fix(player): 优化上下滑切集黑屏

test/player/player_opening_frame_test.dart

@@ -0,0 +1,95 @@
+// Author: fruit.com.cn
+
+import 'dart:ui' as ui;
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:fruitshorts/core/data/services/sprite_thumbnail_asset_loader.dart';
+import 'package:fruitshorts/pages/player/widgets/player_opening_frame.dart';
+
+const String _spriteVtt = '''
+WEBVTT
+
+00:00:00.000 --> 00:00:01.000
+sprite_001.jpg#xywh=0,0,160,90
+''';
+
+void main() {
+  test('should_return_no_opening_frame_when_thumbnails_url_is_empty', () {
+    expect(playerOpeningFrame(null), isNull);
+    expect(playerOpeningFrame('  '), isNull);
+  });
+
+  testWidgets('should_load_and_reuse_first_sprite_frame', (tester) async {
+    final loader = _FakeSpriteLoader();
+    final cache = OpeningFrameCache();
+    addTearDown(cache.clear);
+
+    final first = await cache.load(
+      'https://cdn.test/thumbnails.vtt',
+      loader: loader,
+    );
+    final second = await cache.load(
+      'https://cdn.test/thumbnails.vtt',
+      loader: loader,
+    );
+
+    expect(first, isNotNull);
+    expect(second, same(first));
+    expect(loader.vttCalls, 1);
+    expect(loader.spriteCalls, ['sprite_001.jpg']);
+  });
+
+  testWidgets('should_render_decoded_opening_frame', (tester) async {
+    final loader = _FakeSpriteLoader();
+    final cache = OpeningFrameCache();
+    addTearDown(cache.clear);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: PlayerOpeningFrame(
+          thumbnailsUrl: 'https://cdn.test/thumbnails.vtt',
+          loader: loader,
+          cache: cache,
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.byKey(const Key('player-opening-sprite')), findsOneWidget);
+  });
+}
+
+class _FakeSpriteLoader implements SpriteThumbnailAssetLoader {
+  int vttCalls = 0;
+  final List<String> spriteCalls = [];
+
+  @override
+  Future<String?> loadVtt(Uri uri) async {
+    vttCalls++;
+    return _spriteVtt;
+  }
+
+  @override
+  Future<SpriteThumbnailDecodedImage?> loadSprite(Uri uri) async {
+    spriteCalls.add(uri.pathSegments.last);
+    return SpriteThumbnailDecodedImage(
+      image: await _createImage(320, 180),
+      originalWidth: 320,
+      originalHeight: 180,
+    );
+  }
+}
+
+Future<ui.Image> _createImage(int width, int height) async {
+  final recorder = ui.PictureRecorder();
+  final canvas = Canvas(recorder);
+  canvas.drawRect(
+    Rect.fromLTWH(0, 0, width.toDouble(), height.toDouble()),
+    Paint()..color = Colors.blue,
+  );
+  final picture = recorder.endRecording();
+  final image = await picture.toImage(width, height);
+  picture.dispose();
+  return image;
+}