Bug 1182929 - Work around bug 803703 by refusing to turn heavily-downscaled images into image layers. r=seth

layout/generic/nsImageFrame.cpp

@@ -1430,7 +1430,35 @@ nsDisplayImage::CanOptimizeToImageLayer(LayerManager* aManager,
                  ? imgIContainer::FLAG_SYNC_DECODE
                  : imgIContainer::FLAG_NONE;
 
-  return mImage->IsImageContainerAvailable(aManager, flags);
+  if (!mImage->IsImageContainerAvailable(aManager, flags)) {
+    return false;
+  }
+
+  int32_t imageWidth;
+  int32_t imageHeight;
+  mImage->GetWidth(&imageWidth);
+  mImage->GetHeight(&imageHeight);
+
+  if (imageWidth == 0 || imageHeight == 0) {
+    NS_ASSERTION(false, "invalid image size");
+    return false;
+  }
+
+  const int32_t factor = mFrame->PresContext()->AppUnitsPerDevPixel();
+  const LayoutDeviceRect destRect =
+    LayoutDeviceRect::FromAppUnits(GetDestRect(), factor);
+
+  // Calculate the scaling factor for the frame.
+  const gfxSize scale = gfxSize(destRect.width / imageWidth,
+                                destRect.height / imageHeight);
+
+  if (scale.width < 0.2 || scale.height < 0.2) {
+    // This would look awful as long as we can't use high-quality downscaling
+    // for image layers (bug 803703), so don't turn this into an image layer.
+    return false;
+  }
+
+  return true;
 }
 
 already_AddRefed<ImageContainer>