diff --git a/frontend/src/components/workspace/projects-section.tsx b/frontend/src/components/workspace/projects-section.tsx
index 2c50c0640..35a77e6ed 100644
--- a/frontend/src/components/workspace/projects-section.tsx
+++ b/frontend/src/components/workspace/projects-section.tsx
@@ -91,7 +91,7 @@ function ProjectThreadGroup({
         </CollapsibleTrigger>
       </SidebarMenuItem>
       <CollapsibleContent>
-        <SidebarMenu className="border-sidebar-border ml-4 border-l pl-2">
+        <SidebarMenu className="border-sidebar-border ml-4 w-auto border-l pl-2">
           {branchEntries.map((entry) => (
             <ThreadSidebarItem
               key={entry.thread.thread_id}
@@ -130,7 +130,7 @@ function ArchivedProjectsGroup({
         </CollapsibleTrigger>
       </SidebarMenuItem>
       <CollapsibleContent>
-        <SidebarMenu className="border-sidebar-border ml-4 border-l pl-2">
+        <SidebarMenu className="border-sidebar-border ml-4 w-auto border-l pl-2">
           {projects.map((project) => (
             <ProjectThreadGroup
               key={project.id}
diff --git a/frontend/tests/unit/components/workspace/projects-section-nested-menu.dom.test.tsx b/frontend/tests/unit/components/workspace/projects-section-nested-menu.dom.test.tsx
new file mode 100644
index 000000000..4f1005433
--- /dev/null
+++ b/frontend/tests/unit/components/workspace/projects-section-nested-menu.dom.test.tsx
@@ -0,0 +1,165 @@
+import { afterEach, beforeEach, describe, expect, it, rs } from "@rstest/core";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import type { ReactNode } from "react";
+
+import { SidebarProvider } from "@/components/ui/sidebar";
+import { ProjectsSection } from "@/components/workspace/projects-section";
+import { ThreadDeleteDialogProvider } from "@/components/workspace/thread-delete-dialog";
+import { AuthProvider } from "@/core/auth/AuthProvider";
+import type { User } from "@/core/auth/types";
+import { DEFAULT_LOCALE } from "@/core/i18n";
+import { I18nProvider } from "@/core/i18n/context";
+import { PROJECTS_QUERY_KEY, type Project } from "@/core/projects";
+import { updateLocalSettings } from "@/core/settings/store";
+import { INFINITE_THREADS_QUERY_KEY_PREFIX } from "@/core/threads/hooks";
+import type { AgentThread } from "@/core/threads/types";
+import { THREAD_PROJECT_METADATA_KEY } from "@/core/threads/utils";
+
+rs.mock("next/navigation", () => ({
+  useRouter: () => ({ push: rs.fn(), replace: rs.fn(), refresh: rs.fn() }),
+  usePathname: () => "/workspace",
+  useSearchParams: () => new URLSearchParams(),
+  useParams: () => ({}),
+}));
+
+const ACTIVE_PROJECT_ID = "project-active";
+const ARCHIVED_PROJECT_ID = "project-archived";
+
+function makeProject(
+  id: string,
+  name: string,
+  status: Project["status"],
+): Project {
+  return {
+    id,
+    name,
+    instructions: "",
+    presentation: {},
+    status,
+    created_at: "2026-01-01T00:00:00Z",
+    updated_at: "2026-01-02T00:00:00Z",
+  };
+}
+
+function makeThread(id: string, projectId: string): AgentThread {
+  return {
+    thread_id: id,
+    created_at: "2026-01-01T00:00:00Z",
+    updated_at: "2026-01-02T00:00:00Z",
+    status: "idle",
+    metadata: { [THREAD_PROJECT_METADATA_KEY]: projectId },
+    values: { title: `Chat in ${projectId}` },
+  } as unknown as AgentThread;
+}
+
+const user = {
+  id: "user-1",
+  email: "user@example.test",
+  system_role: "user",
+} as User;
+
+/**
+ * Seed every query `ProjectsSection` reads in grouped mode so the section
+ * renders from cache without touching the network: both project lists and
+ * the non-archived infinite thread search that the groups partition.
