Don't pass subpixel offsets through non-translation transforms

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1182 matching lines @@
                                                   visual_rect_flags))
       return false;
     return true;
   }
 
   // Otherwise, apply the following:
   // 1. Transform.
   // 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.
   TransformationMatrix transform;
   if (Layer() && Layer()->Transform())
     transform.Multiply(Layer()->CurrentTransform());
 
   // 2. Container offset.
   transform.PostTranslate(container_offset.X().ToFloat(),
                           container_offset.Y().ToFloat());
@@ skipping 15 matching lines @@
 
     TransformationMatrix perspective_matrix;
     perspective_matrix.ApplyPerspective(
         container_object->Style()->Perspective());
     perspective_matrix.ApplyTransformOrigin(perspective_origin.X(),
                                             perspective_origin.Y(), 0);
 
     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.
   if (!preserve3D) {
     transform_state.Flatten();
     transform_state.SetQuad(
         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()) {
     return ToLayoutBox(container_object)
         ->ApplyBoxClips(transform_state, accumulation, visual_rect_flags);
   }
 
   return true;
 }
 
@@ skipping 4638 matching lines @@
 void LayoutBox::MutableForPainting::
     SavePreviousContentBoxSizeAndLayoutOverflowRect() {
   auto& rare_data = GetLayoutBox().EnsureRareData();
   rare_data.has_previous_content_box_size_and_layout_overflow_rect_ = true;
   rare_data.previous_content_box_size_ = GetLayoutBox().ContentBoxRect().Size();
   rare_data.previous_layout_overflow_rect_ =
       GetLayoutBox().LayoutOverflowRect();
 }
 
 }  // namespace blink