perf(interface): optimize main-btw alternation, canvas tab switch, and settings traversal

src/web-ui/src/app/components/panels/content-canvas/editor-area/EditorGroup.tsx

@@ -175,17 +175,20 @@ export const EditorGroup: React.FC<EditorGroupProps> = ({
     }
   }, [group.activeTabId, group.tabs, isKeepAliveTerminalTab]);
 
+  // Fast lookup set for cached tabs
+  const cachedTabs = cachedTabsRef.current;
+
   // Tabs to render (active + cached). Hidden terminal tabs stay mounted so
   // reopening a terminal reuses the xterm buffer instead of replaying history.
   // Child-session tabs retain only their lightweight wrapper; that wrapper
   // unmounts its transcript while inactive and owns reading state until close.
   const tabsToRender = useMemo(() => {
     const result = group.tabs.filter(t => 
-        (!t.isHidden && isCanvasTabVisibleForSession(t, activeSessionId) && (t.content.type === 'btw-session' || t.id === effectiveActiveTabId || cachedTabsRef.current.has(t.id))) ||
+        (!t.isHidden && isCanvasTabVisibleForSession(t, activeSessionId) && (t.content.type === 'btw-session' || t.id === effectiveActiveTabId || cachedTabs.has(t.id))) ||
       (t.isHidden && isKeepAliveTerminalTab(t))
     );
     return result;
-  }, [group.tabs, effectiveActiveTabId, activeSessionId, isKeepAliveTerminalTab]);
+  }, [group.tabs, effectiveActiveTabId, activeSessionId, isKeepAliveTerminalTab, cachedTabs]);
 
   const handleContentChange = useCallback((content: PanelContent | null) => {
     if (content && group.activeTabId) {

src/web-ui/src/app/scenes/settings/SettingsNav.tsx

@@ -61,13 +61,18 @@ function resolvePhrases(i18n: I18nApi, phrases: readonly SettingsSearchPhrase[])
   return parts.join(' ');
 }
 
+const searchIndexCache = new WeakMap<I18nApi, SettingsSearchRow[]>();
+
 function buildSettingsSearchIndex(t: SettingsT, i18n: I18nApi): SettingsSearchRow[] {
+  const cached = searchIndexCache.get(i18n);
+  if (cached) return cached;
+
   const categoryLabels = new Map(SETTINGS_CATEGORIES.map((category) => [
     category.id,
     translateString(t, category.labelKey, category.id),
   ]));
 
-  return SETTINGS_PAGE_MANIFESTS.flatMap((page) => {
+  const rows = SETTINGS_PAGE_MANIFESTS.flatMap((page) => {
     const categoryLabel = categoryLabels.get(page.categoryId) ?? page.categoryId;
     const pageLabel = translateString(t, page.labelKey, page.id);
     const description = translateString(t, page.descriptionKey, '');
@@ -106,6 +111,9 @@ function buildSettingsSearchIndex(t: SettingsT, i18n: I18nApi): SettingsSearchRo
       }),
     ];
   });
+
+  searchIndexCache.set(i18n, rows);
+  return rows;
 }
 
 function highlightFirstMatch(text: string, query: string): React.ReactNode {

src/web-ui/src/flow_chat/components/btw/BtwSessionPanel.tsx

@@ -229,7 +229,15 @@ const BtwSessionPanelContent: React.FC<BtwSessionPanelProps & { viewState: BtwPa
       ? t(resolveSubagentNameKey(childSessionId ?? ''))
       : displayTitle?.trim() || resolveSessionTitle(childSession, childTitleFallback);
   const headerModelName = childKind === 'subagent' ? childSession?.config?.modelName?.trim() : undefined;
-  const [projectSession] = useState(createSessionVirtualItemProjector);
+  const projectSession = useMemo(
+    () => {
+      if (!viewState.projectSession) {
+        viewState.projectSession = createSessionVirtualItemProjector();
+      }
+      return viewState.projectSession;
+    },
+    [viewState],
+  );
   const sessionVirtualItems = useMemo(
     () => projectSession(childSession ?? null),
     [childSession, projectSession],

src/web-ui/src/flow_chat/components/btw/btwPanelViewState.ts

@@ -6,6 +6,7 @@ export interface BtwPanelViewState {
   expandedToolCapsules?: ReadonlySet<string>;
   restoredReviewLocation?: string;
   restoring: boolean;
+  projectSession?: (session: any) => any;
 }
 
 export function createBtwPanelViewState(): BtwPanelViewState {