fix(home): 频道切换用封面过渡

test/player/player_video_surface_test.dart

@@ -7,6 +7,24 @@ import 'package:video_player/video_player.dart';
 import 'package:fruitshorts/pages/player/widgets/player_video_surface.dart';
 
 void main() {
+  testWidgets('should_show_poster_when_controller_is_missing', (tester) async {
+    await _pumpBareSurface(tester, posterUrl: '/files/poster.png');
+
+    expect(find.byKey(const Key('player-video-poster')), findsOneWidget);
+    expect(find.byKey(const Key('player-fallback-background')), findsNothing);
+    expect(_opacity(tester), 0);
+  });
+
+  testWidgets('should_keep_black_fallback_when_poster_is_empty', (
+    tester,
+  ) async {
+    await _pumpBareSurface(tester);
+
+    expect(find.byKey(const Key('player-video-poster')), findsNothing);
+    expect(_fallbackColor(tester), Colors.black);
+    expect(_opacity(tester), 0);
+  });
+
   testWidgets('should_keep_fallback_visible_before_video_is_ready', (
     tester,
   ) async {
@@ -96,6 +114,25 @@ Future<VideoPlayerController> _pumpSurface(
   return controller;
 }
 
+Future<void> _pumpBareSurface(
+  WidgetTester tester, {
+  String posterUrl = '',
+}) async {
+  await tester.pumpWidget(
+    MaterialApp(
+      home: Scaffold(
+        body: PlayerVideoSurface(
+          controller: null,
+          videoUrl: '',
+          posterUrl: posterUrl,
+          theater: false,
+          onTap: () {},
+        ),
+      ),
+    ),
+  );
+}
+
 Color _fallbackColor(WidgetTester tester) {
   return tester
       .widget<ColoredBox>(find.byKey(const Key('player-fallback-background')))