diff --git a/packages/ui/src/components/comments/useInlineCommentController.ts b/packages/ui/src/components/comments/useInlineCommentController.ts
index 7009aef0..b8862039 100644
--- a/packages/ui/src/components/comments/useInlineCommentController.ts
+++ b/packages/ui/src/components/comments/useInlineCommentController.ts
@@ -12,7 +12,6 @@ import { useI18n } from '@/lib/i18n';
 import { useEffectiveDirectory } from '@/hooks/useEffectiveDirectory';
 import { getRuntimeKey } from '@/lib/runtime-switch';
 import { focusChatInput } from '@/components/chat/composer/editor/dom';
-import { useUIStore } from '@/stores/useUIStore';
 
 type LineRangeBase = {
   start: number;
@@ -164,7 +163,6 @@ export function useInlineCommentController<TRange extends LineRangeBase>(
 
     reset();
     if (isNewComment) {
-      useUIStore.getState().setActiveMainTab('chat');
       requestAnimationFrame(focusChatInput);
     }
   }, [addDraft, editingDraftId, fileLabel, getCodeForRange, language, reset, selection, source, t, target, toStoreRange, updateDraft]);
diff --git a/packages/ui/src/components/layout/MainLayout.tsx b/packages/ui/src/components/layout/MainLayout.tsx
index 28e44ce8..1a25e06c 100644
--- a/packages/ui/src/components/layout/MainLayout.tsx
+++ b/packages/ui/src/components/layout/MainLayout.tsx
@@ -45,7 +45,7 @@ const SettingsWindow = lazyWithChunkRecovery(() => import('@/components/views/Se
 
 export const MainLayout: React.FC = () => {
     const isSidebarOpen = useUIStore((state) => state.isSidebarOpen);
-    const activeMainTab = useUIStore((state) => state.activeMainTab);
+    const activeSurface = useUIStore((state) => state.activeSurface);
     const setIsMobile = useUIStore((state) => state.setIsMobile);
     const isSessionSwitcherOpen = useUIStore((state) => state.isSessionSwitcherOpen);
     const isSettingsDialogOpen = useUIStore((state) => state.isSettingsDialogOpen);
@@ -83,7 +83,7 @@ export const MainLayout: React.FC = () => {
             if (sessionSelected || draftOpened) closeSurfacePages();
         });
         const unsubscribeTab = useUIStore.subscribe((state, prev) => {
-            if (state.activeMainTab !== prev.activeMainTab) closeSurfacePages();
+            if (state.activeSurface !== prev.activeSurface) closeSurfacePages();
         });
         return () => {
             unsubscribeSession();
@@ -195,7 +195,7 @@ export const MainLayout: React.FC = () => {
     }, [isMobile, setMobileSessionPanelOpen]);
 
     useEffect(() => {
-        if (!isMobile || activeMainTab !== 'chat' || mobileLeftDrawerOpen || mobileRightSidebarOpen || isSettingsDialogOpen) {
+        if (!isMobile || activeSurface !== 'chat' || mobileLeftDrawerOpen || mobileRightSidebarOpen || isSettingsDialogOpen) {
             return;
         }
 
@@ -231,7 +231,7 @@ export const MainLayout: React.FC = () => {
                 window.clearTimeout(timeoutId);
             }
         };
-    }, [activeMainTab, isMobile, isSettingsDialogOpen, mobileLeftDrawerOpen, mobileRightSidebarOpen]);
+    }, [activeSurface, isMobile, isSettingsDialogOpen, mobileLeftDrawerOpen, mobileRightSidebarOpen]);
 
