Phase 3: PWA offline caching (frontend only)

Adds a real Workbox runtime-caching strategy on top of the Phase 1 app-shell
precache: StaleWhileRevalidate (cache-and-refresh) for the five read
endpoints (rooms/mine, open rooms, room messages, room members, invites/mine)
with a bounded/expiring cache per endpoint, while /api/auth/* and all
mutations stay network-only. An OfflineBanner (navigator.onLine-driven) and
a clearer Composer status line ("Connecting..." vs "You're offline") surface
what's actually happening; api/client.ts gains a NetworkError distinct from
ApiError so a genuine cache-miss-while-offline shows a quiet empty state
instead of a red error.

Manual offline testing (backend stopped, `vite preview` against the real
production service worker) surfaced a real gap the plan hadn't accounted
for: GET /api/auth/me is intentionally NetworkOnly, but that meant
ProtectedRoute could never confirm a session while offline and always
bounced to /login -- none of the newly-cached room/message data was ever
reachable. Fixed by caching a minimal, non-sensitive "last known user" in
localStorage (lib/lastUser.ts) and having AuthContext fall back to it for
any *unconfirmed* auth check (network failure, or a down backend answering
through a live reverse proxy with its own 502/503/504 -- both happen in
real deployments, not just literal airplane-mode). Only a server-confirmed
401 still clears it and signs the user out; every real action still
re-checks the actual session cookie server-side, so this can't grant
anything -- it only keeps cached UI reachable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/ChatPane.css

@@ -70,3 +70,7 @@
   padding: var(--sp-2) var(--sp-4) 0;
   margin: 0;
 }
+
+.chat-pane-note {
+  color: var(--ds-muted);
+}

frontend/src/components/ChatPane.tsx

@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useState } from 'react'
 import { useNavigate } from 'react-router-dom'
+import { NetworkError } from '../api/client'
 import { getRoomMessages } from '../api/rooms'
 import { useChatSocket } from '../ws/useChatSocket'
 import type { ChatMessageEnvelope, Message, MyRoomItem, RoomMember, ServerEnvelope } from '../types'
@@ -21,12 +22,22 @@ export function ChatPane({ room, members, isMobile, onBack, onToggleInfo, infoOp
   const [history, setHistory] = useState<Message[]>([])
   const [live, setLive] = useState<ChatMessageEnvelope[]>([])
   const [wsError, setWsError] = useState<string | null>(null)
+  const [historyUnavailableOffline, setHistoryUnavailableOffline] = useState(false)
 
   useEffect(() => {
     setHistory([])
     setLive([])
     setWsError(null)
-    getRoomMessages(room.id).then(setHistory).catch((err) => setWsError(String(err)))
+    setHistoryUnavailableOffline(false)
+    getRoomMessages(room.id)
+      .then(setHistory)
+      .catch((err) => {
+        if (err instanceof NetworkError) {
+          setHistoryUnavailableOffline(true)
+        } else {
+          setWsError(String(err))
+        }
+      })
   }, [room.id])
 
   const onMessage = useCallback((envelope: ServerEnvelope) => {
@@ -71,6 +82,11 @@ export function ChatPane({ room, members, isMobile, onBack, onToggleInfo, infoOp
       </header>
 
       {wsError && <p className="chat-pane-error">{wsError}</p>}
+      {historyUnavailableOffline && (
+        <p className="chat-pane-error chat-pane-note">
+          Message history for this room isn't available offline yet.
+        </p>
+      )}
 
       <MessageList messages={[...history, ...live]} members={members} />
       <Composer roomName={room.name} disabled={!connected} onSend={send} />

frontend/src/components/Composer.css

@@ -2,12 +2,18 @@
   padding: var(--sp-3) var(--sp-4);
   border-top: 1px solid var(--ds-border);
   background: var(--ds-surface);
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.composer-box {
   display: flex;
   gap: var(--sp-2);
   align-items: flex-end;
 }
 
-.composer textarea {
+.composer-box textarea {
   flex: 1;
   resize: none;
   background: var(--ds-surface-2);
@@ -20,7 +26,7 @@
   max-height: 120px;
 }
 
-.composer textarea:focus {
+.composer-box textarea:focus {
   border-color: var(--ds-accent);
 }
 
