test(e2e_ui): cover the Files rail "Working folder" header toggle

Adds a Playwright test that drives the inline desktop Workspace rail and
asserts the working-folder header is a real button: it carries
aria-expanded, collapsing it hides the file-scope content and flips the
attribute to "false", and re-clicking restores it. This is the
browser-level guard for the frameless-vs-drawer header split (the unit
tests pin the render contract; this pins the live interaction the CI
e2e_ui gate requires for ap-web behavior changes). LLM-free.

ap-web/src/shell/FilesPanel.test.tsx

@@ -203,6 +203,62 @@ describe("FilesPanel working folder directory", () => {
   });
 });
 
+describe("FilesPanel working folder header role", () => {
+  // The inline right-rail panel passes `frameless` to fill the rail height
+  // without the card chrome. That must NOT downgrade the "Working folder"
+  // header to a plain label: it stays a collapsible button (accessible name
+  // + aria-expanded) so the rail header is focusable and toggleable, and so
+  // the e2e suite can target it by role. Only the drawer (onClose), which
+  // has its own X close button, uses the static label header.
+  it("renders the header as a collapsible button in the standalone card", () => {
+    renderPanel({ conversationId: "conv_header_card", files: [] });
+    const header = screen.getByRole("button", { name: /working folder/i });
+    expect(header).toHaveAttribute("aria-expanded", "true");
+  });
+
+  it("renders the header as a collapsible button in frameless (inline rail) mode", () => {
+    useAllFilesMock.mockReturnValue(allFilesResult([]));
+    useChangedFilesMock.mockReturnValue(changedFilesResult([]));
+    useDirectoryMock.mockReturnValue(directoryResult());
+    useEnvironmentMock.mockReturnValue(environmentResult("/home/user/workspace"));
+    useSearchMock.mockReturnValue(searchResult());
+
+    render(
+      <MemoryRouter initialEntries={["/c/conv_header_frameless"]}>
+        <Routes>
+          <Route
+            path="/c/:conversationId"
+            element={
+              <FilesPanel
+                sort="recent"
+                onSortChange={vi.fn()}
+                frameless
+                flatView={false}
+                onFileSelect={vi.fn()}
+                onFlatViewChange={vi.fn()}
+                showHidden={false}
+                onShowHiddenChange={vi.fn()}
+              />
+            }
+          />
+        </Routes>
+      </MemoryRouter>,
+    );
+
+    const header = screen.getByRole("button", { name: /working folder/i });
+    expect(header).toHaveAttribute("aria-expanded", "true");
+  });
+
+  it("renders a static label header (no toggle button) in the drawer", () => {
+    renderPanel({ conversationId: "conv_header_drawer", files: [], onClose: vi.fn() });
+    // The drawer has its own X close button, so the title is a plain label,
+    // not a collapse toggle.
+    expect(screen.queryByRole("button", { name: /working folder/i })).toBeNull();
+    expect(screen.getByText("Working folder")).toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "Close files" })).toBeInTheDocument();
+  });
+});
+
 describe("FilesPanel scope switch (Changed | All) visibility", () => {
   it("does not enable the root filesystem listing while showing Changed files", () => {
     renderPanel({