     // Ensure mobile drawers are closed when opening full-screen settings
     useEffect(() => {
@@ -263,10 +263,10 @@ export const MainLayout: React.FC = () => {
         // Desktop surfaces live in the context panel; the only full-view
         // overlays left there are the terminal (promoted by project actions)
         // and the diagram viewer. Mobile keeps the full tab set.
-        if (!isMobile && activeMainTab !== 'terminal' && activeMainTab !== 'diagram') {
+        if (!isMobile && activeSurface !== 'terminal' && activeSurface !== 'diagram') {
             return null;
         }
-        switch (activeMainTab) {
+        switch (activeSurface) {
             case 'plan':
                 return <React.Suspense fallback={null}><PlanView /></React.Suspense>;
             case 'git':
@@ -284,9 +284,9 @@ export const MainLayout: React.FC = () => {
             default:
                 return null;
         }
-    }, [activeMainTab, isMobile, mobileRightSidebarOpen]);
+    }, [activeSurface, isMobile, mobileRightSidebarOpen]);
 
-    const isChatActive = activeMainTab === 'chat';
+    const isChatActive = activeSurface === 'chat';
 
     return (
         <DiffWorkerProvider>
diff --git a/packages/ui/src/components/views/TerminalView.tsx b/packages/ui/src/components/views/TerminalView.tsx
index c97f608e..e4da0d45 100644
--- a/packages/ui/src/components/views/TerminalView.tsx
+++ b/packages/ui/src/components/views/TerminalView.tsx
@@ -147,8 +147,8 @@ export const TerminalView: React.FC<TerminalViewProps> = ({ visible }) => {
         terminalControllerRef.current?.focus();
     }, [useTouchTerminalInput]);
 
-    const activeMainTab = useUIStore((state) => state.activeMainTab);
-    const isTerminalActive = activeMainTab === 'terminal';
+    const activeSurface = useUIStore((state) => state.activeSurface);
+    const isTerminalActive = activeSurface === 'terminal';
     const isTerminalVisible = visible ?? isTerminalActive;
     const [hasOpenedTerminalViewport, setHasOpenedTerminalViewport] = React.useState(isTerminalVisible);
 
diff --git a/packages/ui/src/hooks/useRouter.ts b/packages/ui/src/hooks/useRouter.ts
index fa794ad5..1e34e765 100644
--- a/packages/ui/src/hooks/useRouter.ts
+++ b/packages/ui/src/hooks/useRouter.ts
@@ -3,7 +3,7 @@ import { useSessionUIStore } from '@/sync/session-ui-store';
 import { useUIStore } from '@/stores/useUIStore';
 import { parseRoute, updateBrowserURL, hasRouteParams } from '@/lib/router';
 import type { RouteState, AppRouteState } from '@/lib/router';
-import type { MainTab } from '@/stores/useUIStore';
+import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { resolveSettingsSlug } from '@/lib/settings/metadata';
 import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 
@@ -49,7 +49,7 @@ export function useRouter(): void {
 
   // Get store actions (stable references)
   const setCurrentSession = useSessionUIStore((state) => state.setCurrentSession);
-  const setActiveMainTab = useUIStore((state) => state.setActiveMainTab);
+  const setActiveSurface = useUIStore((state) => state.setActiveSurface);
   const setSettingsDialogOpen = useUIStore((state) => state.setSettingsDialogOpen);
   const setSettingsPage = useUIStore((state) => state.setSettingsPage);
   const navigateToDiff = useUIStore((state) => state.navigateToDiff);
@@ -75,11 +75,11 @@ export function useRouter(): void {
           }
         }
 
-        // 2. Handle settings (takes precedence over tabs - it's a full-screen overlay)
+        // 2. Handle settings first because it is a full-screen overlay.
         if (route.settingsPath) {
           setSettingsPage(resolveSettingsSlug(route.settingsPath));
           setSettingsDialogOpen(true);
-          // Don't process tab when settings is open
+          // Do not process a route view while settings is open.
           return;
         }
 
@@ -88,9 +88,9 @@ export function useRouter(): void {
           setSettingsDialogOpen(false);
         }
 
-        // 3. Apply tab
+        // 3. Apply the view selected by the legacy URL parameter.
         if (route.tab) {
-          setActiveMainTab(route.tab);
+          setActiveSurface(route.tab);
         }
 
