Reland main thread position sticky implementation.

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 25a704b56969c89dc4eeefb4a840f081c2c18e94..dbafbbe174782754ebf591c4bbe814beec98225e 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -99,7 +99,7 @@ static CompositingQueryMode gCompositingQueryMode =
 
 struct SameSizeAsPaintLayer : DisplayItemClient {
     int bitFields;
-    void* pointers[9];
+    void* pointers[10];
     LayoutUnit layoutUnits[4];
     IntSize size;
     Persistent<PaintLayerScrollableArea> scrollableArea;
@@ -172,6 +172,7 @@ PaintLayer::PaintLayer(LayoutBoxModelObject* layoutObject, PaintLayerType type)
     , m_last(0)
     , m_staticInlinePosition(0)
     , m_staticBlockPosition(0)
+    , m_ancestorOverflowLayer(nullptr)
 {
     updateStackingNode();
 
@@ -349,7 +350,8 @@ void PaintLayer::dirtyAncestorChainHasSelfPaintingLayerDescendantStatus()
 
 bool PaintLayer::scrollsWithViewport() const
 {
-    return layoutObject()->style()->position() == FixedPosition && layoutObject()->containerForFixedPosition() == layoutObject()->view();
+    return (layoutObject()->style()->position() == FixedPosition && layoutObject()->containerForFixedPosition() == layoutObject()->view())
+        || (layoutObject()->style()->position() == StickyPosition && !ancestorScrollingLayer());
 }
 
 bool PaintLayer::scrollsWithRespectTo(const PaintLayer* other) const
@@ -747,7 +749,7 @@ bool PaintLayer::update3DTransformedDescendantStatus()
     return has3DTransform();
 }
 
-bool PaintLayer::updateLayerPosition()
+void PaintLayer::updateLayerPosition()
 {
     LayoutPoint localPoint;
     LayoutPoint inlineBoundingBoxOffset; // We don't put this into the Layer x/y for inlines, so we need to subtract it out when done.
@@ -798,10 +800,8 @@ bool PaintLayer::updateLayerPosition()
         localPoint -= scrollOffset;
     }
 
-    bool positionOrOffsetChanged = false;
     if (layoutObject()->isInFlowPositioned()) {
         LayoutSize newOffset = layoutObject()->offsetForInFlowPosition();
-        positionOrOffsetChanged = newOffset != offsetForInFlowPosition();
         if (m_rareData || !newOffset.isZero())
             ensureRareData().offsetForInFlowPosition = newOffset;
         localPoint.move(newOffset);
@@ -813,7 +813,6 @@ bool PaintLayer::updateLayerPosition()
     localPoint.moveBy(-inlineBoundingBoxOffset);
 
     if (m_location != localPoint) {
-        positionOrOffsetChanged = true;
         setNeedsRepaint();
     }
     m_location = localPoint;
@@ -821,7 +820,6 @@ bool PaintLayer::updateLayerPosition()
 #if ENABLE(ASSERT)
     m_needsPositionUpdate = false;
 #endif
-    return positionOrOffsetChanged;
 }
 
 TransformationMatrix PaintLayer::perspectiveTransform() const
@@ -1182,6 +1180,9 @@ void PaintLayer::addChild(PaintLayer* child, PaintLayer* beforeChild)
 
     child->m_parent = this;
 
+    // The ancestor overflow layer is calculated during compositing inputs update and should not be set yet.
+    ASSERT(!child->ancestorOverflowLayer());
+
     setNeedsCompositingInputsUpdate();
 
     if (!child->stackingNode()->isStacked() && !layoutObject()->documentBeingDestroyed())
@@ -1236,6 +1237,10 @@ PaintLayer* PaintLayer::removeChild(PaintLayer* oldChild)
     oldChild->setNextSibling(0);
     oldChild->m_parent = 0;
 
+    // Remove any ancestor overflow layers which descended into the removed child.
+    if (oldChild->ancestorOverflowLayer())
+        oldChild->removeAncestorOverflowLayer(oldChild->ancestorOverflowLayer());
+
     dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
 
     oldChild->updateDescendantDependentFlags();
@@ -2686,6 +2691,23 @@ PaintLayerFilterInfo& PaintLayer::ensureFilterInfo()
     return *rareData.filterInfo;
 }
 
+void PaintLayer::removeAncestorOverflowLayer(const PaintLayer* removedLayer)
+{
+    // If the current ancestor overflow layer does not match the removed layer
+    // the ancestor overflow layer has changed so we can stop searching.
+    if (ancestorOverflowLayer() && ancestorOverflowLayer() != removedLayer)
+        return;
+
+    if (ancestorOverflowLayer())
+        ancestorOverflowLayer()->getScrollableArea()->invalidateStickyConstraintsFor(this);
+    updateAncestorOverflowLayer(nullptr);
+    PaintLayer* current = m_first;
+    while (current) {
+        current->removeAncestorOverflowLayer(removedLayer);
+        current = current->nextSibling();
+    }
+}
+
 void PaintLayer::updateOrRemoveFilterClients()
 {
     const auto& filter = layoutObject()->style()->filter();