From d47ec8dfc5cdfb8878121a3646f672674e3a69f5 Mon Sep 17 00:00:00 2001
From: Markus Stange <mstange@themasta.com>
Date: Tue, 14 Jul 2015 12:06:27 -0400
Subject: [PATCH] Bug 1182929 - Work around bug 803703 by refusing to turn
 heavily-downscaled images into image layers. r=seth

---
 layout/generic/nsImageFrame.cpp | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/layout/generic/nsImageFrame.cpp b/layout/generic/nsImageFrame.cpp
index ecb7a5f00cdc..d8df0809e515 100644
--- a/layout/generic/nsImageFrame.cpp
+++ b/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>