Adapt and reland old 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 c96f42c49d496e00d9ca3538e287babc7d8d072d..988740be5f2f14526785999c6bd7ea62ba1eb589 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;
     OwnPtrWillBePersistent<PaintLayerScrollableArea> scrollableArea;
@@ -173,6 +173,7 @@ PaintLayer::PaintLayer(LayoutBoxModelObject* layoutObject, PaintLayerType type)
     , m_last(0)
     , m_staticInlinePosition(0)
     , m_staticBlockPosition(0)
+    , m_ancestorOverflowLayer(nullptr)
 {
     updateStackingNode();
 
@@ -352,7 +353,8 @@ 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
@@ -750,7 +752,7 @@ 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.
@@ -801,10 +803,8 @@ 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);
@@ -816,7 +816,6 @@ bool PaintLayer::updateLayerPosition()
     localPoint.moveBy(-inlineBoundingBoxOffset);
 
     if (m_location != localPoint) {
-        positionOrOffsetChanged = true;
         setNeedsRepaint();
     }
     m_location = localPoint;
@@ -824,7 +823,6 @@ bool PaintLayer::updateLayerPosition()
 #if ENABLE(ASSERT)
     m_needsPositionUpdate = false;
 #endif
-    return positionOrOffsetChanged;
 }
 
 TransformationMatrix PaintLayer::perspectiveTransform() const
@@ -1187,6 +1185,9 @@ 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())
@@ -1240,6 +1241,9 @@ PaintLayer* PaintLayer::removeChild(PaintLayer* oldChild)
     oldChild->setPreviousSibling(0);
     oldChild->setNextSibling(0);
     oldChild->m_parent = 0;
+    if (oldChild->ancestorOverflowLayer())
+        oldChild->ancestorOverflowLayer()->getScrollableArea()->invalidateStickyConstraintsFor(oldChild);
+    oldChild->updateAncestorOverflowLayer(nullptr);
 
     dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();