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

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

 // Copyright 2014 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/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
 #include "core/paint/FilterPainter.h"
@@ skipping 815 matching lines @@
     GraphicsContext& context,
     const PaintLayerPaintingInfo& painting_info,
     PaintLayerFlags paint_flags,
     const LayoutPoint& fragment_translation) {
   // This involves subtracting out the position of the layer in our current
   // coordinate space, but preserving the accumulated error for sub-pixel
   // layout.
   LayoutPoint delta;
   paint_layer_.ConvertToLayerCoords(painting_info.root_layer, delta);
   delta.MoveBy(fragment_translation);
+  delta += painting_info.sub_pixel_accumulation;
+  IntPoint rounded_delta = RoundedIntPoint(delta);
+
   TransformationMatrix transform(
       paint_layer_.RenderableTransform(painting_info.GetGlobalPaintFlags()));
-  IntPoint rounded_delta = RoundedIntPoint(delta);
   transform.PostTranslate(rounded_delta.X(), rounded_delta.Y());
-  LayoutSize adjusted_sub_pixel_accumulation =
-      painting_info.sub_pixel_accumulation + (delta - rounded_delta);
+
+  LayoutSize new_sub_pixel_accumulation;
+  if (transform.IsIdentityOrTranslation())
+    new_sub_pixel_accumulation += delta - rounded_delta;
+  // Otherwise discard the sub-pixel remainder because paint offset can't be
+  // transformed by a non-translation transform.
 
   // TODO(jbroman): Put the real transform origin here, instead of using a
   // matrix with the origin baked in.
   FloatPoint3D transform_origin;
   Transform3DRecorder transform3d_recorder(
       context, paint_layer_.GetLayoutObject(),
       DisplayItem::kTransform3DElementTransform, transform, transform_origin);
 
   // Now do a paint with the root layer shifted to be us.
   PaintLayerPaintingInfo transformed_painting_info(
       &paint_layer_,
       LayoutRect(EnclosingIntRect(
           transform.Inverse().MapRect(painting_info.paint_dirty_rect))),
-      painting_info.GetGlobalPaintFlags(), adjusted_sub_pixel_accumulation);
+      painting_info.GetGlobalPaintFlags(), new_sub_pixel_accumulation);
   transformed_painting_info.ancestor_has_clip_path_clipping =
       painting_info.ancestor_has_clip_path_clipping;
 
   // Remove skip root background flag when we're painting with a new root.
   if (&paint_layer_ != painting_info.root_layer)
     paint_flags &= ~kPaintLayerPaintingSkipRootBackground;
 
   return PaintLayerContentsCompositingAllPhases(
       context, transformed_painting_info, paint_flags, kForceSingleFragment);
 }
@@ skipping 390 matching lines @@
           context, layout_object, kPaintPhaseClippingMask))
     return;
 
   IntRect snapped_clip_rect = PixelSnappedIntRect(clip_rect.Rect());
   LayoutObjectDrawingRecorder drawing_recorder(
       context, layout_object, kPaintPhaseClippingMask, snapped_clip_rect);
   context.FillRect(snapped_clip_rect, Color::kBlack);
 }
 
 }  // namespace blink