Fix Compositing of Opaque Scrolling Layers and Add Tests

third_party/WebKit/Source/core/paint/PaintLayer.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 37a94ac80e0939e43eb374798ff3684fdb5e86ef..e22e232e35e73be4787d34d29c9f7420d3c2a7fe 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -2334,6 +2334,14 @@ GraphicsLayer* PaintLayer::graphicsLayerBackingForScrolling() const
     }
 }
 
+bool PaintLayer::shouldPaintBackgroundOntoScrollingContentsLayer() const
+{
+    return !isRootLayer()
+        && scrollsOverflow()
+        && layoutObject()->style()->hasEntirelyLocalBackground()
+        && !stackingNode()->hasNegativeZOrderList();
+}
+
 void PaintLayer::ensureCompositedLayerMapping()
 {
     if (m_rareData && m_rareData->compositedLayerMapping)
@@ -2397,9 +2405,6 @@ bool PaintLayer::paintsWithTransform(GlobalPaintFlags globalPaintFlags) const
 
 bool PaintLayer::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect) const
 {
-    if (!isSelfPaintingLayer() && !hasSelfPaintingLayerDescendant())
-        return false;
-
     if (paintsWithTransparency(GlobalPaintNormalPhase))
         return false;
 
@@ -2421,8 +2426,6 @@ bool PaintLayer::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect)
     if (m_stackingNode->zOrderListsDirty())
         return false;
 
-    // FIXME: We currently only check the immediate layoutObject,
-    // which will miss many cases.
     if (layoutObject()->backgroundIsKnownToBeOpaqueInRect(localRect))
         return true;
 
@@ -2431,13 +2434,17 @@ bool PaintLayer::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect)
     if (layoutObject()->hasClipRelatedProperty())
         return false;
 
+    // TODO(schenney): This could be improved by unioning the opaque regions of all the children.
+    // That would require a refactoring because currently children just check they at least
+    // cover the given rect, but a unioning method would require children to compute and report
+    // their rects.
     return childBackgroundIsKnownToBeOpaqueInRect(localRect);
 }
 
 bool PaintLayer::childBackgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect) const
 {
-    PaintLayerStackingNodeReverseIterator revertseIterator(*m_stackingNode, PositiveZOrderChildren | NormalFlowChildren | NegativeZOrderChildren);
-    while (PaintLayerStackingNode* child = revertseIterator.next()) {
+    PaintLayerStackingNodeReverseIterator reverseIterator(*m_stackingNode, PositiveZOrderChildren | NormalFlowChildren | NegativeZOrderChildren);
+    while (PaintLayerStackingNode* child = reverseIterator.next()) {
         const PaintLayer* childLayer = child->layer();
         // Stop at composited paint boundaries.
         if (childLayer->isPaintInvalidationContainer())