         // 4. Apply diff file (only if going to diff tab)
@@ -101,7 +101,7 @@ export function useRouter(): void {
         isApplyingRouteRef.current = false;
       }
     },
-    [setCurrentSession, setActiveMainTab, setSettingsDialogOpen, setSettingsPage, navigateToDiff]
+    [setCurrentSession, setActiveSurface, setSettingsDialogOpen, setSettingsPage, navigateToDiff]
   );
 
   /**
@@ -113,7 +113,7 @@ export function useRouter(): void {
 
     return {
       sessionId: sessionState.currentSessionId,
-      tab: uiState.activeMainTab,
+      tab: uiState.activeSurface,
       isSettingsOpen: uiState.isSettingsDialogOpen,
       settingsPath: uiState.settingsPage,
       diffFile: uiState.pendingDiffFile,
@@ -162,7 +162,7 @@ export function useRouter(): void {
         updateBrowserURL({
           ...getCurrentAppState(),
           sessionId: route.sessionId ?? useSessionUIStore.getState().currentSessionId,
-          tab: route.tab ?? useUIStore.getState().activeMainTab,
+          tab: route.tab ?? useUIStore.getState().activeSurface,
           settingsPath: route.settingsPath ?? useUIStore.getState().settingsPage,
           diffFile: route.diffFile ?? useUIStore.getState().pendingDiffFile,
         }, { replace: true, force: true });
@@ -195,13 +195,13 @@ export function useRouter(): void {
     return unsubscribe;
   }, [isVSCode, isEmbeddedChat, syncURLFromState]);
 
-  // Subscribe to UI store changes (tab, settings)
+  // Subscribe to UI store changes (view, settings)
   React.useEffect(() => {
     if (isVSCode || isEmbeddedChat) {
       return;
     }
 
-    let prevTab: MainTab = useUIStore.getState().activeMainTab;
+    let prevSurface: WorkspaceSurface = useUIStore.getState().activeSurface;
     let prevSettingsOpen: boolean = useUIStore.getState().isSettingsDialogOpen;
     let prevSettingsPath: string = useUIStore.getState().settingsPage;
     let prevDiffFile: string | null = useUIStore.getState().pendingDiffFile;
@@ -212,19 +212,19 @@ export function useRouter(): void {
         return;
       }
 
-      const tabChanged = state.activeMainTab !== prevTab;
+      const surfaceChanged = state.activeSurface !== prevSurface;
       const settingsOpenChanged = state.isSettingsDialogOpen !== prevSettingsOpen;
       const settingsPathChanged = state.settingsPage !== prevSettingsPath;
-      const diffFileChanged = state.pendingDiffFile !== prevDiffFile && state.activeMainTab === 'diff';
+      const diffFileChanged = state.pendingDiffFile !== prevDiffFile && state.activeSurface === 'diff';
 
       // Update tracking vars
-      prevTab = state.activeMainTab;
+      prevSurface = state.activeSurface;
       prevSettingsOpen = state.isSettingsDialogOpen;
       prevSettingsPath = state.settingsPage;
       prevDiffFile = state.pendingDiffFile;
 
