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 1159 matching lines @@
 
   return result;
 }
 
 bool LayoutBox::MapVisualRectToContainer(
     const LayoutObject* container_object,
     const LayoutPoint& container_offset,
     const LayoutObject* ancestor,
     VisualRectFlags visual_rect_flags,
     TransformState& transform_state) const {
-  bool preserve3D = container_object->Style()->Preserves3D();
+  bool container_preserve_3d = container_object->Style()->Preserves3D();
 
   TransformState::TransformAccumulation accumulation =
-      preserve3D ? TransformState::kAccumulateTransform
-                 : TransformState::kFlattenTransform;
+      container_preserve_3d ? TransformState::kAccumulateTransform
+                            : TransformState::kFlattenTransform;
 
   // If there is no transform on this box, adjust for container offset and
   // container scrolling, then apply container clip.
   if (!ShouldUseTransformFromContainer(container_object)) {
     transform_state.MoveBy(container_offset, accumulation);
     if (container_object->IsBox() && container_object != ancestor &&
         !ToLayoutBox(container_object)
              ->MapScrollingContentsRectToBoxSpace(transform_state, accumulation,
                                                   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.
-  // 7. Container clip.
+  // Otherwise, do the following:
+  // 1. Expand for pixel snapping.
+  // 2. Generate transformation matrix by combining:

[chrishtr, 2017/05/01 22:13:42]

"combining, in this order"

[Xianzhu, 2017/05/01 22:59:24]

Done.

+  //    a) transform,
+  //    b) container offset,
+  //    c) container scroll offset,
+  //    d) perspective applied by container.
+  // 3. Apply transform Transform+flattening.
+  // 4. Apply container clip.
 
-  // 1. Transform.
+  // 1. Expand 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. This only makes sense for non-preserve3D, and it's
+  // applicable only for SPv1 paint invalidation use cases when the slow-path
+  // is used to map visual rect/location.
+  if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled() &&
+      !StyleRef().Preserves3D()) {
+    transform_state.Flatten();
+    transform_state.SetQuad(
+        FloatQuad(transform_state.LastPlanarQuad().EnclosingBoundingBox()));
+  }
+
+  // 2. Generate transformation matrix.
+  // a) Transform.
   TransformationMatrix transform;
   if (Layer() && Layer()->Transform())
     transform.Multiply(Layer()->CurrentTransform());
 
-  // 2. Container offset.
+  // b) Container offset.
   transform.PostTranslate(container_offset.X().ToFloat(),
                           container_offset.Y().ToFloat());
 
-  // 3. Container scroll offset.
+  // c) Container scroll offset.
   if (container_object->IsBox() && container_object != ancestor &&
       container_object->HasOverflowClip()) {
     IntSize offset = -ToLayoutBox(container_object)->ScrolledContentOffset();
     transform.PostTranslate(offset.Width(), offset.Height());
   }
 
-  // 4. Perspective applied by container.
+  // d) Perspective applied by container.
   if (container_object && container_object->HasLayer() &&
       container_object->Style()->HasPerspective()) {
     // Perspective on the container affects us, so we have to factor it in here.
     DCHECK(container_object->HasLayer());
     FloatPoint perspective_origin =
         ToLayoutBoxModelObject(container_object)->Layer()->PerspectiveOrigin();
 
     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.
+  // 3. Apply transform and flatten.
   transform_state.ApplyTransform(transform, accumulation);
+  if (!container_preserve_3d)
+    transform_state.Flatten();
 
-  // 6. 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()));
-  }
-
-  // 7. Container clip.
+  // 4. Apply 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;
 }
 
 bool LayoutBox::MapScrollingContentsRectToBoxSpace(
@@ skipping 4634 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