Round the subpixel accumulation used for composited scrolling content

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

 /*
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  *
  * 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
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@@ skipping 3028 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 =

[chrishtr, 2017/05/05 01:14:09]

This will round off the subpixel accumulation for the entire composited
scrolling layer, right? Why is that correct? Now I'm no longer sure
I understand why the background is not being painted to the edge of the
composited layer. Layers are positioned at snapped locations, but they
have subpixel offsets to record the extra offset.

[flackr, 2017/05/05 17:02:19]

The composited scrolling content geometry size does not take into account the
scroller's location, but the subpixel accumulation and painting code does. This
leads to the 1px gap. I'm now thinking the better change is to compute the
composited scrolling layer geometry with the correct size based on the layout
box location and then we could respect the subpixel accumulation. I'll start
doing this but let me know if you have any objections.

+          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