       // Only sync if something relevant changed
-      if (tabChanged || settingsOpenChanged || settingsPathChanged || diffFileChanged) {
+      if (surfaceChanged || settingsOpenChanged || settingsPathChanged || diffFileChanged) {
         syncURLFromState();
       }
     });
@@ -252,9 +252,9 @@ export function useRouter(): void {
         if (uiState.isSettingsDialogOpen) {
           setSettingsDialogOpen(false);
         }
-        // Reset to chat tab if not already there
-        if (uiState.activeMainTab !== 'chat') {
-          setActiveMainTab('chat');
+        // Reset to chat when no route view is specified.
+        if (uiState.activeSurface !== 'chat') {
+          setActiveSurface('chat');
         }
       }
     };
@@ -264,5 +264,5 @@ export function useRouter(): void {
     return () => {
       window.removeEventListener('popstate', handlePopState);
     };
-  }, [applyRoute, isVSCode, isEmbeddedChat, setActiveMainTab, setSettingsDialogOpen]);
+  }, [applyRoute, isVSCode, isEmbeddedChat, setActiveSurface, setSettingsDialogOpen]);
 }
diff --git a/packages/ui/src/lib/router/index.ts b/packages/ui/src/lib/router/index.ts
index c6f2f23c..dc68e1f8 100644
--- a/packages/ui/src/lib/router/index.ts
+++ b/packages/ui/src/lib/router/index.ts
@@ -6,15 +6,15 @@
  *
  * URL Schema:
  * - `?session=<id>` - Navigate to specific session
- * - `?tab=<chat|git|diff|terminal|files>` - Active main tab
+ * - `?tab=<chat|git|diff|terminal|files>` - Legacy URL name for the active workspace surface
  * - `?settings=<section>` - Open settings to specific section
  * - `?file=<path>` - Diff view with file selected
  *
  * Examples:
  * - `/?session=abc123` - Open session abc123
- * - `/?tab=git` - Open git tab
+ * - `/?tab=git` - Open the Git surface
  * - `/?settings=providers` - Open settings to providers section
- * - `/?tab=diff&file=src/main.ts` - Open diff view with file
+ * - `/?tab=diff&file=src/main.ts` - Open the Diff surface with a file
  */
 
 export type { RouteState } from './types';
diff --git a/packages/ui/src/lib/router/parseRoute.ts b/packages/ui/src/lib/router/parseRoute.ts
index 23addc40..458fb4f9 100644
--- a/packages/ui/src/lib/router/parseRoute.ts
+++ b/packages/ui/src/lib/router/parseRoute.ts
@@ -1,4 +1,4 @@
-import type { MainTab } from '@/stores/useUIStore';
+import type { WorkspaceSurface } from '@/stores/useUIStore';
 import {
   type RouteState,
   VALID_TABS,
@@ -52,13 +52,13 @@ function parseSessionId(params: URLSearchParams): string | null {
  * Parse main tab from URL parameters.
  * Returns null if missing or invalid.
  */
-function parseTab(params: URLSearchParams): MainTab | null {
+function parseTab(params: URLSearchParams): WorkspaceSurface | null {
   const value = params.get(ROUTE_PARAMS.TAB);
   if (!value) {
     return null;
   }
 
-  const normalized = value.toLowerCase().trim() as MainTab;
+  const normalized = value.toLowerCase().trim() as WorkspaceSurface;
   if (VALID_TABS.includes(normalized)) {
     return normalized;
   }
diff --git a/packages/ui/src/lib/router/serializeRoute.ts b/packages/ui/src/lib/router/serializeRoute.ts
index c4c27c11..6b6e054c 100644
--- a/packages/ui/src/lib/router/serializeRoute.ts
+++ b/packages/ui/src/lib/router/serializeRoute.ts
@@ -1,4 +1,4 @@
-import type { MainTab } from '@/stores/useUIStore';
+import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 import { ROUTE_PARAMS } from './types';
 
@@ -7,7 +7,7 @@ import { ROUTE_PARAMS } from './types';
  */
 export interface AppRouteState {
   sessionId: string | null;
-  tab: MainTab;
+  tab: WorkspaceSurface;
   isSettingsOpen: boolean;
   settingsPath: string;
   diffFile: string | null;
@@ -16,7 +16,7 @@ export interface AppRouteState {
 /**
  * Default tab when none is specified.
  */
-const DEFAULT_TAB: MainTab = 'chat';
+const DEFAULT_TAB: WorkspaceSurface = 'chat';
 
 /**
  * Serialize application state to URL search parameters.
diff --git a/packages/ui/src/lib/router/types.ts b/packages/ui/src/lib/router/types.ts
index 3978b450..9c5801e2 100644
--- a/packages/ui/src/lib/router/types.ts
+++ b/packages/ui/src/lib/router/types.ts
@@ -1,5 +1,5 @@
 import type { SidebarSection } from '@/constants/sidebar';
-import type { MainTab } from '@/stores/useUIStore';
+import type { WorkspaceSurface } from '@/stores/useUIStore';
 
