fix(player): 优化上下滑切集黑屏

test/playback/playback_preload_test.dart

@@ -91,20 +91,63 @@ void main() {
       expect(session.preloadedIndex, 1);
       final loadingStates = <bool>[];
       session.addListener(() => loadingStates.add(session.loading));
-      await session.select(1, preloadIndex: 2);
+      await session.select(1, preloadIndex: 2, previousIndex: 0);
       await session.preloadSettled;
       expect(session.active, same(warm));
       expect(warm.initializeCalls, 1);
       expect(warm.playCalls, 1);
       expect(loadingStates, isNot(contains(true)));
       expect(activated, ['ep-0', 'ep-1']);
-      expect(factory.peakLive, 2);
+      expect(session.preparedPlayerAt(0), isNotNull);
+      expect(session.preparedPlayerAt(2), isNotNull);
+      expect(factory.peakLive, lessThanOrEqualTo(3));
       expect(factory.peakPlaying, 1);
       await session.close();
       expect(factory.live, 0);
     },
   );
 
+  test('should_prepare_previous_neighbor_when_selected', () async {
+    final factory = _Factory();
+    final session = PlaybackSession(
+      episodes: episodes,
+      factory: factory.create,
+      canPlay: (_) => true,
+    );
+    await session.select(1, preloadIndex: 2, previousIndex: 0);
+    await session.preloadSettled;
+    expect(session.preparedPlayerAt(0), isNotNull);
+    expect(session.preparedPlayerAt(2), isNotNull);
+    expect(factory.players.map((player) => player.id).toSet(), {
+      'ep-0',
+      'ep-1',
+      'ep-2',
+    });
+    expect(factory.peakLive, 3);
+    await session.close();
+  });
+
+  test('should_warm_previous_without_waiting_for_slow_next', () async {
+    final nextGate = Completer<void>();
+    final factory = _Factory(gates: {'ep-2': nextGate});
+    final session = PlaybackSession(
+      episodes: episodes,
+      factory: factory.create,
+      canPlay: (_) => true,
+    );
+
+    await session.select(1, preloadIndex: 2, previousIndex: 0);
+    await _tick();
+
+    expect(session.preparedPlayerAt(0), isNotNull);
+    expect(session.preparedPlayerAt(2), isNull);
+
+    nextGate.complete();
+    await session.preloadSettled;
+    expect(session.preparedPlayerAt(2), isNotNull);
+    await session.close();
+  });
+
   test('should_reuse_in_flight_preload_when_swiped_before_ready', () async {
     final gate = Completer<void>();
     final factory = _Factory(gates: {'ep-1': gate});
@@ -124,7 +167,7 @@ void main() {
     await session.preloadSettled;
     expect(session.active, same(warm));
     expect(warm.initializeCalls, 1);
-    expect(factory.peakLive, lessThanOrEqualTo(2));
+    expect(factory.peakLive, lessThanOrEqualTo(3));
     expect(factory.peakPlaying, 1);
     await session.close();
   });
@@ -149,7 +192,7 @@ void main() {
     expect(session.preloadedIndex, 2);
     expect(stale.playCalls, 0);
     expect(stale.disposed, isTrue);
-    expect(factory.peakLive, lessThanOrEqualTo(2));
+    expect(factory.peakLive, lessThanOrEqualTo(3));
     expect(factory.peakPlaying, 1);
     await session.close();
   });