--- a/image/test/gtest/TestAnimationFrameBuffer.cpp
+++ b/image/test/gtest/TestAnimationFrameBuffer.cpp
@@ -6,8 +6,283 @@
 #include "gtest/gtest.h"
 
 #include "mozilla/Move.h"
 #include "AnimationFrameBuffer.h"
 
 using namespace mozilla;
 using namespace mozilla::image;
 
+static already_AddRefed<imgFrame>
+CreateEmptyFrame()
+{
+  RefPtr<imgFrame> frame = new imgFrame();
+  nsresult rv = frame->InitForAnimator(nsIntSize(1, 1), SurfaceFormat::B8G8R8A8);
+  EXPECT_TRUE(NS_SUCCEEDED(rv));
+  RawAccessFrameRef frameRef = frame->RawAccessRef();
+  frame->Finish();
+  return frame.forget();
+}
+
+class ImageAnimationFrameBuffer : public ::testing::Test
+{
+public:
+  ImageAnimationFrameBuffer()
+  { }
+
+private:
+  AutoInitializeImageLib mInit;
+};
+
+TEST_F(ImageAnimationFrameBuffer, RetainedInitialState)
+{
+  const size_t kThreshold = 800;
+  const size_t kBatch = 100;
+  AnimationFrameRetainedBuffer buffer(kThreshold, kBatch, 0);
+
+  EXPECT_EQ(kThreshold, buffer.Threshold());
+  EXPECT_EQ(kBatch, buffer.Batch());
+  EXPECT_EQ(size_t(0), buffer.Displayed());
+  EXPECT_EQ(kBatch * 2, buffer.PendingDecode());
+  EXPECT_EQ(size_t(0), buffer.PendingAdvance());
+  EXPECT_FALSE(buffer.MayDiscard());
+  EXPECT_FALSE(buffer.SizeKnown());
+  EXPECT_EQ(size_t(0), buffer.Size());
+}
+
+TEST_F(ImageAnimationFrameBuffer, ThresholdTooSmall)
+{
+  const size_t kThreshold = 0;
+  const size_t kBatch = 10;
+  AnimationFrameRetainedBuffer buffer(kThreshold, kBatch, 0);
+
+  EXPECT_EQ(kBatch * 2 + 1, buffer.Threshold());
+  EXPECT_EQ(kBatch, buffer.Batch());
+  EXPECT_EQ(kBatch * 2, buffer.PendingDecode());
+  EXPECT_EQ(size_t(0), buffer.PendingAdvance());
+}
+
+TEST_F(ImageAnimationFrameBuffer, BatchTooSmall)
+{
+  const size_t kThreshold = 10;
+  const size_t kBatch = 0;
+  AnimationFrameRetainedBuffer buffer(kThreshold, kBatch, 0);
+
+  EXPECT_EQ(kThreshold, buffer.Threshold());
+  EXPECT_EQ(size_t(1), buffer.Batch());
+  EXPECT_EQ(size_t(2), buffer.PendingDecode());
+  EXPECT_EQ(size_t(0), buffer.PendingAdvance());
+}
+
+TEST_F(ImageAnimationFrameBuffer, BatchTooBig)
+{
+  const size_t kThreshold = 50;
+  const size_t kBatch = SIZE_MAX;
+  AnimationFrameRetainedBuffer buffer(kThreshold, kBatch, 0);
+
+  // The rounding is important here (e.g. SIZE_MAX/4 * 2 != SIZE_MAX/2).