 /**
  * Represents the current route state derived from URL parameters.
@@ -8,8 +8,8 @@ import type { MainTab } from '@/stores/useUIStore';
 export interface RouteState {
   /** Session ID to navigate to */
   sessionId: string | null;
-  /** Main tab to display (chat, git, diff, terminal, files) */
-  tab: MainTab | null;
+  /** View selected through the legacy `tab` URL parameter. */
+  tab: WorkspaceSurface | null;
   /** Settings section - when non-null, settings dialog should be open */
   settingsPath: string | null;
   /** File path for diff view */
@@ -17,9 +17,9 @@ export interface RouteState {
 }
 
 /**
- * Valid main tab values for URL routing.
+ * Valid values for the legacy `tab` URL parameter.
  */
-export const VALID_TABS: readonly MainTab[] = ['chat', 'git', 'diff', 'terminal', 'files', 'diagram'] as const;
+export const VALID_TABS: readonly WorkspaceSurface[] = ['chat', 'git', 'diff', 'terminal', 'files', 'diagram'] as const;
 
 /**
  * Valid settings section values for URL routing.
diff --git a/packages/ui/src/stores/DOCUMENTATION.md b/packages/ui/src/stores/DOCUMENTATION.md
index 6ef48876..49ed4a87 100644
--- a/packages/ui/src/stores/DOCUMENTATION.md
+++ b/packages/ui/src/stores/DOCUMENTATION.md
@@ -38,7 +38,7 @@ Examples:
 - `useFeatureFlagsStore.ts`
 - `useUpdateStore.ts`
 
-These stores coordinate visible app state, navigation, selected tabs, dialogs, and lightweight feature flags.
+These stores coordinate visible app state, navigation, selected context-panel tabs, dialogs, and lightweight feature flags. `useUIStore.activeSurface` selects the primary mobile view and the few desktop views that are promoted out of the context panel. It is not a desktop tab selection.
 
 Context-panel session chats mount only the active chat iframe. After installing
 its message listener, the iframe requests its authoritative visibility from the
diff --git a/packages/ui/src/stores/useUIStore.ts b/packages/ui/src/stores/useUIStore.ts
index 24235b3b..7c772122 100644
--- a/packages/ui/src/stores/useUIStore.ts
+++ b/packages/ui/src/stores/useUIStore.ts
@@ -13,7 +13,13 @@ import { useFilesViewTabsStore } from './useFilesViewTabsStore';
 import { isWindowsArm64 } from '@/lib/platform';
 import { isVSCodeRuntime } from '@/lib/desktop';
 
-export type MainTab = 'chat' | 'plan' | 'git' | 'diff' | 'terminal' | 'files' | 'context' | 'diagram';
+/**
+ * The primary view on mobile and the desktop's promoted full-screen view.
+ * Desktop context-panel content is not represented here.
+ */
+export type WorkspaceSurface = 'chat' | 'plan' | 'git' | 'diff' | 'terminal' | 'files' | 'context' | 'diagram';
+/** @deprecated Use WorkspaceSurface. */
+export type MainTab = WorkspaceSurface;
 export type PendingDiffScope = 'working' | 'staged' | 'turn';
 export type ContextPanelMode = 'diff' | 'walkthrough' | 'file' | 'context' | 'plan' | 'chat' | 'browser' | 'git' | 'pr' | 'notes' | 'terminal';
 export type MermaidRenderingMode = 'svg' | 'ascii';
@@ -77,7 +83,9 @@ type PendingFileNavigation = {
   column: number;
 };
 
