepseek-ai/dsh-client-ui-conversation/client'
@@ -44,7 +45,10 @@ async function bench() {
   runtime.ctx.provide('layout', { openRightbar: vi.fn(), closeRightbar: vi.fn() } as never)
   runtime.ctx.provide('sidebarRight', { openResource: vi.fn() } as never)
   runtime.ctx.provide('uiWorkspace', {
-    connectWorkspace: vi.fn(async () => SID),
+    openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+      beforeOpen(SID)
+      runtime.sessions.open(SID)
+    }),
     openSession: (id: SessionId) => { runtime.sessions.open(id) },
   } as never)
   new TestRemote(runtime.ctx, {
diff --git a/packages/client/ui-conversation/src/client/apply.ts b/packages/client/ui-conversation/src/client/apply.ts
index 7827186ede..7eba82cfc0 100644
--- a/packages/client/ui-conversation/src/client/apply.ts
+++ b/packages/client/ui-conversation/src/client/apply.ts
@@ -86,9 +86,10 @@ const ABSENT_FILE_UPLOADS = {
 
 interface WorkspaceNavigation {
   openSession(sessionId: SessionId): void
-  connectWorkspace(
+  openWorkspace(
     workspaceId: Parameters<ConversationInjected['selectWorkspace']>[0],
-  ): Promise<SessionId>
+    beforeOpen: (sessionId: SessionId) => void,
+  ): Promise<void>
 }
 
 /** Resolve the session-scoped Conversation action face, failing loud. */
@@ -232,8 +233,7 @@ export function apply(ctx: Context, config: Config = Config({})): void {
       hooks: {
         composerBlock: sessionId === undefined ? ABSENT_BLOCK : composerBlocks.storeFor(sessionId),
       },
-      selectWorkspace: async (workspaceId) => {
-        const nextId = await workspaceNavigation.connectWorkspace(workspaceId)
+      selectWorkspace: workspaceId => workspaceNavigation.openWorkspace(workspaceId, (nextId) => {
         if (sessionId !== undefined && nextId !== sessionId) {
           const from = inputHub.shell(sessionId)
           const draft = from.snapshot.draft
@@ -253,8 +253,7 @@ export function apply(ctx: Context, config: Config = Config({})): void {
             }
           }
         }
-        workspaceNavigation.openSession(nextId)
-      },
+      }),
     }),
   }, ConversationRoot)
 
diff --git a/packages/client/ui-conversation/tests/apply-inject.client.spec.tsx b/packages/client/ui-conversation/tests/apply-inject.client.spec.tsx
index aa50a19ba2..c6d6226bfe 100644
--- a/packages/client/ui-conversation/tests/apply-inject.client.spec.tsx
+++ b/packages/client/ui-conversation/tests/apply-inject.client.spec.tsx
@@ -50,7 +50,11 @@ async function bench() {
   runtime.ctx.provide('settingsScope', { bind: () => stubSettingsScope().scope } as never)
   const connectWorkspace = vi.fn(async () => ROOT)
   runtime.ctx.provide('uiWorkspace', {
-    connectWorkspace,
+    openWorkspace: async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+      const id = await connectWorkspace()
+      beforeOpen(id)
+      runtime.sessions.open(id)
+    },
     openSession: (id: SessionId) => { runtime.sessions.open(id) },
   } as never)
   const sessionFake = sessionFakeFor()
diff --git a/packages/client/ui-conversation/tests/apply-wiring.client.spec.tsx b/packages/client/ui-conversation/tests/apply-wiring.client.spec.tsx
index a71103fb32..3c59deb1c2 100644
--- a/packages/client/ui-conversation/tests/apply-wiring.client.spec.tsx
+++ b/packages/client/ui-conversation/tests/apply-wiring.client.spec.tsx
@@ -15,7 +15,10 @@ const SID = 'session-1' as SessionId
 async function bench(options: { declareConversation?: boolean } = {}) {
   const runtime = await SlotTestRuntime.create()
   runtime.ctx.provide('uiWorkspace', {
-    connectWorkspace: vi.fn(async () => SID),
+    openWorkspace: vi.fn(async (_workspaceId: unknown, beforeOpen: (id: SessionId) => void) => {
+      beforeOpen(SID)
+      runtime.sessions.open(SID)
+    }),
     openSession: (id: SessionId) => { runtime.sessions.open(id) },
   } as never)
   runtime.ctx.provide('settingsScope', { bind: () => stubSettingsScope().scope } as never)
diff --git a/packages/client/ui-conversation/tests/assembly-surfaces.client.spec.tsx b/packages/client/ui-conversation/tests/assembly-surfaces.client.spec.tsx
index fc4eccb528..573a732780 100644
--- a/packages/client/ui-conversation/tests/assembly-surfaces.client.spec.tsx
+++ b/packages/client/ui-conversation/tests/assembly-surfaces.client.spec.tsx
@@ -12,6 +12,7 @@ import {
 import { InputHub } from '../src/client/input/hub.ts'
 import { apply, inject, type EmptyWorkspaceOwnerProps } from '@deepseek-ai/dsh-client-ui-conversation/client'
 import type { SessionId } from '@deepseek-ai/dsh-session/types'
+import type { WorkspaceId } from '@deepseek-ai/dsh-workspace/types'
 
