Don't pass subpixel offsets through non-translation transforms

third_party/WebKit/Source/core/layout/LayoutBox.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.cpp b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
index 5ebf1b6cbcdb5e3a88e03274d743659dfdd54307..5cf983a5aec154f4cb1dd002c8bc4a3df5f0db85 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -1200,8 +1200,8 @@ bool LayoutBox::MapVisualRectToContainer(
   // 2. Container offset.
   // 3. Container scroll offset.
   // 4. Perspective applied by container.
-  // 5. Transform flattening.
-  // 6. Expansion for pixel snapping.
+  // 5. Expansion for pixel snapping.
+  // 6. Transform flattening.
   // 7. Container clip.
 
   // 1. Transform.
@@ -1237,11 +1237,8 @@ bool LayoutBox::MapVisualRectToContainer(
     transform = perspective_matrix * transform;
   }
 
-  // 5. Transform flattening.
-  transform_state.ApplyTransform(transform, accumulation);
-
-  // 6. Expansion for pixel snapping.
-  // Use enclosingBoundingBox because we cannot properly compute pixel
+  // 5. Expansion for pixel snapping.
+  // Use EnclosingBoundingBox because we cannot properly compute pixel
   // snapping for painted elements within the transform since we don't know
   // the desired subpixel accumulation at this point, and the transform may
   // include a scale.
@@ -1251,6 +1248,13 @@ bool LayoutBox::MapVisualRectToContainer(
         FloatQuad(transform_state.LastPlanarQuad().EnclosingBoundingBox()));

[Xianzhu, 2017/04/28 19:35:27]

I'm not sure if this is correct:
1. Is it OK to flatten before applying the transform?
2. What if preserve3D and the painter snaps to pixels?

   }
 
+  // 6. Transform flattening.
+  transform_state.ApplyTransform(transform, accumulation);
+  if (!preserve3D) {
+    transform_state.Flatten();
+    transform_state.SetQuad(FloatQuad(transform_state.LastPlanarQuad()));
+  }
+
   // 7. Container clip.
   if (container_object->IsBox() && container_object != ancestor &&
       container_object->HasClipRelatedProperty()) {