+  EXPECT_EQ(SIZE_MAX/4, buffer.Batch());
+  EXPECT_EQ(buffer.Batch() * 2 + 1, buffer.Threshold());
+  EXPECT_EQ(buffer.Batch() * 2, buffer.PendingDecode());
+  EXPECT_EQ(size_t(0), buffer.PendingAdvance());
+}
+
+TEST_F(ImageAnimationFrameBuffer, FinishUnderBatchAndThreshold)
+{
+  const size_t kThreshold = 30;
+  const size_t kBatch = 10;
+  AnimationFrameRetainedBuffer buffer(kThreshold, kBatch, 0);
+  const auto& frames = buffer.Frames();
+
+  EXPECT_EQ(kBatch * 2, buffer.PendingDecode());
+
+  RefPtr<imgFrame> firstFrame;
+  for (size_t i = 0; i < 5; ++i) {
+    RefPtr<imgFrame> frame = CreateEmptyFrame();
+    auto status = buffer.Insert(RefPtr<imgFrame>(frame));
+    EXPECT_EQ(status, AnimationFrameBuffer::InsertStatus::CONTINUE);
+    EXPECT_FALSE(buffer.SizeKnown());
+    EXPECT_EQ(buffer.Size(), i + 1);
+
+    if (i == 4) {
+      EXPECT_EQ(size_t(15), buffer.PendingDecode());
+      bool keepDecoding = buffer.MarkComplete(IntRect(0, 0, 1, 1));
+      EXPECT_FALSE(keepDecoding);
+      EXPECT_TRUE(buffer.SizeKnown());
+      EXPECT_EQ(size_t(0), buffer.PendingDecode());
+      EXPECT_FALSE(buffer.HasRedecodeError());
+    }
+
+    EXPECT_FALSE(buffer.MayDiscard());
+
+    imgFrame* gotFrame = buffer.Get(i, false);
+    EXPECT_EQ(frame.get(), gotFrame);
+    ASSERT_EQ(i + 1, frames.Length());
+    EXPECT_EQ(frame.get(), frames[i].get());
+
+    if (i == 0) {
+      firstFrame = std::move(frame);
+      EXPECT_EQ(size_t(0), buffer.Displayed());
+    } else {
+      EXPECT_EQ(i - 1, buffer.Displayed());
+      bool restartDecoder = buffer.AdvanceTo(i);
+      EXPECT_FALSE(restartDecoder);
+      EXPECT_EQ(i, buffer.Displayed());
+    }
+
+    gotFrame = buffer.Get(0, false);
+    EXPECT_EQ(firstFrame.get(), gotFrame);
+  }
+
+  // Loop again over the animation and make sure it is still all there.
+  for (size_t i = 0; i < frames.Length(); ++i) {
+    EXPECT_TRUE(buffer.Get(i, false) != nullptr);
+
+    bool restartDecoder = buffer.AdvanceTo(i);
+    EXPECT_FALSE(restartDecoder);
+  }
+}
+
+TEST_F(ImageAnimationFrameBuffer, FinishMultipleBatchesUnderThreshold)
+{
+  const size_t kThreshold = 30;
+  const size_t kBatch = 2;
+  AnimationFrameRetainedBuffer buffer(kThreshold, kBatch, 0);
+  const auto& frames = buffer.Frames();
+
+  EXPECT_EQ(kBatch * 2, buffer.PendingDecode());
+
+  // Add frames until it tells us to stop.
+  AnimationFrameBuffer::InsertStatus status;
+  do {
+    status = buffer.Insert(CreateEmptyFrame());
+    EXPECT_FALSE(buffer.SizeKnown());
+    EXPECT_FALSE(buffer.MayDiscard());
+  } while (status == AnimationFrameBuffer::InsertStatus::CONTINUE);
+
+  EXPECT_EQ(size_t(0), buffer.PendingDecode());
+  EXPECT_EQ(size_t(4), frames.Length());
+  EXPECT_EQ(status, AnimationFrameBuffer::InsertStatus::YIELD);
+
+  // Progress through the animation until it lets us decode again.
+  bool restartDecoder = false;
+  size_t i = 0;
+  do {
+    EXPECT_TRUE(buffer.Get(i, false) != nullptr);
+    if (i > 0) {
+      restartDecoder = buffer.AdvanceTo(i);
+    }
+    ++i;
+  } while (!restartDecoder);
+
+  EXPECT_EQ(size_t(2), buffer.PendingDecode());
+  EXPECT_EQ(size_t(2), buffer.Displayed());
+
+  // Add the last frame.
+  status = buffer.Insert(CreateEmptyFrame());
+  EXPECT_EQ(status, AnimationFrameBuffer::InsertStatus::CONTINUE);
+  bool keepDecoding = buffer.MarkComplete(IntRect(0, 0, 1, 1));
+  EXPECT_FALSE(keepDecoding);
+  EXPECT_TRUE(buffer.SizeKnown());
+  EXPECT_EQ(size_t(0), buffer.PendingDecode());
+  EXPECT_EQ(size_t(5), frames.Length());
+  EXPECT_FALSE(buffer.HasRedecodeError());
+
+  // Finish progressing through the animation.
+  for ( ; i < frames.Length(); ++i) {
+    EXPECT_TRUE(buffer.Get(i, false) != nullptr);
+    restartDecoder = buffer.AdvanceTo(i);
+    EXPECT_FALSE(restartDecoder);
+  }
+
+  // Loop again over the animation and make sure it is still all there.
