fix: keep Tab and the / shortcut out of a message list hidden by the open message

components/mail/__tests__/mail-app-return-to-list.test.tsx

@@ -273,6 +273,24 @@ describe.each([
   });
 });
 
+describe('A list collapsed under an open message (focused layout)', () => {
+  const list = () => screen.getByText('message list').closest('section')!;
+
+  it('is out of the Tab order while the message is open', async () => {
+    render(<MailApp />);
+    await screen.findByText('stub archive', {}, { timeout: 5000 });
+    expect(list()).toHaveAttribute('inert');
+  });
+
+  it('comes back for the / shortcut, whose search field lives in it', async () => {
+    render(<MailApp />);
+    await screen.findByText('stub archive', {}, { timeout: 5000 });
+    act(() => { fireEvent.keyDown(window, { key: '/' }); });
+    await waitFor(() => expect(selectedId()).toBeNull());
+    await waitFor(() => expect(list()).not.toHaveAttribute('inert'));
+  });
+});
+
 describe('The e shortcut in Mountain View', () => {
   beforeEach(() => {
     useSettingsStore.setState({ interfaceLayout: 'mountain-view' } as never);

components/mail/mail-app.tsx

@@ -1109,10 +1109,19 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
     },
     onFocusSearch: () => {
       if (isScheduledView) return;
-      const searchInput = document.querySelector('[data-search-input]') as HTMLInputElement;
-      if (searchInput) {
-        searchInput.focus();
-        searchInput.select();
+      const focusSearch = () => {
+        const searchInput = document.querySelector('[data-search-input]') as HTMLInputElement;
+        if (searchInput) {
+          searchInput.focus();
+          searchInput.select();
+        }
+      };
+      // The field is in the list, which an open message can have collapsed.
+      if (shouldCollapseListPane) {
+        handleMobileBack();
+        requestAnimationFrame(() => requestAnimationFrame(focusSearch));
+      } else {
+        focusSearch();
       }
     },
     onShowHelp: () => {
@@ -5089,6 +5098,9 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
           <section
             ref={listPaneRef}
             aria-labelledby={listHeadingId}
+            // Collapsed to no width under the reading pane: Tab must not walk
+            // through its toolbar and rows, which nobody can see.
+            inert={shouldCollapseListPane}
             className={cn(
               "relative flex flex-col bg-background min-w-0",
               isHorizontalMailLayout