-export type MainTabGuard = (nextTab: MainTab) => boolean;
+export type WorkspaceSurfaceGuard = (nextSurface: WorkspaceSurface) => boolean;
+/** @deprecated Use WorkspaceSurfaceGuard. */
+export type MainTabGuard = WorkspaceSurfaceGuard;
 export type EventStreamStatus =
   | 'idle'
   | 'connecting'
@@ -129,7 +137,7 @@ const CONTEXT_PANEL_MAX_WIDTH = 1400;
 const CONTEXT_PANEL_MAX_TABS = 12;
 const CONTEXT_PANEL_MAX_LABEL_LENGTH = 120;
 const LEFT_SIDEBAR_MIN_WIDTH = 280;
-const activeMainTabByRuntime = new Map<string, MainTab>();
+const activeSurfaceByRuntime = new Map<string, WorkspaceSurface>();
 /** Separates browser tabs opened in the same millisecond. */
 let browserTabSequence = 0;
 
@@ -648,8 +656,12 @@ interface UIStore {
   workStatusHiddenSections: string[];
   isSessionSwitcherOpen: boolean;
   isSessionDropdownOpen: boolean;
-  activeMainTab: MainTab;
-  mainTabGuard: MainTabGuard | null;
+  activeSurface: WorkspaceSurface;
+  surfaceGuard: WorkspaceSurfaceGuard | null;
+  /** @deprecated Use activeSurface. */
+  activeMainTab: WorkspaceSurface;
+  /** @deprecated Use surfaceGuard. */
+  mainTabGuard: WorkspaceSurfaceGuard | null;
   sidebarOpenBeforeFullscreenTab: boolean | null;
   pendingDiffFile: string | null;
   pendingDiffStaged: boolean;
@@ -834,10 +846,14 @@ interface UIStore {
   setWorkStatusHiddenSections: (sectionIds: string[]) => void;
   setSessionSwitcherOpen: (open: boolean) => void;
   setSessionDropdownOpen: (open: boolean) => void;
-  setActiveMainTab: (tab: MainTab) => void;
+  setActiveSurface: (surface: WorkspaceSurface) => void;
+  /** @deprecated Use setActiveSurface. */
+  setActiveMainTab: (surface: WorkspaceSurface) => void;
   prepareForRuntimeSwitch: (runtimeKey?: string | null) => void;
   restoreForRuntimeSwitch: (runtimeKey?: string | null) => void;
-  setMainTabGuard: (guard: MainTabGuard | null) => void;
+  setSurfaceGuard: (guard: WorkspaceSurfaceGuard | null) => void;
+  /** @deprecated Use setSurfaceGuard. */
+  setMainTabGuard: (guard: WorkspaceSurfaceGuard | null) => void;
   setPendingDiffFile: (filePath: string | null, staged?: boolean, scope?: PendingDiffScope | null) => void;
   setPendingDiagramFile: (filePath: string | null) => void;
   setPendingFileNavigation: (navigation: PendingFileNavigation | null) => void;
@@ -1011,6 +1027,8 @@ export const useUIStore = create<UIStore>()(
         workStatusHiddenSections: [],
         isSessionSwitcherOpen: false,
         isSessionDropdownOpen: false,
+        activeSurface: 'chat',
+        surfaceGuard: null,
         activeMainTab: 'chat',
         mainTabGuard: null,
         sidebarOpenBeforeFullscreenTab: null,
@@ -1625,29 +1643,33 @@ export const useUIStore = create<UIStore>()(
           set({ isSessionDropdownOpen: open });
         },
 
-        setMainTabGuard: (guard) => {
-          if (get().mainTabGuard === guard) {
+        setSurfaceGuard: (guard) => {
+          if (get().surfaceGuard === guard) {
             return;
           }
-          set({ mainTabGuard: guard });
+          set({ surfaceGuard: guard, mainTabGuard: guard });
         },
 
-        setActiveMainTab: (tab) => {
-          const guard = get().mainTabGuard;
-          if (guard && !guard(tab)) {
+        setMainTabGuard: (guard) => get().setSurfaceGuard(guard),
+
+        setActiveSurface: (surface) => {
+          const guard = get().surfaceGuard;
+          if (guard && !guard(surface)) {
             return;
           }
-          activeMainTabByRuntime.set(runtimeMemoryKey(), tab);
-          set({ activeMainTab: tab });
+          activeSurfaceByRuntime.set(runtimeMemoryKey(), surface);
+          set({ activeSurface: surface, activeMainTab: surface });
         },
 
