Invalidate scrolling contents on scrolling contents layer only

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
index dcf30ed39c937159c2dcbdb4bf999cbe9cc18526..f28e398770de00e5a40ef5dab21f26244e4c333d 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -407,7 +407,10 @@ void LayoutBoxModelObject::invalidateDisplayItemClientOnBacking(const DisplayIte
         if (GraphicsLayer* squashingLayer = layer()->groupedMapping()->squashingLayer())
             squashingLayer->invalidateDisplayItemClient(displayItemClient, invalidationReason, enclosingIntRect(previousPaintInvalidationRect), enclosingIntRect(newPaintInvalidationRect));
     } else if (CompositedLayerMapping* compositedLayerMapping = layer()->compositedLayerMapping()) {
-        compositedLayerMapping->invalidateDisplayItemClient(displayItemClient, invalidationReason, previousPaintInvalidationRect, newPaintInvalidationRect);
+        if (this->displayItemClient() != displayItemClient.displayItemClient() && isBox() && toLayoutBox(this)->usesCompositedScrolling())
+            compositedLayerMapping->invalidateDisplayItemClientOnScrollingContentsLayer(displayItemClient, invalidationReason, previousPaintInvalidationRect, newPaintInvalidationRect);

[chrishtr, 2015/10/16 00:55:38]

This conditional doesn't need to be protected by
SlimmingPaintSynchronizedPaintingEnabled? Why is it correct to not invalidate
even without the flag?

[Xianzhu, 2015/10/16 17:38:05]

A scrolling content is on the scrolling contents layer, so it's correct not to
invalidate the scrolling container layer, regardless of the flag.

Invalidating on both layers is a bug (crbug.com/416535) which is not fixed
because of crbug.com/429845 and because it is not a big issue. With this CL, we
still invalidate both layers if the invalidation is issued by the container
(while the rect may be for contents but we can't distinguish) to avoid
crbug.com/429845. If an invalidation is issued by scrolling content, we
invalidate the scrolling contents layer only.

crbug.com/416535 doesn't apply for spv2.

+        else
+            compositedLayerMapping->invalidateDisplayItemClient(displayItemClient, invalidationReason, previousPaintInvalidationRect, newPaintInvalidationRect);
     }
 }