 // jsdom implements no Range geometry (Lexical's scroll-into-view measures the
 // caret with one once the surface is genuinely contenteditable).
@@ -61,7 +62,10 @@ function WorkspaceProbe({ open }: EmptyWorkspaceOwnerProps) {
 async function bench(opts?: { blank?: boolean }) {
   const runtime = await SlotTestRuntime.create()
   runtime.ctx.provide('uiWorkspace', {
-    connectWorkspace: vi.fn(async () => SID),
+    openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+      beforeOpen(SID)
+      runtime.sessions.open(SID)
+    }),
     openSession: (id: SessionId) => { runtime.sessions.open(id) },
   } as never)
   runtime.ctx.provide('settingsScope', { bind: () => stubSettingsScope().scope } as never)
@@ -86,7 +90,10 @@ describe('resident composer', () => {
   it('renders the locked view state while no session exists at all', async () => {
     const runtime = await SlotTestRuntime.create()
     runtime.ctx.provide('uiWorkspace', {
-      connectWorkspace: vi.fn(async () => SID),
+      openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+        beforeOpen(SID)
+        runtime.sessions.open(SID)
+      }),
       openSession: (id: SessionId) => { runtime.sessions.open(id) },
     } as never)
     runtime.ctx.provide('settingsScope', { bind: () => stubSettingsScope().scope } as never)
@@ -116,7 +123,10 @@ describe('resident composer', () => {
   it('keeps the complete Hero tree mounted when the first Workspace session appears', async () => {
     const runtime = await SlotTestRuntime.create()
     runtime.ctx.provide('uiWorkspace', {
-      connectWorkspace: vi.fn(async () => SID),
+      openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+        beforeOpen(SID)
+        runtime.sessions.open(SID)
+      }),
       openSession: (id: SessionId) => { runtime.sessions.open(id) },
     } as never)
     runtime.ctx.provide('settingsScope', { bind: () => stubSettingsScope().scope } as never)
@@ -184,7 +194,10 @@ describe('prompt rejection through the assembled composer', () => {
   it('renders the promptError alert strip and keeps the draft in the machine', async () => {
     const runtime = await SlotTestRuntime.create()
     runtime.ctx.provide('uiWorkspace', {
-      connectWorkspace: vi.fn(async () => SID),
+      openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+        beforeOpen(SID)
+        runtime.sessions.open(SID)
+      }),
       openSession: (id: SessionId) => { runtime.sessions.open(id) },
     } as never)
     runtime.ctx.provide('settingsScope', { bind: () => stubSettingsScope().scope } as never)
diff --git a/packages/client/ui-layout/src/client/index.ts b/packages/client/ui-layout/src/client/index.ts
index 83439c9d8e..ffbbd68bb9 100644
--- a/packages/client/ui-layout/src/client/index.ts
+++ b/packages/client/ui-layout/src/client/index.ts
@@ -133,7 +133,8 @@ export function apply(ctx: ClientContext): void {
     const handle = createLayoutStore()
     const instance = handle.create()
     const store: typeof handle = { ...handle, create: () => instance }
-    const layout = new LayoutController(instance.actions)
+    const layout = new LayoutController(instance.actions, id =>
+      ctx.slots.entries('main').some(entry => entry.options.key === id))
     const retainMainPanels = (): void => {
       instance.actions.retainMainPanels(ctx.slots.entries('main').flatMap(entry =>
         entry.options.key === undefined ? [] : [entry.options.key]))
@@ -158,6 +159,7 @@ export function apply(ctx: ClientContext): void {
     const disposePanels = ctx.slots.subscribe('main', retainMainPanels)
     retainMainPanels()
     return () => {
+      layout.dispose()
       disposePanels()
       disposeRegistration()
       disposePanelInfo()
diff --git a/packages/client/ui-layout/src/client/service.ts b/packages/client/ui-layout/src/client/service.ts
index 7835f3aa20..14a3924d63 100644
--- a/packages/client/ui-layout/src/client/service.ts
+++ b/packages/client/ui-layout/src/client/service.ts
@@ -29,8 +29,14 @@ export interface ILayout {
   /**
    * Select a global central panel without changing the current Session.
    * @param panelId - registered main key, or null to show the Conversation.
+   * @throws if the selected main key is not registered; preserves the current selection.
    */
   selectPanel(panelId: MainPanelId | null): void
+  /**
+   * Start an asynchronous navigation, superseding any earlier pending navigation.
+   * @returns a signal aborted by the next navigation or layout disposal; check it before committing UI state.
+   */
+  beginNavigation(): AbortSignal
   /** Toggle the sidebar panel (closed ⟷ contract default width). */
   toggleSidebar(): void
   /**
@@ -47,14 +53,38 @@ export interface ILayout {
 