+        setActiveMainTab: (surface) => get().setActiveSurface(surface),
+
         prepareForRuntimeSwitch: (runtimeKey?: string | null) => {
-          activeMainTabByRuntime.set(runtimeMemoryKey(runtimeKey), get().activeMainTab);
+          activeSurfaceByRuntime.set(runtimeMemoryKey(runtimeKey), get().activeSurface);
         },
 
         restoreForRuntimeSwitch: (runtimeKey?: string | null) => {
-          const restored = activeMainTabByRuntime.get(runtimeMemoryKey(runtimeKey)) ?? 'chat';
-          set({ activeMainTab: restored });
+          const restored = activeSurfaceByRuntime.get(runtimeMemoryKey(runtimeKey)) ?? 'chat';
+          set({ activeSurface: restored, activeMainTab: restored });
         },
 
         setPendingDiffFile: (filePath, staged = false, scope = null) => {
@@ -1671,11 +1693,11 @@ export const useUIStore = create<UIStore>()(
         },
 
         navigateToDiff: (filePath, staged = false, scope = null) => {
-          const guard = get().mainTabGuard;
+          const guard = get().surfaceGuard;
           if (guard && !guard('diff')) {
             return;
           }
-          set({ pendingDiffFile: filePath, pendingDiffStaged: staged, pendingDiffScope: scope, activeMainTab: 'diff' });
+          set({ pendingDiffFile: filePath, pendingDiffStaged: staged, pendingDiffScope: scope, activeSurface: 'diff', activeMainTab: 'diff' });
         },
 
         consumePendingDiffFile: () => {
@@ -1687,11 +1709,11 @@ export const useUIStore = create<UIStore>()(
         },
 
         navigateToDiagram: (filePath) => {
-          const guard = get().mainTabGuard;
+          const guard = get().surfaceGuard;
           if (guard && !guard('diagram')) {
             return;
           }
-          set({ pendingDiagramFile: filePath, activeMainTab: 'diagram' });
+          set({ pendingDiagramFile: filePath, activeSurface: 'diagram', activeMainTab: 'diagram' });
         },
 
         consumePendingDiagramFile: () => {
@@ -2462,13 +2484,20 @@ export const useUIStore = create<UIStore>()(
       {
         name: 'ui-store',
         storage: createDeferredSafeJSONStorage(),
-        version: 14,
+        version: 15,
         migrate: (persistedState, version) => {
           if (!persistedState || typeof persistedState !== 'object') {
             return persistedState;
           }
           const state = persistedState as Record<string, unknown>;
 
+          // v14 -> v15: rename the historic main-tab field. The selected
+          // mobile or promoted desktop view remains unchanged.
+          if (version < 15) {
+            state.activeSurface = state.activeMainTab;
+            state.activeMainTab = state.activeSurface;
+          }
+
           // v13 -> v14: the separate 'preview' surface merged into 'browser'.
           // Stored preview tabs keep their URL and become browser tabs; their
           // id encodes the mode, so it is rebuilt rather than left dangling.
@@ -2674,7 +2703,9 @@ export const useUIStore = create<UIStore>()(
           workStatusPanelEnabled: state.workStatusPanelEnabled,
           workStatusHiddenSections: state.workStatusHiddenSections,
           isSessionSwitcherOpen: state.isSessionSwitcherOpen,
-          activeMainTab: state.activeMainTab,
+          activeSurface: state.activeSurface,
+          // Keep the deprecated mirror synchronized while consumers migrate.
+          activeMainTab: state.activeSurface,
           sidebarSection: state.sidebarSection,
           settingsPage: state.settingsPage,
           settingsHasOpenedOnce: state.settingsHasOpenedOnce,