Fix getChildPeriod logic if the child is a TimeOffsetMediaPeriod

The current instanceof check accidentally unpacks the child,
returning the inner period instead of the one passed to the source.

PiperOrigin-RevId: 746556825
(cherry picked from commit 68538029c926642da42a7ebf40543f480c38146e)

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/MergingMediaPeriod.java

@@ -41,6 +41,7 @@ import java.util.List;
 /* package */ final class MergingMediaPeriod implements MediaPeriod, MediaPeriod.Callback {
 
   private final MediaPeriod[] periods;
+  private final boolean[] periodsWithTimeOffsets;
   private final IdentityHashMap<SampleStream, Integer> streamPeriodIndices;
   private final CompositeSequenceableLoaderFactory compositeSequenceableLoaderFactory;
   private final ArrayList<MediaPeriod> childrenPendingPreparation;
@@ -62,8 +63,10 @@ import java.util.List;
     compositeSequenceableLoader = compositeSequenceableLoaderFactory.empty();
     streamPeriodIndices = new IdentityHashMap<>();
     enabledPeriods = new MediaPeriod[0];
+    periodsWithTimeOffsets = new boolean[periods.length];
     for (int i = 0; i < periods.length; i++) {
       if (periodTimeOffsetsUs[i] != 0) {
+        periodsWithTimeOffsets[i] = true;
         this.periods[i] = new TimeOffsetMediaPeriod(periods[i], periodTimeOffsetsUs[i]);
       }
     }
@@ -75,7 +78,7 @@ import java.util.List;
    * specified index.
    */
   public MediaPeriod getChildPeriod(int index) {
-    return periods[index] instanceof TimeOffsetMediaPeriod
+    return periodsWithTimeOffsets[index]
         ? ((TimeOffsetMediaPeriod) periods[index]).getWrappedMediaPeriod()
         : periods[index];
   }

libraries/exoplayer/src/test/java/androidx/media3/exoplayer/source/MergingMediaPeriodTest.java

@@ -22,6 +22,7 @@ import static com.google.common.truth.Truth.assertThat;
 
 import androidx.media3.common.C;
 import androidx.media3.common.Format;
+import androidx.media3.common.MimeTypes;
 import androidx.media3.common.TrackGroup;
 import androidx.media3.common.util.NullableType;
 import androidx.media3.decoder.DecoderInputBuffer;
@@ -33,6 +34,7 @@ import androidx.media3.exoplayer.source.MediaSource.MediaPeriodId;
 import androidx.media3.exoplayer.source.MediaSourceEventListener.EventDispatcher;
 import androidx.media3.exoplayer.trackselection.ExoTrackSelection;
 import androidx.media3.exoplayer.trackselection.FixedTrackSelection;
+import androidx.media3.exoplayer.upstream.Allocator;
 import androidx.media3.exoplayer.upstream.DefaultAllocator;
 import androidx.media3.test.utils.FakeMediaPeriod;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
@@ -270,6 +272,39 @@ public final class MergingMediaPeriodTest {
     assertThat(inputBuffer.timeUs).isEqualTo(456_000 - 3000);
   }
 
+  @Test
+  public void
+      getChildPeriod_withTimeOffsetsAndTimeOffsetPeriodChildren_returnsCorrectChildPeriod() {
+    TrackGroupArray trackGroupArray =
+        new TrackGroupArray(
+            new TrackGroup(new Format.Builder().setSampleMimeType(MimeTypes.VIDEO_AV1).build()));
+    Allocator allocator =
+        new DefaultAllocator(/* trimOnReset= */ false, /* individualAllocationSize= */ 1024);
+    MediaPeriod childPeriod0 =
+        new FakeMediaPeriod(
+            trackGroupArray, allocator, /* singleSampleTimeUs= */ 0, new EventDispatcher());
+    MediaPeriod childPeriod1 =
+        new TimeOffsetMediaPeriod(
+            new FakeMediaPeriod(
+                trackGroupArray, allocator, /* singleSampleTimeUs= */ 300, new EventDispatcher()),
+            /* timeOffsetUs= */ -300);
+    MediaPeriod childPeriod2 =
+        new FakeMediaPeriod(
+            trackGroupArray, allocator, /* singleSampleTimeUs= */ -500, new EventDispatcher());
+
+    MergingMediaPeriod mergingMediaPeriod =
+        new MergingMediaPeriod(
+            new DefaultCompositeSequenceableLoaderFactory(),
+            /* periodTimeOffsetsUs= */ new long[] {0, 0, 500},
+            childPeriod0,
+            childPeriod1,
+            childPeriod2);
+
+    assertThat(mergingMediaPeriod.getChildPeriod(0)).isEqualTo(childPeriod0);
+    assertThat(mergingMediaPeriod.getChildPeriod(1)).isEqualTo(childPeriod1);
+    assertThat(mergingMediaPeriod.getChildPeriod(2)).isEqualTo(childPeriod2);
+  }
+
   private MergingMediaPeriod prepareMergingPeriod(MergingPeriodDefinition... definitions)
       throws Exception {
     return prepareMergingPeriod(/* singleTrackGroup= */ false, definitions);