 /** Cross-plugin panel-action face (ctx.layout). */
 export class LayoutController implements ILayout {
-  /** @param panels - actions of the instance shared with the root entry. */
-  constructor(private readonly panels: PanelActions) {}
+  private navigation = new AbortController()
+
+  /**
+   * @param panels - actions of the instance shared with the root entry.
+   * @param hasMainPanel - checks the live main-slot registry for a panel id.
+   */
+  constructor(
+    private readonly panels: PanelActions,
+    private readonly hasMainPanel: (id: MainPanelId) => boolean,
+  ) {}
 
   /** Select a global panel or return to the Conversation. */
   selectPanel(panelId: MainPanelId | null): void {
+    if (panelId !== null && !this.hasMainPanel(panelId)) {
+      throw new Error(`layout.selectPanel: main panel "${panelId}" is not registered`)
+    }
+    this.navigation.abort()
     this.panels.selectPanel(panelId)
   }
 
+  /** @returns the new pending navigation's cancellation signal. */
+  beginNavigation(): AbortSignal {
+    this.navigation.abort()
+    this.navigation = new AbortController()
+    return this.navigation.signal
+  }
+
+  /** Invalidate pending navigations when the layout owner is unloaded. */
+  dispose(): void {
+    this.navigation.abort()
+  }
+
   /** Toggle the sidebar panel (closed ⟷ contract default width). */
   toggleSidebar(): void {
     this.panels.toggleSidebar()
diff --git a/packages/client/ui-layout/tests/apply.client.spec.ts b/packages/client/ui-layout/tests/apply.client.spec.ts
index 94124c2805..74c58022c9 100644
--- a/packages/client/ui-layout/tests/apply.client.spec.ts
+++ b/packages/client/ui-layout/tests/apply.client.spec.ts
@@ -101,6 +101,7 @@ describe('ui-layout client apply', () => {
     const instance = handle.create()
     expect(handle.create()).toBe(instance)
     const layout = ctx.get('layout') as LayoutController
+    expect(() => { layout.selectPanel('missing' as MainPanelId) }).toThrow('main panel "missing" is not registered')
     layout.toggleSidebar()
     expect(instance.getSnapshot().layoutInfo.sidebar).toBe(0)
     const host = rendererHost()
@@ -113,7 +114,11 @@ describe('ui-layout client apply', () => {
     expect(panelInfo.getSnapshot()).toEqual({ activePanelId: panelId })
     disposePanel()
     await vi.waitFor(() => { expect(panelInfo.getSnapshot()).toEqual({ activePanelId: null }) })
+    expect(() => { layout.selectPanel(panelId) }).toThrow('main panel "panel-a" is not registered')
     expect(instance.getSnapshot().layoutInfo.sidebar).toBe(0)
+    const pending = layout.beginNavigation()
+    await fiber.dispose()
+    expect(pending.aborted).toBe(true)
   })
 
   it('theme presenter applies the initial snapshot, follows theme/change, and unwinds on dispose', async () => {
diff --git a/packages/client/ui-layout/tests/service.client.spec.ts b/packages/client/ui-layout/tests/service.client.spec.ts
index 14c3fb479e..7ccadb5c3c 100644
--- a/packages/client/ui-layout/tests/service.client.spec.ts
+++ b/packages/client/ui-layout/tests/service.client.spec.ts
@@ -18,7 +18,7 @@ function fakePanels(): PanelActions {
 describe('LayoutController', () => {
   it('forwards right column transitions to the constructor-supplied actions', () => {
     const panels = fakePanels()
-    const service = new LayoutController(panels)
+    const service = new LayoutController(panels, () => true)
 
