diff --git a/dom/media/gtest/MockMediaResource.cpp b/dom/media/gtest/MockMediaResource.cpp
index b8b1ebaf92c..03cc0e5e273 100644
--- a/dom/media/gtest/MockMediaResource.cpp
+++ b/dom/media/gtest/MockMediaResource.cpp
@@ -60,7 +60,8 @@ MockMediaResource::ReadAt(int64_t aOffset, char* aBuffer, uint32_t aCount,
 int64_t
 MockMediaResource::GetLength()
 {
-  return fseek(mFileHandle, 0, SEEK_END);
+  fseek(mFileHandle, 0, SEEK_END);
+  return ftell(mFileHandle);
 }
 
 void
diff --git a/dom/media/gtest/TestMP4Demuxer.cpp b/dom/media/gtest/TestMP4Demuxer.cpp
index 9559eb6a670..0aee4190cd1 100644
--- a/dom/media/gtest/TestMP4Demuxer.cpp
+++ b/dom/media/gtest/TestMP4Demuxer.cpp
@@ -331,3 +331,34 @@ TEST(MP4Demuxer, CENCFrag)
   }
   EXPECT_EQ(ArrayLength(audio), i);
 }
+
+TEST(MP4Demuxer, GetNextKeyframe)
+{
+  nsRefPtr<MP4DemuxerBinding> b = new MP4DemuxerBinding("gizmo-frag.mp4");
+  MonitorAutoLock mon(b->mMonitor);
+  MP4Demuxer* d = b->demuxer;
+
+  EXPECT_TRUE(d->Init());
+
+  // Insert a [0,end] buffered range, to simulate Moof's being buffered
+  // via MSE.
+  auto len = b->resource->GetLength();
+  b->resource->MockAddBufferedRange(0, len);
+
+  // Rebuild the index so that it can be used to find the keyframes.
+  nsTArray<MediaByteRange> ranges;
+  EXPECT_TRUE(NS_SUCCEEDED(b->resource->GetCachedRanges(ranges)));
+  d->UpdateIndex(ranges);
+
+  // gizmp-frag has two keyframes; one at dts=cts=0, and another at
+  // dts=cts=1000000. Verify we get expected results.
+
+  MP4Sample* sample;
+  size_t i = 0;
+  const int64_t keyframe = 1000000;
+  while (!!(sample = d->DemuxVideoSample())) {
+    int64_t expected = (sample->decode_timestamp < keyframe) ? keyframe : -1;
+    EXPECT_EQ(d->GetNextKeyframeTime(), expected);
+    i++;
+  }
+}