feat(mobile): right-rail as drawer on mobile, header toggle button

Reverts v1.6.1's max-md:hidden wrapper around RightRail. On mobile,
RightRail now renders as a fixed right-side drawer (w-[85vw],
max-w-sm) toggled by a new FolderTree button in the Session header.

- New useRightRailDrawer hook mirrors useSidebarDrawer (Context +
  auto-close on route change).
- New MobileRightRailBackdrop component in App.tsx mirrors the
  existing MobileBackdrop for the left sidebar.
- RightRail computes an isOpen synthesis: on mobile, reads the
  drawer Context; on desktop, reads the persistent internal state.
  The existing tree-load effect and open_file_in_browser
  subscription share this plumbing via openRail / closeRail
  helpers.
- The desktop floating chevron handle is hidden on mobile (the
  Session header's FolderTree button replaces it).
- Session header gains a mobile-only FolderTree button after the
  ModelPicker, calling toggle() on the drawer Context.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/App.tsx

@@ -9,6 +9,7 @@ import { Session } from '@/pages/Session';
 import { Toaster } from '@/components/ui/sonner';
 import { useUserEvents } from '@/hooks/useUserEvents';
 import { SidebarDrawerProvider, useSidebarDrawer } from '@/hooks/useSidebarDrawer';
+import { RightRailDrawerProvider, useRightRailDrawer } from '@/hooks/useRightRailDrawer';
 import { useViewport } from '@/hooks/useViewport';
 
 function SessionRightRail() {
@@ -26,13 +27,11 @@ function RightRailForSession({ sessionId }: { sessionId: string }) {
       .catch((err) => console.warn('RightRail: failed to fetch session', err));
   }, [sessionId]);
   if (!projectId) return null;
-  // Hidden entirely below md breakpoint; mobile users get the file browser
+  // v1.6.2: rendered on all viewports. On mobile, RightRail itself renders as
-  // via the FileBrowserPane infrastructure if/when it lands in workspace panes.
+  // a right-side drawer toggled by the header's FolderTree button (via
-  return (
+  // useRightRailDrawer). On desktop, it renders inline as before with its
-    <div className="max-md:hidden contents">
+  // own internal open/close state.
-      <RightRail projectId={projectId} />
+  return <RightRail projectId={projectId} />;
-    </div>
-  );
 }
 
 function MobileBackdrop() {
@@ -48,6 +47,19 @@ function MobileBackdrop() {
   );
 }
 
+function MobileRightRailBackdrop() {
+  const { open, setOpen } = useRightRailDrawer();
+  const { isMobile } = useViewport();
+  if (!isMobile || !open) return null;
+  return (
+    <div
+      className="fixed inset-0 z-30 bg-black/40 md:hidden"
+      onClick={() => setOpen(false)}
+      aria-hidden="true"
+    />
+  );
+}
+
 function AppShell() {
   useUserEvents();
   return (
@@ -61,6 +73,7 @@ function AppShell() {
           <Route path="/session/:id" element={<Session />} />
         </Routes>
       </main>
+      <MobileRightRailBackdrop />
       <Routes>
         <Route path="/session/:id" element={<SessionRightRail />} />
       </Routes>
@@ -73,7 +86,9 @@ export default function App() {
   return (
     <BrowserRouter>
       <SidebarDrawerProvider>
-        <AppShell />
+        <RightRailDrawerProvider>
+          <AppShell />
+        </RightRailDrawerProvider>
       </SidebarDrawerProvider>
     </BrowserRouter>
   );

apps/web/src/components/RightRail.tsx

@@ -4,8 +4,11 @@ import { api } from '@/api/client';
 import type { FileEntry } from '@/api/types';
 import { inferLanguage } from '@/lib/attachments';
 import { sessionEvents } from '@/hooks/sessionEvents';
+import { useRightRailDrawer } from '@/hooks/useRightRailDrawer';
+import { useViewport } from '@/hooks/useViewport';
 import { FileViewerOverlay } from '@/components/FileViewerOverlay';
 import { Input } from '@/components/ui/input';
+import { cn } from '@/lib/utils';
 
 interface Props {
   projectId: string;
@@ -25,6 +28,8 @@ function joinPath(parent: string, name: string): string {
 }
 
 export function RightRail({ projectId }: Props) {
+  const { isMobile } = useViewport();
+  const { open: drawerOpen, setOpen: setDrawerOpen } = useRightRailDrawer();
   const [open, setOpen] = useState(() => {
     try { return localStorage.getItem(`${STORAGE_KEY}.open`) !== 'false'; } catch { return true; }
   });
@@ -34,6 +39,19 @@ export function RightRail({ projectId }: Props) {
   const [fullFileList, setFullFileList] = useState<string[] | null>(null);
   const [viewerFile, setViewerFile] = useState<{ path: string; content: string } | null>(null);
 