ap-web/src/shell/FilesPanel.tsx

@@ -61,10 +61,11 @@ interface FilesPanelProps {
   onClose?: () => void;
   /**
    * Frameless mode: drops the rounded card chrome and fills the parent
-   * container's height, just like `onClose` (fullScreen) mode — but
-   * without rendering a close button. Used by the inline right panel
-   * where the panel is embedded in a split layout rather than a drawer.
-   * The collapse chevron is also hidden in this mode.
+   * container's height (like the `onClose` drawer) — but without a close
+   * button. Used by the inline right panel where the panel is embedded in a
+   * split layout rather than a drawer. Unlike the drawer, it keeps the
+   * collapsible "Working folder" button header (with its chevron and
+   * `aria-expanded`), so the header stays a focusable, toggleable control.
    */
   frameless?: boolean;
 }
@@ -299,10 +300,17 @@ export function FilesPanel({
   const [treeExclude, setTreeExclude] = useState("");
   const [debouncedTreeExclude, setDebouncedTreeExclude] = useState("");
   const [showSearchFilters, setShowSearchFilters] = useState(false);
-  // Full-screen drawer mode: forced-open content, no rounded card,
-  // section grows to fill the parent rather than capping at max-h.
-  const fullScreen = onClose !== undefined || frameless === true;
-  const contentVisible = !collapsed || fullScreen;
+  // The drawer (onClose) owns the full viewport, so it gets a static,
+  // always-open header with its own X close button — a collapse chevron there
+  // would be a no-op. The inline rail (frameless) and the standalone card keep
+  // the collapsible "Working folder" *button* header (accessible name +
+  // aria-expanded), which is what lets it be focused/toggled and asserted on.
+  const isDrawer = onClose !== undefined;
+  // Both the drawer and the inline rail fill their parent's height and drop the
+  // rounded card chrome; only the standalone card caps content at max-h.
+  const fillHeight = isDrawer || frameless === true;
+  // The drawer is always open; everywhere else the header chevron toggles it.
+  const contentVisible = isDrawer || !collapsed;
   const changedQuery = useWorkspaceChangedFiles(conversationId, {
     enabled: contentVisible,
   });
@@ -357,12 +365,12 @@ export function FilesPanel({
     <div
       className={cn(
         "@container/filespanel overflow-hidden bg-card",
-        fullScreen ? "flex h-full min-h-0 flex-col" : "flex min-h-0 flex-col",
+        fillHeight ? "flex h-full min-h-0 flex-col" : "flex min-h-0 flex-col",
       )}
     >
       {/* Header — single row: [title · workingDir] [eye] [chevron / close] */}
       <div className="flex shrink-0 items-center gap-2 px-3 py-2">
-        {fullScreen ? (
+        {isDrawer ? (
           <>
             <span className="shrink-0 font-medium text-sm">Working folder</span>
             {workingDir && <WorkingDirLabel dir={workingDir} />}
@@ -517,7 +525,7 @@ export function FilesPanel({
             className={cn(
               "overflow-y-auto px-2 pb-2",
               flatView ? "pt-1" : "pt-2",
-              fullScreen ? "min-h-0 flex-1" : "max-h-72",
+              fillHeight ? "min-h-0 flex-1" : "max-h-72",
             )}
           >
             {flatView ? (

tests/e2e_ui/files/test_files_panel_header.py

@@ -0,0 +1,64 @@
+"""E2E: the Files rail "Working folder" header is a collapsible button.
+
+The desktop Workspace rail renders ``FilesPanel`` in its ``frameless``
+(inline) mode. That must NOT downgrade the working-folder header to a plain
+label: it stays an interactive ``button`` carrying ``aria-expanded`` so it is
+focusable and toggles the file list. Only the mobile/full-screen drawer
+(which has its own X close button) uses a static label header.
+
+This is the regression guard for that distinction: ``frameless`` once folded
+into a ``fullScreen`` flag that swapped the button for a ``<span>``, so the
+rail header silently stopped being a button. No message is sent — the header
+and its collapse state are rail state, not a function of any turn — so this
+stays a fast, LLM-free check.
+"""
+
+from __future__ import annotations
+
+import re
+
+from playwright.sync_api import Page, expect
+
+from tests.e2e_ui.conftest import open_right_rail
+
+
+def test_files_rail_working_folder_header_is_a_toggle_button(
+    page: Page,
+    seeded_session: tuple[str, str],
+) -> None:
+    """The rail's "Working folder" header is a button whose chevron collapses
+    the file list and flips ``aria-expanded``."""
+    base_url, session_id = seeded_session
+    page.goto(f"{base_url}/c/{session_id}")
+
+    # The rail defaults open but is remembered per session; ensure it is open
+    # so the Files panel header below is reachable. Scope every lookup to the
+    # desktop "Workspace" rail so it never matches the hidden mobile drawer
+    # that mirrors the same markup.
+    open_right_rail(page)
+    rail = page.get_by_role("complementary", name="Workspace")
+
+    # Files is the default rail tab; click it explicitly so the assertion does
+    # not depend on the remembered tab from a prior session.
+    rail.get_by_role("tab", name=re.compile("^Files")).click()
+
+    # The header is a BUTTON (not a label): substring-matching "Working folder"
+    # tolerates the trailing working-directory basename the header also renders.
+    header = rail.get_by_role("button", name=re.compile("Working folder"))
+    expect(header).to_be_visible(timeout=30_000)
+    expect(header).to_have_attribute("aria-expanded", "true")
+
+    # Expanded: the file-scope switch (Changed | All) is part of the content.
+    scope = rail.get_by_role("radiogroup", name="File scope")
+    expect(scope).to_be_visible()
+
+    # Collapsing via the header hides the content and flips aria-expanded.
+    header.click()
+    expect(header).to_have_attribute("aria-expanded", "false")
+    expect(scope).to_have_count(0)
+
+    # Expanding again restores the content — proving the header drives a real
+    # collapse toggle, not a one-way no-op.
+    header.click()
+    expect(header).to_have_attribute("aria-expanded", "true")
+    expect(rail.get_by_role("radiogroup", name="File scope")).to_be_visible()