Don't add subpixel accumulations for squashed layers when invalidating them.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * 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
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 576 matching lines @@
 
         squashedBounds.move(offsetFromSquashingLayer);
         totalSquashBounds.unite(squashedBounds);
     }
 
     // The totalSquashBounds is positioned with respect to referenceLayer of this CompositedLayerMapping.
     // But the squashingLayer needs to be positioned with respect to the ancestor CompositedLayerMapping.
     // The conversion between referenceLayer and the ancestor CLM is already computed as
     // offsetFromReferenceLayerToParentGraphicsLayer.
     totalSquashBounds.moveBy(offsetFromReferenceLayerToParentGraphicsLayer);
-    IntRect squashLayerBounds = enclosingIntRect(totalSquashBounds);
-    IntPoint squashLayerOrigin = squashLayerBounds.location();
-    LayoutSize squashLayerOriginInOwningLayerSpace = squashLayerOrigin - offsetFromReferenceLayerToParentGraphicsLayer;
+    const IntRect squashLayerBounds = enclosingIntRect(totalSquashBounds);
+    const IntPoint squashLayerOrigin = squashLayerBounds.location();
+    const LayoutSize squashLayerOriginInOwningLayerSpace = squashLayerOrigin - offsetFromReferenceLayerToParentGraphicsLayer;
 
     // Now that the squashing bounds are known, we can convert the RenderLayer painting offsets
     // from CLM owning layer space to the squashing layer space.
     //
     // The painting offset we want to compute for each squashed RenderLayer is essentially the position of
-    // the squashed RenderLayer described w.r.t. referenceLayer's origin. For this purpose we already cached
-    // offsetFromSquashingCLM before, which describes where the squashed RenderLayer is located w.r.t.
-    // referenceLayer. So we just need to convert that point from referenceLayer space to referenceLayer
+    // the squashed RenderLayer described w.r.t. referenceLayer's origin.
+    // So we just need to convert that point from referenceLayer space to the squashing layer's
     // space. This is simply done by subtracing squashLayerOriginInOwningLayerSpace, but then the offset
     // overall needs to be negated because that's the direction that the painting code expects the
     // offset to be.
     for (size_t i = 0; i < layers.size(); ++i) {
-        LayoutPoint offsetFromTransformedAncestorForSquashedLayer = layers[i].renderLayer->computeOffsetFromTransformedAncestor();
-        LayoutSize offsetFromSquashLayerOrigin = (offsetFromTransformedAncestorForSquashedLayer - referenceOffsetFromTransformedAncestor) - squashLayerOriginInOwningLayerSpace;
+        const LayoutPoint offsetFromTransformedAncestorForSquashedLayer = layers[i].renderLayer->computeOffsetFromTransformedAncestor();
+        const LayoutSize offsetFromSquashLayerOrigin = (offsetFromTransformedAncestorForSquashedLayer - referenceOffsetFromTransformedAncestor) - squashLayerOriginInOwningLayerSpace;
 
-        // It is ok to issue paint invalidation here, because all of the geometry needed to correctly invalidate paint is computed by this point.
         IntSize newOffsetFromRenderer = -IntSize(offsetFromSquashLayerOrigin.width().round(), offsetFromSquashLayerOrigin.height().round());
         LayoutSize subpixelAccumulation = offsetFromSquashLayerOrigin + newOffsetFromRenderer;
         if (layers[i].offsetFromRendererSet && layers[i].offsetFromRenderer != newOffsetFromRenderer) {
+            // It is ok to issue paint invalidation here, because all of the geometry needed to correctly invalidate paint is computed by this point.
             layers[i].renderLayer->renderer()->invalidatePaintIncludingNonCompositingDescendants();
 
             TRACE_LAYER_INVALIDATION(layers[i].renderLayer, InspectorLayerInvalidationTrackingEvent::SquashingLayerGeometryWasUpdated);
             layersNeedingPaintInvalidation.append(layers[i].renderLayer);
         }
         layers[i].offsetFromRenderer = newOffsetFromRenderer;
         layers[i].offsetFromRendererSet = true;
 
         layers[i].renderLayer->setSubpixelAccumulation(subpixelAccumulation);
     }
@@ skipping 1745 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink