perf(ui): narrow session switch fanout

Keep sidebar rows independent from the active sync directory by moving export history loading behind an explicit-directory command.

Make active-project selection preserve project topology and remove activeProjectId from mounted group render props. Reuse a per-session-array ID index so subscription snapshots do not repeatedly scan session lists.

packages/ui/src/stores/useProjectsStore.test.ts

@@ -20,6 +20,26 @@ describe("useProjectsStore settings synchronization", () => {
   })
 })
 
+describe("useProjectsStore selection identity", () => {
+  test("changes only the active project id", () => {
+    const first = { id: "project-a", path: "/repo-a", lastOpenedAt: 10 } as ProjectEntry
+    const second = { id: "project-b", path: "/repo-b", lastOpenedAt: 20 } as ProjectEntry
+    const projects = [first, second]
+    useProjectsStore.setState({
+      projects,
+      activeProjectId: first.id,
+      manualProjectOrder: projects.map((project) => project.id),
+    })
+
+    useProjectsStore.getState().setActiveProjectIdOnly(second.id)
+
+    const state = useProjectsStore.getState()
+    expect(state.activeProjectId).toBe(second.id)
+    expect(state.projects).toBe(projects)
+    expect(state.projects.map((project) => project.lastOpenedAt)).toEqual([10, 20])
+  })
+})
+
 describe("useProjectsStore default model and thinking level", () => {
   const seed = (project: ProjectEntry) => {
     useProjectsStore.setState({

packages/ui/src/stores/useProjectsStore.ts

@@ -357,13 +357,7 @@ const readPersistedActiveProjectId = (): string | null => {
   return null;
 };
 
-const cacheProjects = (projects: ProjectEntry[], activeProjectId: string | null) => {
-  try {
-    safeStorage.setItem(getProjectsStorageKey(), JSON.stringify(projects));
-  } catch {
-    // ignored
-  }
-
+const cacheActiveProjectId = (activeProjectId: string | null) => {
   try {
     const activeProjectStorageKey = getActiveProjectStorageKey();
     if (activeProjectId) {
@@ -376,6 +370,15 @@ const cacheProjects = (projects: ProjectEntry[], activeProjectId: string | null)
   }
 };
 
+const cacheProjects = (projects: ProjectEntry[], activeProjectId: string | null) => {
+  try {
+    safeStorage.setItem(getProjectsStorageKey(), JSON.stringify(projects));
+  } catch {
+    // ignored
+  }
+  cacheActiveProjectId(activeProjectId);
+};
+
 const persistProjects = (projects: ProjectEntry[], activeProjectId: string | null, manualOrder?: string[]) => {
   cacheProjects(projects, activeProjectId);
   if (manualOrder) {
@@ -693,18 +696,13 @@ export const useProjectsStore = create<ProjectsStore>()(
       if (activeProjectId === id) {
         return;
       }
-      const target = projects.find((project) => project.id === id);
-      if (!target) {
+      if (!projects.some((project) => project.id === id)) {
         return;
       }
 
-      const now = Date.now();
-      const nextProjects = projects.map((project) =>
-        project.id === id ? { ...project, lastOpenedAt: now } : project
-      );
-
-      set({ projects: nextProjects, activeProjectId: id });
-      persistProjects(nextProjects, id, get().manualProjectOrder);
+      set({ activeProjectId: id });
+      cacheActiveProjectId(id);
+      void updateDesktopSettings({ activeProjectId: id });
     },
 
     renameProject: (id: string, label: string) => {