     service.openRightbar(true, false)
     service.openRightbar(true, true)
@@ -35,7 +35,7 @@ describe('LayoutController', () => {
 
   it('can toggle the sidebar immediately after construction', () => {
     const panels = fakePanels()
-    const service = new LayoutController(panels)
+    const service = new LayoutController(panels, () => true)
 
     service.toggleSidebar()
 
@@ -45,7 +45,7 @@ describe('LayoutController', () => {
 
   it('forwards panel selection and returning to the Conversation without changing geometry', () => {
     const panels = fakePanels()
-    const service = new LayoutController(panels)
+    const service = new LayoutController(panels, () => true)
     const panelId = 'panel-a' as MainPanelId
     service.selectPanel(panelId)
     service.selectPanel(panelId)
@@ -61,8 +61,8 @@ describe('LayoutController', () => {
   it('keeps separately constructed controllers bound to their own instances', () => {
     const first = fakePanels()
     const second = fakePanels()
-    const firstService = new LayoutController(first)
-    const secondService = new LayoutController(second)
+    const firstService = new LayoutController(first, () => true)
+    const secondService = new LayoutController(second, () => true)
     firstService.toggleSidebar()
     expect(first.toggleSidebar).toHaveBeenCalledTimes(1)
     expect(second.toggleSidebar).not.toHaveBeenCalled()
@@ -70,4 +70,35 @@ describe('LayoutController', () => {
     expect(first.closeRightbar).not.toHaveBeenCalled()
     expect(second.closeRightbar).toHaveBeenCalledTimes(1)
   })
+
+  it('rejects an absent main entry without changing selection or cancelling pending navigation', () => {
+    const panels = fakePanels()
+    const present = new Set(['panel-a'])
+    const service = new LayoutController(panels, id => present.has(id))
+    service.selectPanel('panel-a' as MainPanelId)
+    const navigation = service.beginNavigation()
+    present.delete('panel-a')
+    expect(() => { service.selectPanel('panel-a' as MainPanelId) }).toThrow('main panel "panel-a" is not registered')
+    expect(panels.selectPanel).toHaveBeenCalledOnce()
+    expect(navigation.aborted).toBe(false)
+    service.selectPanel(null)
+    expect(navigation.aborted).toBe(true)
+  })
+
+  it('supersedes asynchronous navigation on another request, any valid selection, and disposal', () => {
+    const service = new LayoutController(fakePanels(), () => true)
+    const first = service.beginNavigation()
+    const second = service.beginNavigation()
+    expect(first.aborted).toBe(true)
+    expect(second.aborted).toBe(false)
+    service.selectPanel('panel-a' as MainPanelId)
+    expect(second.aborted).toBe(true)
+    const repeated = service.beginNavigation()
+    service.selectPanel('panel-a' as MainPanelId)
+    expect(repeated.aborted).toBe(true)
+    const pending = service.beginNavigation()
+    service.dispose()
+    expect(pending.aborted).toBe(true)
+    service.dispose()
+  })
 })
diff --git a/packages/client/ui-tool/tests/assembly-surfaces.client.spec.tsx b/packages/client/ui-tool/tests/assembly-surfaces.client.spec.tsx
index 99ee175099..7959b9c60a 100644
--- a/packages/client/ui-tool/tests/assembly-surfaces.client.spec.tsx
+++ b/packages/client/ui-tool/tests/assembly-surfaces.client.spec.tsx
@@ -6,6 +6,7 @@ import { cleanup, fireEvent, waitFor } from '@testing-library/react'
 import { LocaleRuntime } from '@deepseek-ai/dsh-client-locale/client'
 import type { ISession } from '@deepseek-ai/dsh-api-session-controller/client'
 import type { SessionId } from '@deepseek-ai/dsh-session/types'
+import type { WorkspaceId } from '@deepseek-ai/dsh-workspace/types'
 import type { TodoItem } from '@deepseek-ai/dsh-client-ui-conversation/client'
 import {
   apply as applyChat, inject as injectChat, type ToolResultNode,
@@ -79,7 +80,10 @@ async function bench(nodes: ToolResultNode[]) {
   runtime.ctx.provide('layout', { openDetails: vi.fn(), closeDetails: vi.fn() })
   runtime.ctx.provide('sidebarRight', { openResource: vi.fn() } as never)
   runtime.ctx.provide('uiWorkspace', {
-    connectWorkspace: vi.fn(async () => SID),
+    openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+      beforeOpen(SID)
+      runtime.sessions.open(SID)
+    }),
     openSession: (id: SessionId) => { runtime.sessions.open(id) },