+ */
+function renderGroupedSection(): ReturnType<typeof render> {
+  const queryClient = new QueryClient({
+    defaultOptions: {
+      queries: { retry: false, staleTime: Infinity },
+      mutations: { retry: false },
+    },
+  });
+  queryClient.setQueryData(
+    [...PROJECTS_QUERY_KEY, { status: "active" }],
+    [makeProject(ACTIVE_PROJECT_ID, "Active project", "active")],
+  );
+  queryClient.setQueryData(
+    [...PROJECTS_QUERY_KEY, { status: "archived" }],
+    [makeProject(ARCHIVED_PROJECT_ID, "Archived project", "archived")],
+  );
+  queryClient.setQueryData(
+    [...INFINITE_THREADS_QUERY_KEY_PREFIX, { archived: false }],
+    {
+      pages: [
+        [
+          makeThread("thread-active", ACTIVE_PROJECT_ID),
+          makeThread("thread-archived", ARCHIVED_PROJECT_ID),
+        ],
+      ],
+      pageParams: [0],
+    },
+  );
+  const tree: ReactNode = (
+    <I18nProvider initialLocale={DEFAULT_LOCALE}>
+      <QueryClientProvider client={queryClient}>
+        <AuthProvider initialUser={user}>
+          <SidebarProvider>
+            <ThreadDeleteDialogProvider>
+              <ProjectsSection />
+            </ThreadDeleteDialogProvider>
+          </SidebarProvider>
+        </AuthProvider>
+      </QueryClientProvider>
+    </I18nProvider>
+  );
+  return render(tree);
+}
+
+function menuOf(element: Element): HTMLElement | null {
+  return element.closest<HTMLElement>('[data-sidebar="menu"]');
+}
+
+beforeEach(() => {
+  updateLocalSettings("projectsDisplayMode", "grouped");
+});
+
+afterEach(() => {
+  updateLocalSettings("projectsDisplayMode", "flat");
+  rs.restoreAllMocks();
+  cleanup();
+});
+
+describe("ProjectsSection grouped-mode nested menus", () => {
+  // happy-dom has no layout engine, so the regression is pinned at the class
+  // level: `SidebarMenu` defaults to `w-full`, and an indented (`ml-4`) menu
+  // that keeps it is 16px wider than its container — the absolutely
+  // positioned row kebab (`right-1`) then lands outside the sidebar and gets
+  // clipped. Every indented menu must swap `w-full` for `w-auto`.
+  it("indents thread rows without overflowing the sidebar width", async () => {
+    const { container } = renderGroupedSection();
+
+    // The Archived group starts collapsed; expand it so its doubly nested
+    // menus (group → project → rows) mount and get checked too.
+    fireEvent.click(screen.getByRole("button", { name: "Archived" }));
+    const kebabs = await screen.findAllByRole("button", { name: "More" });
+    expect(kebabs).toHaveLength(2);
+
+    for (const kebab of kebabs) {
+      const menu = menuOf(kebab);
+      expect(menu).not.toBeNull();
+      expect(menu?.classList.contains("ml-4")).toBe(true);
+      expect(menu?.classList.contains("w-auto")).toBe(true);
+      expect(menu?.classList.contains("w-full")).toBe(false);
+    }
+
+    const menus = [
+      ...container.querySelectorAll<HTMLElement>('[data-sidebar="menu"]'),
+    ];
+    const nestedMenus = menus.filter((menu) =>
+      Boolean(menu.parentElement && menuOf(menu.parentElement)),
+    );
+    // Active project rows, the Archived group, and the archived project rows.
+    expect(nestedMenus).toHaveLength(3);
+    for (const menu of nestedMenus) {
+      expect(menu.classList.contains("w-auto")).toBe(true);
+      expect(menu.classList.contains("w-full")).toBe(false);
+    }
+
+    // The outermost menu is not indented and keeps the primitive's full width.
+    const rootMenus = menus.filter((menu) => !nestedMenus.includes(menu));
+    expect(rootMenus).toHaveLength(1);
+    expect(rootMenus[0]?.classList.contains("w-full")).toBe(true);
+  });
+});