diff --git a/ui/src/lib/telegram.ts b/ui/src/lib/telegram.ts
index cbc1889..9946896 100644
--- a/ui/src/lib/telegram.ts
+++ b/ui/src/lib/telegram.ts
@@ -14,6 +14,10 @@ interface TelegramWebApp {
   themeParams?: TelegramThemeParams;
   colorScheme?: 'light' | 'dark';
   isFullscreen?: boolean;
+  isExpanded?: boolean;
+  viewportHeight?: number;
+  viewportStableHeight?: number;
+  exitFullscreen?: () => void;
   safeAreaInset?: { top: number; bottom: number; left: number; right: number };
   contentSafeAreaInset?: { top: number; bottom: number; left: number; right: number };
   ready?: () => void;
@@ -469,6 +473,25 @@ export function collectTelegramDiag(): TelegramDiag {
   };
 }
 
+/**
+ * telegramChromeDiag is a TEMPORARY on-device readout of Telegram's viewport / fullscreen state,
+ * rendered in the lobby to debug the Android fullscreen-persistence issue (the app still opens
+ * fullscreen on Android even though requestFullscreen is now iOS-only). REMOVE before merge.
+ */
+export function telegramChromeDiag(): string {
+  const w = webApp();
+  if (!w) return 'no telegram';
+  const ih = typeof window === 'undefined' ? 0 : window.innerHeight;
+  const sh = typeof screen === 'undefined' ? 0 : screen.height;
+  return [
+    `platform: ${w.platform ?? '—'}  version: ${w.version ?? '—'}`,
+    `isFullscreen: ${w.isFullscreen}  isExpanded: ${w.isExpanded}`,
+    `viewportH: ${w.viewportHeight ?? '—'}  stableH: ${w.viewportStableHeight ?? '—'}`,
+    `innerH: ${ih}  screenH: ${sh}`,
+    `contentSafeTop: ${w.contentSafeAreaInset?.top ?? '—'}  safeTop: ${w.safeAreaInset?.top ?? '—'}`,
+  ].join('\n');
+}
+
 // --- Login Widget (web sign-in for account linking) ---
 
 // The Login Widget is the web (non-Mini-App) Telegram sign-in. It is used only to
diff --git a/ui/src/screens/Lobby.svelte b/ui/src/screens/Lobby.svelte
index b06ba42..af3fd3e 100644
--- a/ui/src/screens/Lobby.svelte
+++ b/ui/src/screens/Lobby.svelte
@@ -9,6 +9,7 @@
   import { gateway } from '../lib/gateway';
   import { navigate } from '../lib/router.svelte';
   import { t } from '../lib/i18n/index.svelte';
+  import { insideTelegram, telegramChromeDiag } from '../lib/telegram'; // TEMP diag — remove before merge
   import { resultBadge } from '../lib/result';
   import { badgeKind } from '../lib/unread';
   import { getLobby, setLobby } from '../lib/lobbycache';
@@ -223,6 +224,12 @@
 
 <Screen title={app.profile?.displayName ?? t('app.title')}>
   <div class="lobby">
+    <!-- TEMP DIAGNOSTIC (Android fullscreen debug) — REMOVE before merge -->
+    {#if insideTelegram()}
+      <pre
+        style="margin:0 0 8px;padding:8px;font:11px/1.4 ui-monospace,monospace;white-space:pre-wrap;overflow-wrap:anywhere;background:var(--bg-elev);border:1px solid var(--border);border-radius:var(--radius-sm);color:var(--text)"
+      >{telegramChromeDiag()}</pre>
+    {/if}
     {#if invitations.length}
       <section>
         <h2>{t('lobby.invitations')}</h2>