Stage 17 (#4): in-memory lobby cache — render instantly on the back-slide, refresh in background

ui/src/lib/app.svelte.ts

@@ -13,6 +13,7 @@ import { insideTelegram, onTelegramPath, telegramColorScheme, telegramLaunch, te
 import { parseStartParam } from './deeplink';
 import { clearSession, loadPrefs, loadSession, saveSession, savePrefs } from './session';
 import { clearGameCache } from './gamecache';
+import { clearLobby } from './lobbycache';
 import type { BoardLabelMode } from './boardlabels';
 
 export interface Toast {
@@ -311,6 +312,7 @@ export async function loginEmail(email: string, code: string): Promise<void> {
 export async function logout(): Promise<void> {
   closeStream();
   clearGameCache();
+  clearLobby();
   gateway.setToken(null);
   await clearSession();
   app.session = null;

ui/src/lib/lobbycache.ts

@@ -0,0 +1,30 @@
+// In-memory lobby snapshot, the lobby counterpart of gamecache.ts. The lobby re-fetches
+// its lists on every entry, so without a cache the screen renders blank and "draws in"
+// during the back-slide from a game. Caching the last lists lets the lobby render
+// instantly (before/under the transition) and refresh in the background. Process-memory
+// only; cleared on logout.
+
+import type { AccountRef, GameView, Invitation } from './model';
+
+interface LobbySnapshot {
+  games: GameView[];
+  invitations: Invitation[];
+  incoming: AccountRef[];
+}
+
+let snapshot: LobbySnapshot | null = null;
+
+/** getLobby returns the last lobby lists, or null before the first load. */
+export function getLobby(): LobbySnapshot | null {
+  return snapshot;
+}
+
+/** setLobby stores the latest lobby lists. */
+export function setLobby(s: LobbySnapshot): void {
+  snapshot = s;
+}
+
+/** clearLobby drops the cached lobby (called on logout). */
+export function clearLobby(): void {
+  snapshot = null;
+}

ui/src/screens/Lobby.svelte

@@ -8,6 +8,7 @@
   import { navigate } from '../lib/router.svelte';
   import { t, type MessageKey } from '../lib/i18n/index.svelte';
   import { resultBadge } from '../lib/result';
+  import { getLobby, setLobby } from '../lib/lobbycache';
   import type { AccountRef, GameView, Invitation } from '../lib/model';
 
   let games = $state<GameView[]>([]);
@@ -23,12 +24,23 @@
         [invitations, incoming] = await Promise.all([gateway.invitationsList(), gateway.friendsIncoming()]);
         app.notifications = invitations.length + incoming.length;
       }
+      setLobby({ games, invitations, incoming });
     } catch (e) {
       handleError(e);
     }
   }
 
-  onMount(load);
+  onMount(() => {
+    // Render instantly from the cached lists (so the screen does not "draw in" during
+    // the back-slide), then refresh in the background.
+    const cached = getLobby();
+    if (cached) {
+      games = cached.games;
+      invitations = cached.invitations;
+      incoming = cached.incoming;
+    }
+    void load();
+  });
   $effect(() => {
     if (app.lastEvent) void load();
   });