+  // Combined open state: on mobile use the global drawer state (toggled by
+  // the Session header's FolderTree button); on desktop use the persistent
+  // internal state.
+  const isOpen = isMobile ? drawerOpen : open;
+  const closeRail = useCallback(() => {
+    if (isMobile) setDrawerOpen(false);
+    else setOpen(false);
+  }, [isMobile, setDrawerOpen]);
+  const openRail = useCallback(() => {
+    if (isMobile) setDrawerOpen(true);
+    else setOpen(true);
+  }, [isMobile, setDrawerOpen]);
+
   useEffect(() => {
     // best-effort; ignore failure because localStorage may be unavailable (quota, private mode)
     try { localStorage.setItem(`${STORAGE_KEY}.open`, String(open)); } catch {}
@@ -56,9 +74,9 @@ export function RightRail({ projectId }: Props) {
   }, [projectId]);
 
   useEffect(() => {
-    if (!open) return;
+    if (!isOpen) return;
     if (!cache.has('')) void loadDir('');
-  }, [open, cache, loadDir]);
+  }, [isOpen, cache, loadDir]);
 
   function toggleDir(dirPath: string) {
     setExpandedDirs((prev) => {
@@ -108,12 +126,14 @@ export function RightRail({ projectId }: Props) {
   useEffect(() => {
     return sessionEvents.subscribe((event) => {
       if (event.type !== 'open_file_in_browser') return;
-      if (!open) setOpen(true);
+      if (!isOpen) openRail();
       void openFile(event.path);
     });
-  }, [open, projectId]);
+  }, [isOpen, openRail, projectId]);
 
-  if (!open) {
+  // Desktop closed state: render the floating chevron handle. Mobile never
+  // shows the handle — the toggle lives in the Session header on mobile.
+  if (!isMobile && !open) {
     return (
       <button
         type="button"
@@ -128,15 +148,25 @@ export function RightRail({ projectId }: Props) {
 
   const rootEntries = cache.get('') ?? [];
 
+  // Mobile: render as fixed-position right-side drawer (always mounted so
+  // the transform transition can animate in/out). Desktop: inline aside.
+  const asideCls = isMobile
+    ? cn(
+        'fixed inset-y-0 right-0 z-40 w-[85vw] max-w-sm border-l bg-sidebar flex flex-col overflow-hidden',
+        'transition-transform duration-200 ease-out',
+        drawerOpen ? 'translate-x-0' : 'translate-x-full',
+      )
+    : 'w-64 shrink-0 border-l bg-sidebar flex flex-col h-full overflow-hidden';
+
   return (
     <>
-      <aside className="w-64 shrink-0 border-l bg-sidebar flex flex-col h-full overflow-hidden">
+      <aside className={asideCls}>
         <div className="flex items-center gap-2 px-3 py-2 border-b shrink-0">
           <span className="text-xs font-medium flex-1">Files</span>
           <button
             type="button"
-            onClick={() => setOpen(false)}
+            onClick={closeRail}
-            className="p-1 rounded hover:bg-muted text-muted-foreground"
+            className="p-1 rounded hover:bg-muted text-muted-foreground max-md:min-h-[44px] max-md:min-w-[44px]"
             aria-label="Close file browser"
           >
             <PanelRightClose size={14} />

apps/web/src/hooks/useRightRailDrawer.tsx

@@ -0,0 +1,35 @@
+import { createContext, useCallback, useContext, useEffect, useState } from 'react';
+import type { ReactNode } from 'react';
+import { useLocation } from 'react-router-dom';
+
+interface RightRailDrawerState {
+  open: boolean;
+  setOpen: (open: boolean) => void;
+  toggle: () => void;
+}
+
+const Ctx = createContext<RightRailDrawerState | null>(null);
+
+export function RightRailDrawerProvider({ children }: { children: ReactNode }) {
+  const [open, setOpen] = useState(false);
+  const location = useLocation();
+
+  // Auto-close on route change. Same pattern as useSidebarDrawer — keeps the
+  // drawer from leaking between sessions when the user navigates.
+  useEffect(() => {
+    setOpen(false);
+  }, [location.pathname]);
+
+  const toggle = useCallback(() => setOpen((v) => !v), []);
+
+  return <Ctx.Provider value={{ open, setOpen, toggle }}>{children}</Ctx.Provider>;
+}
+
+export function useRightRailDrawer(): RightRailDrawerState {
+  const ctx = useContext(Ctx);
+  if (!ctx) {
+    // Soft fallback so consumers don't crash if rendered outside a provider.
+    return { open: false, setOpen: () => {}, toggle: () => {} };
+  }
+  return ctx;
+}

apps/web/src/pages/Session.tsx

@@ -1,11 +1,12 @@
 import { useEffect, useState } from 'react';
 import { Link, useNavigate, useParams } from 'react-router-dom';
-import { ChevronRight, Menu } from 'lucide-react';
+import { ChevronRight, FolderTree, Menu } from 'lucide-react';
 import { api } from '@/api/client';
 import type { Project, Session as SessionType } from '@/api/types';
 import { sessionEvents } from '@/hooks/sessionEvents';
 import { useActivePane } from '@/hooks/useActivePane';
 import { useSidebarDrawer } from '@/hooks/useSidebarDrawer';
+import { useRightRailDrawer } from '@/hooks/useRightRailDrawer';
 import { useViewport } from '@/hooks/useViewport';
 import { Workspace } from '@/components/Workspace';
 import { ModelPicker } from '@/components/ModelPicker';
@@ -19,6 +20,7 @@ export function Session() {
   const [editingName, setEditingName] = useState(false);
   const active = useActivePane();
   const { setOpen: setDrawerOpen } = useSidebarDrawer();
+  const { toggle: toggleRightRail } = useRightRailDrawer();
   const { isMobile } = useViewport();
 
   useEffect(() => {
@@ -176,6 +178,18 @@ export function Session() {
             </div>
           )}
         </div>
+
+        {/* File browser toggle — mobile only */}
+        {isMobile && (
+          <button
+            type="button"
+            onClick={toggleRightRail}
+            className="inline-flex items-center justify-center -mr-1 min-w-[44px] min-h-[44px] rounded text-muted-foreground hover:bg-muted hover:text-foreground shrink-0"
+            aria-label="Toggle file browser"
+          >
+            <FolderTree className="size-5" />
+          </button>
+        )}
       </header>
 
       {id && session && (