diff --git a/image/Downscaler.cpp b/image/Downscaler.cpp
index d83141b93e84..d44e6b8a9c2c 100644
--- a/image/Downscaler.cpp
+++ b/image/Downscaler.cpp
@@ -74,8 +74,8 @@ Downscaler::BeginFrame(const nsIntSize& aOriginalSize,
 
   mFrameRect = aFrameRect.valueOr(nsIntRect(nsIntPoint(), aOriginalSize));
   MOZ_ASSERT(mFrameRect.x >= 0 && mFrameRect.y >= 0 &&
-             mFrameRect.width > 0 && mFrameRect.height > 0,
-             "Frame rect must have positive components");
+             mFrameRect.width >= 0 && mFrameRect.height >= 0,
+             "Frame rect must have non-negative components");
   MOZ_ASSERT(nsIntRect(0, 0, aOriginalSize.width, aOriginalSize.height)
                .Contains(mFrameRect),
              "Frame rect must fit inside image");
@@ -159,8 +159,13 @@ Downscaler::ResetForNextProgressivePass()
   mCurrentInLine = 0;
   mLinesInBuffer = 0;
 
-  // If we have a vertical offset, commit rows to shift us past it.
-  SkipToRow(mFrameRect.y);
+  if (mFrameRect.IsEmpty()) {
+    // Our frame rect is zero size; commit rows until the end of the image.
+    SkipToRow(mOriginalSize.height - 1);
+  } else {
+    // If we have a vertical offset, commit rows to shift us past it.
+    SkipToRow(mFrameRect.y);
+  }
 }
 
 static void
diff --git a/image/Downscaler.h b/image/Downscaler.h
index e2942cc43e31..64ab1fe91137 100644
--- a/image/Downscaler.h
+++ b/image/Downscaler.h
@@ -83,6 +83,8 @@ public:
                       bool aHasAlpha,
                       bool aFlipVertically = false);
 
+  bool IsFrameComplete() const { return mCurrentInLine >= mOriginalSize.height; }
+
   /// Retrieves the buffer into which the Decoder should write each row.
   uint8_t* RowBuffer()
   {
@@ -161,6 +163,7 @@ public:
     return NS_ERROR_FAILURE;
   }
 
+  bool IsFrameComplete() const { return false; }
   uint8_t* RowBuffer() { return nullptr; }
   void ClearRow(uint32_t = 0) { }
   void CommitRow() { }