   } as never)
   const locale = new LocaleRuntime(runtime.ctx)
diff --git a/packages/client/ui-tool/tests/toolview-slot.client.spec.tsx b/packages/client/ui-tool/tests/toolview-slot.client.spec.tsx
index 2f72a5d2b7..67417ba4a3 100644
--- a/packages/client/ui-tool/tests/toolview-slot.client.spec.tsx
+++ b/packages/client/ui-tool/tests/toolview-slot.client.spec.tsx
@@ -4,6 +4,7 @@ import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { cleanup } from '@testing-library/react'
 import type { ISession } from '@deepseek-ai/dsh-api-session-controller/client'
 import type { SessionId } from '@deepseek-ai/dsh-session/types'
+import type { WorkspaceId } from '@deepseek-ai/dsh-workspace/types'
 import {
   apply as applyChat, inject as injectChat, type ToolResultNode,
 } from '@deepseek-ai/dsh-client-ui-chat/client'
@@ -66,7 +67,10 @@ async function bench(nodes: ToolResultNode[]) {
   const sidebarRight = { openResource: vi.fn<(address: string) => void>() }
   runtime.ctx.provide('sidebarRight', sidebarRight as never)
   runtime.ctx.provide('uiWorkspace', {
-    connectWorkspace: vi.fn(async () => SID),
+    openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+      beforeOpen(SID)
+      runtime.sessions.open(SID)
+    }),
     openSession: (id: SessionId) => { runtime.sessions.open(id) },
   } as never)
   const locale = new LocaleRuntime(runtime.ctx)
@@ -213,7 +217,10 @@ describe('registrant declaration injection', () => {
     runtime.ctx.provide('layout', { openDetails: vi.fn(), closeDetails: vi.fn() })
     runtime.ctx.provide('sidebarRight', { openResource: vi.fn() } as never)
     runtime.ctx.provide('uiWorkspace', {
-      connectWorkspace: vi.fn(async () => SID),
+      openWorkspace: vi.fn(async (_workspaceId: WorkspaceId, beforeOpen: (id: SessionId) => void) => {
+        beforeOpen(SID)
+        runtime.sessions.open(SID)
+      }),
       openSession: (id: SessionId) => { runtime.sessions.open(id) },
     } as never)
     const locale = new LocaleRuntime(runtime.ctx)
diff --git a/packages/client/ui-workspace/src/client/index.ts b/packages/client/ui-workspace/src/client/index.ts
index 0633e96b85..9204fe7774 100644
--- a/packages/client/ui-workspace/src/client/index.ts
+++ b/packages/client/ui-workspace/src/client/index.ts
@@ -115,8 +115,7 @@ export function apply(ctx: Context): void {
       if (!result.ok) throw new Error(result.error.message)
     },
     forkSession: (sessionId) => {
-      sessions.fork({ sessionId, increaseTitle: true })
-        .then(openSession)
+      uiWorkspace.forkSession(sessionId)
         .catch(() => {
           // Fork or child-rename failure keeps the current selection.
         })
diff --git a/packages/client/ui-workspace/src/client/navigation.ts b/packages/client/ui-workspace/src/client/navigation.ts
index c9dcc5e984..6d86beb826 100644
--- a/packages/client/ui-workspace/src/client/navigation.ts
+++ b/packages/client/ui-workspace/src/client/navigation.ts
@@ -19,6 +19,19 @@ export interface UiWorkspace {
    * @param sessionId - listed or retained Session to display.
    */
   openSession(sessionId: SessionId): void
+  /**
+   * Connect a Workspace and open its Session unless a later navigation supersedes it.
+   * @param workspaceId - target Workspace.
+   * @param beforeOpen - optional synchronous preparation for the selected Session, skipped after supersession.
+   * @returns completion; a superseded request may create a Session but does not open it.
+   */
+  openWorkspace(workspaceId: WorkspaceId, beforeOpen?: (sessionId: SessionId) => void): Promise<void>
+  /**
+   * Fork a Session and open the child unless a later navigation supersedes it.
+   * @param sessionId - source Session.
+   * @returns completion; a superseded request leaves its child available without selecting it.
+   */
+  forkSession(sessionId: SessionId): Promise<void>
   /**
    * Resolve the reusable or newly created blank Session for a Workspace.
    * @param workspaceId - target Workspace.
@@ -76,6 +89,7 @@ export class DirectoryBrowseError extends Error {
 /** Implements Workspace archive and directory UI operations. */
 class UiWorkspaceService extends Service implements UiWorkspace {
   private readonly connecting = new Map<WorkspaceId, Promise<SessionId>>()
+  private readonly lifetime = new AbortController()
 