@@ -42,3 +48,9 @@
   opacity: 0.5;
   cursor: not-allowed;
 }
+
+.composer-status {
+  font-size: 0.76rem;
+  color: var(--ds-muted);
+  padding-left: 2px;
+}

frontend/src/components/Composer.tsx

@@ -1,4 +1,5 @@
 import { useRef, useState, type KeyboardEvent } from 'react'
+import { useOnlineStatus } from '../hooks/useOnlineStatus'
 import './Composer.css'
 
 interface ComposerProps {
@@ -10,6 +11,7 @@ interface ComposerProps {
 export function Composer({ roomName, disabled, onSend }: ComposerProps) {
   const [value, setValue] = useState('')
   const textareaRef = useRef<HTMLTextAreaElement>(null)
+  const online = useOnlineStatus()
 
   function autoGrow() {
     const el = textareaRef.current
@@ -35,29 +37,34 @@ export function Composer({ roomName, disabled, onSend }: ComposerProps) {
 
   return (
     <div className="composer">
-      <textarea
-        ref={textareaRef}
-        rows={1}
-        value={value}
-        disabled={disabled}
-        onChange={(e) => {
-          setValue(e.target.value)
-          autoGrow()
-        }}
-        onKeyDown={handleKeyDown}
-        placeholder={`Message #${roomName}`}
-      />
-      <button
-        type="button"
-        className="composer-send"
-        onClick={handleSend}
-        disabled={disabled || !value.trim()}
-        aria-label="Send message"
-      >
-        <svg width="15" height="15" viewBox="0 0 20 20" aria-hidden="true">
-          <polygon points="2,2 18,10 2,18 6,10" fill="currentColor" />
-        </svg>
-      </button>
+      <div className="composer-box">
+        <textarea
+          ref={textareaRef}
+          rows={1}
+          value={value}
+          disabled={disabled}
+          onChange={(e) => {
+            setValue(e.target.value)
+            autoGrow()
+          }}
+          onKeyDown={handleKeyDown}
+          placeholder={disabled ? (online ? 'Connecting…' : "You're offline") : `Message #${roomName}`}
+        />
+        <button
+          type="button"
+          className="composer-send"
+          onClick={handleSend}
+          disabled={disabled || !value.trim()}
+          aria-label="Send message"
+        >
+          <svg width="15" height="15" viewBox="0 0 20 20" aria-hidden="true">
+            <polygon points="2,2 18,10 2,18 6,10" fill="currentColor" />
+          </svg>
+        </button>
+      </div>
+      {disabled && (
+        <div className="composer-status">{online ? 'Connecting…' : "You're offline — messages can't be sent right now"}</div>
+      )}
     </div>
   )
 }

frontend/src/components/OfflineBanner.css

@@ -0,0 +1,10 @@
+.offline-banner {
+  flex: none;
+  background: color-mix(in srgb, var(--ds-highlight) 16%, var(--ds-surface));
+  color: var(--ds-text);
+  border-bottom: 1px solid var(--ds-border);
+  font-size: 0.8rem;
+  font-weight: 600;
+  text-align: center;
+  padding: 6px var(--sp-4);
+}

frontend/src/components/OfflineBanner.tsx

@@ -0,0 +1,14 @@
+import { useOnlineStatus } from '../hooks/useOnlineStatus'
+import './OfflineBanner.css'
+
+export function OfflineBanner() {
+  const online = useOnlineStatus()
+
+  if (online) return null
+
+  return (
+    <div className="offline-banner" role="status">
+      You're offline — showing cached data.
+    </div>
+  )
+}

frontend/src/components/Sidebar.css

@@ -103,3 +103,11 @@
   color: var(--ds-muted);
   text-transform: uppercase;
 }
+
+.sidebar-offline-note {
+  color: var(--ds-muted);
+  font-size: 0.82rem;
+  padding: var(--sp-4);
+  line-height: 1.5;
+  margin: 0;
+}

frontend/src/components/Sidebar.tsx

@@ -11,6 +11,7 @@ interface SidebarProps {
   onOpenBrowse: () => void
   onOpenInvites: () => void
   inviteCount: number
+  unavailableOffline?: boolean
 }
 
 export function Sidebar({
@@ -22,6 +23,7 @@ export function Sidebar({
   onOpenBrowse,
   onOpenInvites,
   inviteCount,
+  unavailableOffline,
 }: SidebarProps) {
   const query = searchQuery.trim().toLowerCase()
   const filtered = query ? rooms.filter((r) => r.name.toLowerCase().includes(query)) : rooms
@@ -67,17 +69,25 @@ export function Sidebar({
           Browse rooms
         </button>
 
-        {filtered.length > 0 && <div className="sidebar-section-label">Rooms</div>}
-        <nav>
-          {filtered.map((room, i) => (
-            <RoomRow
-              key={room.id}
-              room={room}
-              colorIndex={i}
-              active={room.id === activeRoomId}
-            />
-          ))}
-        </nav>
+        {unavailableOffline ? (
+          <p className="sidebar-offline-note">
+            Your rooms aren't available offline yet. Reconnect to load them.
+          </p>
+        ) : (
+          <>
+            {filtered.length > 0 && <div className="sidebar-section-label">Rooms</div>}
+            <nav>
+              {filtered.map((room, i) => (
+                <RoomRow
+                  key={room.id}
+                  room={room}
+                  colorIndex={i}
+                  active={room.id === activeRoomId}
+                />
+              ))}
+            </nav>
+          </>
+        )}
       </div>
     </aside>
   )