Round the subpixel accumulation used for composited scrolling content

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 307 matching lines @@
       m_paintLayer.layoutObject().view()->frame()->shouldThrottleRendering())
     return result;
 
   // Ensure our lists are up to date.
   m_paintLayer.stackingNode()->updateLayerListsIfNeeded();
 
   LayoutSize subpixelAccumulation =
       m_paintLayer.compositingState() == PaintsIntoOwnBacking
           ? m_paintLayer.subpixelAccumulation()
           : paintingInfoArg.subPixelAccumulation;
+  // The composited scrolling contents layer position rounds the subpixel
+  // accumulation.
+  if (isPaintingScrollingContent)
+    subpixelAccumulation = LayoutSize(roundedIntSize(subpixelAccumulation));

[flackr, 2017/03/14 21:07:19]

I found where we seem to first make the incorrect calculation. If we're painting
composited scrolling content
CompositedLayerMapping::updateScrollingLayerGeometry rounds the subpixel
accumulation offset applied to the layer position (see
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/co...
).

   ShouldRespectOverflowClipType respectOverflowClip =
       shouldRespectOverflowClip(paintFlags, m_paintLayer.layoutObject());
 
   Optional<SubsequenceRecorder> subsequenceRecorder;
   bool shouldClearEmptyPaintPhaseFlags = false;
   if (shouldCreateSubsequence(m_paintLayer, context, paintingInfoArg,
                               paintFlags)) {
     if (!shouldRepaintSubsequence(m_paintLayer, paintingInfoArg,
                                   respectOverflowClip, subpixelAccumulation,
                                   shouldClearEmptyPaintPhaseFlags) &&
@@ skipping 871 matching lines @@
           context, layoutObject, PaintPhaseClippingMask))
     return;
 
   IntRect snappedClipRect = pixelSnappedIntRect(clipRect.rect());
   LayoutObjectDrawingRecorder drawingRecorder(
       context, layoutObject, PaintPhaseClippingMask, snappedClipRect);
   context.fillRect(snappedClipRect, Color::black);
 }
 
 }  // namespace blink