+  for (i = 0; i < frames.Length(); ++i) {
+    EXPECT_TRUE(buffer.Get(i, false) != nullptr);
+    restartDecoder = buffer.AdvanceTo(i);
+    EXPECT_FALSE(restartDecoder);
+  }
+
+  // Loop to the third frame and then reset the animation.
+  for (i = 0; i < 3; ++i) {
+    EXPECT_TRUE(buffer.Get(i, false) != nullptr);
+    restartDecoder = buffer.AdvanceTo(i);
+    EXPECT_FALSE(restartDecoder);
+  }
+
+  // Since we are below the threshold, we can reset the get index only.
+  // Nothing else should have changed.
+  restartDecoder = buffer.Reset();
+  EXPECT_FALSE(restartDecoder);
+  for (i = 0; i < 5; ++i) {
+    EXPECT_TRUE(buffer.Get(i, false) != nullptr);
+  }
+  EXPECT_EQ(size_t(0), buffer.PendingDecode());
+  EXPECT_EQ(size_t(0), buffer.PendingAdvance());
+  EXPECT_EQ(size_t(0), buffer.Displayed());
+}
+
+TEST_F(ImageAnimationFrameBuffer, StartAfterBeginning)
+{
+  const size_t kThreshold = 30;
+  const size_t kBatch = 2;
+  const size_t kStartFrame = 7;
+  AnimationFrameRetainedBuffer buffer(kThreshold, kBatch, kStartFrame);
+
+  EXPECT_EQ(kStartFrame, buffer.PendingAdvance());
+
+  // Add frames until it tells us to stop. It should be later than before,
+  // because it auto-advances until its displayed frame is kStartFrame.
+  AnimationFrameBuffer::InsertStatus status;
+  size_t i = 0;
+  do {
+    status = buffer.Insert(CreateEmptyFrame());
+    EXPECT_FALSE(buffer.SizeKnown());
+    EXPECT_FALSE(buffer.MayDiscard());
+
+    if (i <= kStartFrame) {
+      EXPECT_EQ(i, buffer.Displayed());
+      EXPECT_EQ(kStartFrame - i, buffer.PendingAdvance());
+    } else {
+      EXPECT_EQ(kStartFrame, buffer.Displayed());
+      EXPECT_EQ(size_t(0), buffer.PendingAdvance());
+    }
+
+    i++;
+  } while (status == AnimationFrameBuffer::InsertStatus::CONTINUE);
+
+  EXPECT_EQ(size_t(0), buffer.PendingDecode());
+  EXPECT_EQ(size_t(0), buffer.PendingAdvance());
+  EXPECT_EQ(size_t(10), buffer.Size());
+}
+
+TEST_F(ImageAnimationFrameBuffer, StartAfterBeginningAndReset)
+{
+  const size_t kThreshold = 30;
+  const size_t kBatch = 2;
+  const size_t kStartFrame = 7;
+  AnimationFrameRetainedBuffer buffer(kThreshold, kBatch, kStartFrame);
+
+  EXPECT_EQ(kStartFrame, buffer.PendingAdvance());
+
+  // Add frames until it tells us to stop. It should be later than before,
+  // because it auto-advances until its displayed frame is kStartFrame.
+  for (size_t i = 0; i < 5; ++i) {
+    AnimationFrameBuffer::InsertStatus status =
+      buffer.Insert(CreateEmptyFrame());
+    EXPECT_EQ(status, AnimationFrameBuffer::InsertStatus::CONTINUE);
+    EXPECT_FALSE(buffer.SizeKnown());
+    EXPECT_FALSE(buffer.MayDiscard());
+    EXPECT_EQ(i, buffer.Displayed());
+    EXPECT_EQ(kStartFrame - i, buffer.PendingAdvance());
+  }
+
+  // When we reset the animation, it goes back to the beginning. That means
+  // we can forget about what we were told to advance to at the start. While
+  // we have plenty of frames in our buffer, we still need one more because
+  // in the real scenario, the decoder thread is still running and it is easier
+  // to let it insert its last frame than to coordinate quitting earlier.
+  buffer.Reset();
+  EXPECT_EQ(size_t(0), buffer.Displayed());
+  EXPECT_EQ(size_t(1), buffer.PendingDecode());
+  EXPECT_EQ(size_t(0), buffer.PendingAdvance());
+  EXPECT_EQ(size_t(5), buffer.Size());
+}
+