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 2d0b917413a35611db9079fff8995dca642b36f9..9f9909c630febc58101727b7dd9bbe5cbc1ab67b 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -47,6 +47,8 @@ void PaintLayerPainter::Paint(GraphicsContext& context,
                               const LayoutRect& damage_rect,
                               const GlobalPaintFlags global_paint_flags,
                               PaintLayerFlags paint_flags) {
+  DCHECK(paint_layer_.GetCompositingState() != kPaintsIntoOwnBacking ||
+         paint_layer_.SubpixelAccumulation().IsZero());
   PaintLayerPaintingInfo painting_info(
       &paint_layer_, LayoutRect(EnclosingIntRect(damage_rect)),
       global_paint_flags, LayoutSize());
@@ -324,10 +326,6 @@ PaintResult PaintLayerPainter::PaintLayerContents(
   // Ensure our lists are up to date.
   paint_layer_.StackingNode()->UpdateLayerListsIfNeeded();
 
-  LayoutSize subpixel_accumulation =
-      paint_layer_.GetCompositingState() == kPaintsIntoOwnBacking
-          ? paint_layer_.SubpixelAccumulation()
-          : painting_info_arg.sub_pixel_accumulation;
   ShouldRespectOverflowClipType respect_overflow_clip =
       ShouldRespectOverflowClip(paint_flags, paint_layer_.GetLayoutObject());
 
@@ -336,7 +334,8 @@ PaintResult PaintLayerPainter::PaintLayerContents(
   if (ShouldCreateSubsequence(paint_layer_, context, painting_info_arg,
                               paint_flags)) {
     if (!ShouldRepaintSubsequence(paint_layer_, painting_info_arg,
-                                  respect_overflow_clip, subpixel_accumulation,
+                                  respect_overflow_clip,
+                                  painting_info_arg.sub_pixel_accumulation,
                                   should_clear_empty_paint_phase_flags) &&
         SubsequenceRecorder::UseCachedSubsequenceIfPossible(context,
                                                             paint_layer_))
@@ -357,7 +356,7 @@ PaintResult PaintLayerPainter::PaintLayerContents(
 
   LayoutPoint offset_from_root;
   paint_layer_.ConvertToLayerCoords(painting_info.root_layer, offset_from_root);
-  offset_from_root.Move(subpixel_accumulation);
+  offset_from_root.Move(painting_info_arg.sub_pixel_accumulation);
 
   LayoutRect bounds = paint_layer_.PhysicalBoundingBox(offset_from_root);
   if (!painting_info.paint_dirty_rect.Contains(bounds))
@@ -413,8 +412,6 @@ PaintResult PaintLayerPainter::PaintLayerContents(
   }
 
   PaintLayerPaintingInfo local_painting_info(painting_info);
-  local_painting_info.sub_pixel_accumulation = subpixel_accumulation;
-
   bool should_paint_content = paint_layer_.HasVisibleContent() &&
                               is_self_painting_layer &&
                               !is_painting_overlay_scrollbars;
@@ -823,7 +820,9 @@ PaintResult PaintLayerPainter::PaintFragmentByApplyingTransform(
   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);
+      paint_layer_.GetCompositingState() == kPaintsIntoOwnBacking

[chrishtr, 2017/04/21 15:50:51]

Why is this change needed?

[flackr, 2017/05/04 17:38:41]

This was to make the code behave the same as before (i.e. the removed / moved
code from PaintLayerPainter::Paint) but I'm not sure if it's necessary yet.
Without this change the bug is still fixed and my tests pass.

+          ? paint_layer_.SubpixelAccumulation()
+          : painting_info.sub_pixel_accumulation + (delta - rounded_delta);
 
   // TODO(jbroman): Put the real transform origin here, instead of using a
   // matrix with the origin baked in.
@@ -1226,6 +1225,8 @@ void PaintLayerPainter::PaintOverlayScrollbars(
   if (!paint_layer_.ContainsDirtyOverlayScrollbars())
     return;
 
+  DCHECK(paint_layer_.GetCompositingState() != kPaintsIntoOwnBacking ||
+         paint_layer_.SubpixelAccumulation().IsZero());
   PaintLayerPaintingInfo painting_info(
       &paint_layer_, LayoutRect(EnclosingIntRect(damage_rect)), paint_flags,
       LayoutSize());