Clear scrollbar damages in time

Source/core/rendering/RenderBox.cpp

Index: Source/core/rendering/RenderBox.cpp
diff --git a/Source/core/rendering/RenderBox.cpp b/Source/core/rendering/RenderBox.cpp
index 9fbbb8b7a364b3f66cce6f67fb94f2afd0f1b294..57f465026ef4aceeb56f744614157a59a5a7fa06 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -1603,10 +1603,32 @@ void RenderBox::invalidatePaintIfNeeded(const PaintInvalidationState& paintInval
                 invalidatePaintRectangle(area->verticalBarDamage());
             if (area->hasHorizontalBarDamage())
                 invalidatePaintRectangle(area->horizontalBarDamage());
+        }
+    }
+}
+
+void RenderBox::clearPaintInvalidationState()
+{
+    RenderBoxModelObject::clearPaintInvalidationState();
+
+    if (enclosingLayer()) {

[Julien - ping for review, 2014/08/01 18:13:45]

I missed this in our current code: we will end up invalidating / clearing the
same scrollbars several times as enclosingLayer can return the same RenderLayer
in a subtree.

It also potentially makes this algorithm supra-linear as we walk up the tree :-(

[Xianzhu, 2014/08/01 18:20:01]

Thanks for catching this. I think we should use hasLayer() here. Will try.

+        if (RenderLayerScrollableArea* area = enclosingLayer()->scrollableArea())
             area->resetScrollbarDamage();
+    }
+}
+
+#if ENABLE(ASSERT)
+bool RenderBox::paintInvalidationStateIsDirty() const
+{
+    if (enclosingLayer()) {
+        if (RenderLayerScrollableArea* area = enclosingLayer()->scrollableArea()) {
+            if (area->hasVerticalBarDamage() || area->hasHorizontalBarDamage())
+                return true;
         }
     }
+    return RenderBoxModelObject::paintInvalidationStateIsDirty();
 }
+#endif
 
 bool RenderBox::pushContentsClip(PaintInfo& paintInfo, const LayoutPoint& accumulatedOffset, ContentsClipBehavior contentsClipBehavior)
 {