   /**
    * @param ctx - Client root Context.
@@ -122,6 +136,20 @@ class UiWorkspaceService extends Service implements UiWorkspace {
     this.ctx.layout.selectPanel(null)
   }
 
+  async openWorkspace(workspaceId: WorkspaceId, beforeOpen?: (sessionId: SessionId) => void): Promise<void> {
+    const navigation = AbortSignal.any([this.ctx.layout.beginNavigation(), this.lifetime.signal])
+    const sessionId = await this.connectWorkspace(workspaceId)
+    if (navigation.aborted) return
+    beforeOpen?.(sessionId)
+    if (!navigation.aborted) this.openSession(sessionId)
+  }
+
+  async forkSession(sessionId: SessionId): Promise<void> {
+    const navigation = AbortSignal.any([this.ctx.layout.beginNavigation(), this.lifetime.signal])
+    const childId = await this.sessions.fork({ sessionId, increaseTitle: true })
+    if (!navigation.aborted) this.openSession(childId)
+  }
+
   startSession(workspaceId?: WorkspaceId): void {
     const workspace = this.workspaces.list.getSnapshot()
     const sessions = this.sessions.list.getSnapshot()
@@ -138,8 +166,7 @@ class UiWorkspaceService extends Service implements UiWorkspace {
       this.ctx.layout.selectPanel(null)
       return
     }
-    void this.connectWorkspace(target).then(
-      (sessionId) => { this.openSession(sessionId) },
+    void this.openWorkspace(target).catch(
       (reason: unknown) => { console.warn('new session failed:', reason) },
     )
   }
@@ -168,9 +195,8 @@ class UiWorkspaceService extends Service implements UiWorkspace {
 
   private watchNavigation(): () => void {
     let initial: 'waiting' | 'connecting' | 'done' = 'waiting'
-    let disposed = false
     const reconcile = (): void => {
-      if (disposed) return
+      if (this.lifetime.signal.aborted) return
       if (this.clearArchivedCurrent()) return
       if (initial !== 'waiting') return
       const workspace = this.workspaces.list.getSnapshot()
@@ -188,14 +214,14 @@ class UiWorkspaceService extends Service implements UiWorkspace {
       initial = 'connecting'
       void this.connectWorkspace(target).then(
         (sessionId) => {
-          if (disposed) return
+          if (this.lifetime.signal.aborted) return
           if (this.sessions.list.getSnapshot().current === undefined) {
             this.sessions.open(sessionId)
           }
           initial = 'done'
         },
         (reason: unknown) => {
-          if (disposed) return
+          if (this.lifetime.signal.aborted) return
           initial = 'waiting'
           console.warn('initial workspace selection failed:', reason)
         },
@@ -205,7 +231,7 @@ class UiWorkspaceService extends Service implements UiWorkspace {
     const disposeSessions = this.sessions.list.subscribe(reconcile)
     reconcile()
     return () => {
-      disposed = true
+      this.lifetime.abort()
       disposeSessions()
       disposeWorkspaces()
     }
diff --git a/packages/client/ui-workspace/tests/apply.client.spec.ts b/packages/client/ui-workspace/tests/apply.client.spec.ts
index d35e29c81a..8035b099a1 100644
--- a/packages/client/ui-workspace/tests/apply.client.spec.ts
+++ b/packages/client/ui-workspace/tests/apply.client.spec.ts
@@ -22,7 +22,7 @@ async function bench() {
   const open = vi.fn()
   const clear = vi.fn()
   const selectPanel = vi.fn()
-  ctx.provide('layout', { selectPanel })
+  ctx.provide('layout', { selectPanel, beginNavigation: () => new AbortController().signal })
   const search = vi.fn(async () => ({
     ok: true as const,
     value: { items: [{ sessionId: 'session' as never, snippet: 'match' }], hasMore: false },
diff --git a/packages/client/ui-workspace/tests/workspaces-service.client.spec.ts b/packages/client/ui-workspace/tests/workspaces-service.client.spec.ts
index 7bc3b537c3..52f106f0f0 100644
--- a/packages/client/ui-workspace/tests/workspaces-service.client.spec.ts
+++ b/packages/client/ui-workspace/tests/workspaces-service.client.spec.ts
@@ -10,6 +10,8 @@ import type { ClientRemote, DirectoryListing } from '@deepseek-ai/dsh-api-remote
 import { RemoteError } from '@deepseek-ai/dsh-client-test-runtime'
 import type { RemoteResult } from '@deepseek-ai/dsh-api-remotes/client'
 import { SessionId } from '@deepseek-ai/dsh-session/types'
+import { LayoutController } from '@deepseek-ai/dsh-client-ui-layout/client'
+import type { MainPanelId } from '@deepseek-ai/dsh-client-ui-layout/client'
 import { DirectoryBrowseError, UiWorkspaceService } from '../src/client/navigation.ts'
 
