Round the subpixel accumulation used for composited scrolling content

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 3017 matching lines @@
       !GetLayoutObject().View()->GetFrame()->ShouldThrottleRendering())
     paint_info.paint_layer->GetLayoutObject().AssertSubtreeIsLaidOut();
 #endif
 
   float device_scale_factor = blink::DeviceScaleFactorDeprecated(
       paint_info.paint_layer->GetLayoutObject().GetFrame());
   context.SetDeviceScaleFactor(device_scale_factor);
 
   if (paint_info.paint_layer->GetCompositingState() !=
       kPaintsIntoGroupedBacking) {
+    LayoutSize sub_pixel_accumulation =
+        paint_info.paint_layer->SubpixelAccumulation();
+    if (paint_layer_flags & kPaintLayerPaintingCompositingScrollingPhase) {
+      // The composited scrolling contents layer position rounds the subpixel
+      // accumulation.
+      sub_pixel_accumulation =
+          LayoutSize(RoundedIntSize(sub_pixel_accumulation));
+    }
     // FIXME: GraphicsLayers need a way to split for multicol.
     PaintLayerPaintingInfo painting_info(
         paint_info.paint_layer, LayoutRect(dirty_rect), kGlobalPaintNormalPhase,
-        paint_info.paint_layer->SubpixelAccumulation());
+        sub_pixel_accumulation);
     PaintLayerPainter(*paint_info.paint_layer)
         .PaintLayerContents(context, painting_info, paint_layer_flags);
 
     if (paint_info.paint_layer->ContainsDirtyOverlayScrollbars())
       PaintLayerPainter(*paint_info.paint_layer)
           .PaintLayerContents(
               context, painting_info,
               paint_layer_flags | kPaintLayerPaintingOverlayScrollbars);
   } else {
     PaintLayerPaintingInfo painting_info(
@@ skipping 552 matching lines @@
   } else if (graphics_layer == decoration_outline_layer_.get()) {
     name = "Decoration Layer";
   } else {
     NOTREACHED();
   }
 
   return name;
 }
 
 }  // namespace blink