Reland of 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 dbf533cd0cb10eed0c1323ae9409b1a2c23b8edd..b444f085dec4cc13af15ad17437124e04d3f59c5 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -1177,11 +1177,11 @@ bool LayoutBox::MapVisualRectToContainer(
     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.
@@ -1195,32 +1195,45 @@ bool LayoutBox::MapVisualRectToContainer(
     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 combining, in this order
+  //    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.
+  if (!StyleRef().Preserves3D()) {

[chrishtr, 2017/05/05 16:04:44]

Why remove the SPv2 condition here?

[Xianzhu, 2017/05/05 16:14:51]

MapToVisualRectInAncestorSpace() is still used on SPv2 for non-painting
purposes. Without EnclosingBoundingBox() the result will mismatch where we
paint.

For example,
  <div style="transform: scale(50)">
    <div style="position: relative; left: 0.7px; width: 100px; height:
100px"></div>
  </div>
we'll paint the inner div at (50,0 100x100) which is snapped to pixels in the
transformed space. Without EnclosingBoundingBox() here we'll get visual rect
(35,0 100x100) which mismatches where we paint. With EnclosingBoundingBox()
we'll get (0,0 200x100) which covers the painted area.

In some cases PaintPropertyTreeBulderTest also expects this behavior with
slop_factor.

[Xianzhu, 2017/05/05 16:16:02]

Correction: with width and height of the inner div should be 2px instead of
100px.

[chrishtr, 2017/05/05 16:17:19]

Ah, right. Ok.

+    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.
@@ -1237,21 +1250,12 @@ bool LayoutBox::MapVisualRectToContainer(
     transform = perspective_matrix * transform;
   }
 
-  // 5. Transform flattening.
+  // 3. Apply transform and flatten.
   transform_state.ApplyTransform(transform, accumulation);
-
-  // 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) {
+  if (!container_preserve_3d)
     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)