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 302 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)

[chrishtr, 2017/04/07 21:03:12]

I think you can move the logic determining subpixel accumulation entirely
up into CompositedLayerMapping, and pass it through paintingInfoArg.
Then the conditional on lines 320-322 can be removed, and the code can always
obey paintingInfoArg.subPixelAccumulation.

[flackr, 2017/04/21 03:43:44]

Done. The conditional on 320 was strange since it seems that
paintingInfoArg.subPixelAccumulation is passed in from CLM as the paint layers
subpixel accumulation (i.e. the same value). There are a couple of other
callsites where the passed in accumulation value is (0, 0), so perhaps the
conditional is to make sure we respect the layer accumulation in those cases? In
any event I've added checks to these callsites to see if this change would cause
any accumulation changes, I suspect not, but I'm running tryjobs to see if it
triggers.

+    subpixelAccumulation = LayoutSize(roundedIntSize(subpixelAccumulation));
   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 865 matching lines @@
           context, layoutObject, PaintPhaseClippingMask))
     return;
 
   IntRect snappedClipRect = pixelSnappedIntRect(clipRect.rect());
   LayoutObjectDrawingRecorder drawingRecorder(
       context, layoutObject, PaintPhaseClippingMask, snappedClipRect);
   context.fillRect(snappedClipRect, Color::black);
 }
 
 }  // namespace blink