Reland of Don't pass subpixel offsets through non-translation transforms

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintPropertyTreeBuilder.h"
 
 #include <memory>
 #include "core/dom/DOMNodeIds.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 251 matching lines @@
     // regardless of whether a transform is present. If there is a transform
     // we round the paint offset but keep around the residual fractional
     // component for the transformed content to paint with.  In spv1 this was
     // called "subpixel accumulation". For more information, see
     // PaintLayer::subpixelAccumulation() and
     // PaintLayerPainter::paintFragmentByApplyingTransform.
     IntPoint rounded_paint_offset =
         RoundedIntPoint(context.current.paint_offset);
     LayoutPoint fractional_paint_offset =
         LayoutPoint(context.current.paint_offset - rounded_paint_offset);
+    if (fractional_paint_offset != LayoutPoint()) {
+      // If the object has a non-translation transform, discard the fractional
+      // paint offset which can't be transformed by the transform.
+      TransformationMatrix matrix;
+      object.StyleRef().ApplyTransform(
+          matrix, LayoutSize(), ComputedStyle::kExcludeTransformOrigin,
+          ComputedStyle::kIncludeMotionPath,
+          ComputedStyle::kIncludeIndependentTransformProperties);
+      if (!matrix.IsIdentityOrTranslation())
+        fractional_paint_offset = LayoutPoint();
+    }
 
     force_subtree_update |= properties.UpdatePaintOffsetTranslation(
         context.current.transform,
         TransformationMatrix().Translate(rounded_paint_offset.X(),
                                          rounded_paint_offset.Y()),
         FloatPoint3D(), context.current.should_flatten_inherited_transform,
         context.current.rendering_context_id);
 
     context.current.transform = properties.PaintOffsetTranslation();
     context.current.paint_offset = fractional_paint_offset;
     if (RuntimeEnabledFeatures::rootLayerScrollingEnabled() &&
         object.IsLayoutView()) {
       context.absolute_position.transform = properties.PaintOffsetTranslation();
       context.fixed_position.transform = properties.PaintOffsetTranslation();
-      context.absolute_position.paint_offset = LayoutPoint();
-      context.fixed_position.paint_offset = LayoutPoint();
+      context.absolute_position.paint_offset = fractional_paint_offset;
+      context.fixed_position.paint_offset = fractional_paint_offset;
     }
   } else {
     if (auto* properties = object.GetMutableForPainting().PaintProperties())
       force_subtree_update |= properties->ClearPaintOffsetTranslation();
   }
 }
 
 static bool NeedsTransformForNonRootSVG(const LayoutObject& object) {
   // TODO(pdr): Check for the presence of a transform instead of the value.
   // Checking for an identity matrix will cause the property tree structure
@@ skipping 972 matching lines @@
                            context.force_subtree_update);
 
     context.force_subtree_update |= object.SubtreeNeedsPaintPropertyUpdate();
   }
 
   if (object.CanContainAbsolutePositionObjects())
     context.container_for_absolute_position = &object;
 }
 
 }  // namespace blink