--- a/image/SurfacePipe.h
+++ b/image/SurfacePipe.h
@@ -315,17 +315,18 @@ public:
 
     AdvanceRow();
 
     return IsSurfaceFinished() ? WriteState::FINISHED
                                : WriteState::NEED_MORE_DATA;
   }
 
   /**
-   * Write an empty row to the surface.
+   * Write an empty row to the surface. If some pixels have already been written
+   * to this row, they'll be discarded.
    *
    * @return WriteState::FINISHED if the entire surface has been written to.
    *         Otherwise, returns WriteState::NEED_MORE_DATA.
    */
   WriteState WriteEmptyRow()
   {
     if (IsSurfaceFinished()) {
       return WriteState::FINISHED;  // Already done.
@@ -570,17 +571,18 @@ public:
   WriteState WriteBuffer(const PixelType* aSource,
                          const size_t aStartColumn,
                          const size_t aLength)
   {
     return mHead->WriteBuffer<PixelType>(aSource, aStartColumn, aLength);
   }
 
   /**
-   * Write an empty row to the surface.
+   * Write an empty row to the surface. If some pixels have already been written
+   * to this row, they'll be discarded.
    *
    * @see SurfaceFilter::WriteEmptyRow() for the canonical documentation.
    */
   WriteState WriteEmptyRow()
   {
     return mHead->WriteEmptyRow();
   }
 

--- a/image/test/gtest/TestSurfaceSink.cpp
+++ b/image/test/gtest/TestSurfaceSink.cpp
@@ -371,16 +371,41 @@ TEST(ImageSurfaceSink, SurfaceSinkWriteE
       CheckIterativeWrite(aDecoder, aSink, IntRect(0, 0, 0, 0), [&]{
         return aSink->WriteEmptyRow();
       });
     }
 
     ResetForNextPass(aSink);
 
     {
+      // Write a partial row before we begin calling WriteEmptyRow(). We check
+      // that the generated image is entirely transparent, which is to be
+      // expected since WriteEmptyRow() overwrites the current row even if some
+      // data has already been written to it.
+      uint32_t count = 0;
+      auto result = aSink->WritePixels<uint32_t>([&]() -> NextPixel<uint32_t> {
+        if (count == 50) {
+          return AsVariant(WriteState::NEED_MORE_DATA);
+        }
+        ++count;
+        return AsVariant(BGRAColor::Green().AsPixel());
+      });
+
+      EXPECT_EQ(WriteState::NEED_MORE_DATA, result);
+      EXPECT_EQ(50u, count);
+      EXPECT_FALSE(aSink->IsSurfaceFinished());
+
+      CheckIterativeWrite(aDecoder, aSink, IntRect(0, 0, 0, 0), [&]{
+        return aSink->WriteEmptyRow();
+      });
+    }
+
+    ResetForNextPass(aSink);
+
+    {
       // Create a buffer the same size as one row of the surface, containing all
       // green pixels.
       uint32_t buffer[100];
       for (int i = 0; i < 100; ++i) {
         buffer[i] = BGRAColor::Green().AsPixel();
       }
 
       // Write an empty row to the middle 60 rows of the surface. The first 20
@@ -927,16 +952,41 @@ TEST(ImageSurfaceSink, PalettedSurfaceSi
       CheckPalettedIterativeWrite(aDecoder, aSink, IntRect(0, 0, 0, 0), [&]{
         return aSink->WriteEmptyRow();
       });
     }
 
     ResetForNextPass(aSink);
 
     {
+      // Write a partial row before we begin calling WriteEmptyRow(). We check
+      // that the generated image is entirely 0, which is to be expected since
+      // WriteEmptyRow() overwrites the current row even if some data has
+      // already been written to it.
+      uint32_t count = 0;
+      auto result = aSink->WritePixels<uint8_t>([&]() -> NextPixel<uint8_t> {
+        if (count == 50) {
+          return AsVariant(WriteState::NEED_MORE_DATA);
+        }
+        ++count;
+        return AsVariant(uint8_t(255));
+      });
+
+      EXPECT_EQ(WriteState::NEED_MORE_DATA, result);
+      EXPECT_EQ(50u, count);
+      EXPECT_FALSE(aSink->IsSurfaceFinished());
+
+      CheckPalettedIterativeWrite(aDecoder, aSink, IntRect(0, 0, 0, 0), [&]{
+        return aSink->WriteEmptyRow();
+      });
+    }
+
+    ResetForNextPass(aSink);
+
+    {
       // Create a buffer the same size as one row of the surface, containing all
       // 255 pixels.
       uint8_t buffer[100];
       for (int i = 0; i < 100; ++i) {
         buffer[i] = 255;
       }
 
       // Write an empty row to the middle 60 rows of the surface. The first 20