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 9fa0a8e32d6fdd95aeccbb84c1e4ee9423d139bb..bd6279a3ae29c8ff13295bb007c0ac061db95100 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;

[Stephen Chennney, 2016/08/29 20:49:52]

This check is redundant in all pre-existing callers except the one below in
childBackgroundIsKnownToBeOpaqueInRect. All the other cases are GraphicsLayers
calling it on their m_owningLayer, so it must be self painting. I verified that
with CHECKs.

-
     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())

[Stephen Chennney, 2016/08/29 20:49:52]

I think this means we do not need to check that the child is a
selfPaintingLayer, per the previous logic. Regardless, that logic seems wrong.
As far as I can tell, we should return false here is the child IS a self
painting layer because it will not then contribute to the opacity of it's
parent's layer.

The code hasn't changed wince it was originally added in 2013, back in WebKit.
So it doesn't surprise me if it's out of data.