Round the subpixel accumulation used for composited scrolling content

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

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index 8aa1b45ddc3d32f32fa3d49a291bd3182cd6b108..680abaf07b2155e308a4b506162bc0684258ad91 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -325,6 +325,10 @@ PaintResult PaintLayerPainter::paintLayerContents(
       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());