 const sid = (id: string): SessionId => SessionId(id)
@@ -108,6 +110,7 @@ class FakeSessions {
   readonly create: ReturnType<typeof vi.fn<ISessions['create']>>
   readonly open: ReturnType<typeof vi.fn<(id: SessionId) => void>>
   readonly clear: ReturnType<typeof vi.fn<() => void>>
+  readonly fork = vi.fn<ISessions['fork']>(async () => sid('forked'))
 
   constructor(initial: SessionListState) {
     this.list = new MutableSource(initial)
@@ -189,8 +192,14 @@ interface BenchOptions {
 
 function bench(options: BenchOptions = {}) {
   const ctx = new Context()
-  const selectPanel = vi.fn()
-  ctx.provide('layout', { selectPanel })
+  const layout = new LayoutController({
+    selectPanel: vi.fn(), retainMainPanels: vi.fn(),
+    setSidebar: vi.fn(), toggleSidebar: vi.fn(), setViewportWidth: vi.fn(),
+    setRightbar: vi.fn(), openRightbar: vi.fn(), closeRightbar: vi.fn(),
+  }, () => true)
+  const selectPanel = vi.spyOn(layout, 'selectPanel')
+  ctx.provide('layout', layout)
+  ctx.effect(() => () => { layout.dispose() })
   const directoryPicker = new FakeDirectoryPicker()
   const workspaces = new FakeWorkspaces(options.workspaces ?? workspaceState([], [], 'pending'))
   const sessions = new FakeSessions(options.sessions ?? sessionState([], undefined, 'pending'))
@@ -200,7 +209,7 @@ function bench(options: BenchOptions = {}) {
     workspaces,
     sessions as unknown as ISessions,
   )
-  return { ctx, directoryPicker, sessions, uiWorkspace, workspaces, selectPanel }
+  return { ctx, directoryPicker, sessions, uiWorkspace, workspaces, layout, selectPanel }
 }
 
 async function flush(): Promise<void> {
@@ -228,6 +237,107 @@ describe('UiWorkspaceService', () => {
     expect(b.selectPanel).not.toHaveBeenCalled()
   })
 
+  it('leaves a later panel selection in place when New Session finishes', async () => {
+    const b = bench({
+      sessions: sessionState([summary('current')], sid('current')),
+      workspaces: workspaceState([workspace('alpha')]),
+    })
+    const created = Promise.withResolvers<SessionId>()
+    b.sessions.create.mockReturnValue(created.promise)
+    const opening = vi.spyOn(b.uiWorkspace, 'openWorkspace')
+    b.uiWorkspace.startSession(wid('alpha'))
+    b.layout.selectPanel('panel-a' as MainPanelId)
+    created.resolve(sid('late'))
+    await opening.mock.results[0]!.value
+    expect(b.sessions.open).not.toHaveBeenCalled()
+    expect(b.selectPanel).toHaveBeenCalledExactlyOnceWith('panel-a')
+    expect(b.sessions.list.getSnapshot().current).toBe(sid('current'))
+  })
+
+  it('opens only the latest Workspace request when creation completes out of order', async () => {
+    const b = bench({
+      sessions: sessionState([summary('current')], sid('current')),
+      workspaces: workspaceState([workspace('alpha'), workspace('beta')]),
+    })
+    const older = Promise.withResolvers<SessionId>()
+    const newer = Promise.withResolvers<SessionId>()
+    b.sessions.create.mockImplementation(options => options?.workspaceId === wid('alpha') ? older.promise : newer.promise)
+    const oldDraft = vi.fn()
+    const newDraft = vi.fn()
+    const first = b.uiWorkspace.openWorkspace(wid('alpha'), oldDraft)
+    const second = b.uiWorkspace.openWorkspace(wid('beta'), newDraft)
+    newer.resolve(sid('newer'))
+    await second
+    older.resolve(sid('older'))
+    await first
+    expect(oldDraft).not.toHaveBeenCalled()
+    expect(newDraft).toHaveBeenCalledExactlyOnceWith(sid('newer'))
+    expect(b.sessions.open).toHaveBeenCalledExactlyOnceWith(sid('newer'))
+  })
+
+  it('does not move drafts or reopen a Workspace after reselecting the current Session', async () => {
+    const b = bench({
+      sessions: sessionState([summary('current')], sid('current')),
+      workspaces: workspaceState([workspace('alpha')]),
+    })
+    const created = Promise.withResolvers<SessionId>()
+    b.sessions.create.mockReturnValue(created.promise)
+    const moveDraft = vi.fn()
+    const pending = b.uiWorkspace.openWorkspace(wid('alpha'), moveDraft)
+    b.uiWorkspace.openSession(sid('current'))
+    created.resolve(sid('late'))
+    await pending
+    expect(moveDraft).not.toHaveBeenCalled()
+    expect(b.sessions.open).toHaveBeenCalledExactlyOnceWith(sid('current'))
+  })
+
+  it('keeps navigation performed by preparation and propagates preparation failures', async () => {
+    const b = bench({
+      sessions: sessionState([summary('current')], sid('current')),
+      workspaces: workspaceState([workspace('alpha')]),
+    })
+    await b.uiWorkspace.openWorkspace(wid('alpha'), () => { b.layout.selectPanel('panel-a' as MainPanelId) })
+    expect(b.sessions.open).not.toHaveBeenCalled()
+    await expect(b.uiWorkspace.openWorkspace(wid('alpha'), () => { throw new Error('draft failed') })).rejects.toThrow('draft failed')
+    expect(b.sessions.open).not.toHaveBeenCalled()
+  })
+
+  it('opens a fork only while its navigation is current and preserves fork failures', async () => {
+    const b = bench()
+    await b.uiWorkspace.forkSession(sid('source'))
+    expect(b.sessions.fork).toHaveBeenCalledWith({ sessionId: sid('source'), increaseTitle: true })
+    expect(b.sessions.open).toHaveBeenCalledExactlyOnceWith(sid('forked'))
+    const forked = Promise.withResolvers<SessionId>()
+    b.sessions.fork.mockReturnValueOnce(forked.promise)
+    const pending = b.uiWorkspace.forkSession(sid('source'))
+    b.layout.selectPanel('panel-a' as MainPanelId)
+    forked.resolve(sid('late-fork'))
+    await pending
+    expect(b.sessions.open).toHaveBeenCalledOnce()
+    b.sessions.fork.mockRejectedValueOnce(new Error('fork failed'))
+    await expect(b.uiWorkspace.forkSession(sid('source'))).rejects.toThrow('fork failed')
+    expect(b.sessions.open).toHaveBeenCalledOnce()
+  })
+
+  it('does not open a pending Workspace or fork after the navigation owner is disposed', async () => {
+    for (const kind of ['workspace', 'fork'] as const) {
+      const b = bench({
+        sessions: sessionState([summary('current')], sid('current')),
+        workspaces: workspaceState([workspace('alpha')]),
+      })
+      const resolved = Promise.withResolvers<SessionId>()
+      b.sessions.create.mockReturnValue(resolved.promise)
+      b.sessions.fork.mockReturnValue(resolved.promise)
+      const pending = kind === 'workspace'
+        ? b.uiWorkspace.openWorkspace(wid('alpha'))
+        : b.uiWorkspace.forkSession(sid('current'))
+      await b.ctx.fiber.dispose()
+      resolved.resolve(sid('late'))
+      await pending
+      expect(b.sessions.open).not.toHaveBeenCalled()
+    }
+  })
+
   it('reuses only an unarchived member blank and coalesces concurrent creation', async () => {
     const b = bench()
     const memberBlank = sid('member-blank')