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 7db7ff8cfcba5dc1d6b77569bd23183fc424a520..8fe03615a776995f6d2b9c1c093d22bba8309257 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
@@ -771,7 +773,7 @@ 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.
@@ -822,10 +824,8 @@ 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);
@@ -837,7 +837,6 @@ bool PaintLayer::updateLayerPosition()
     localPoint.moveBy(-inlineBoundingBoxOffset);
 
     if (m_location != localPoint) {
-        positionOrOffsetChanged = true;
         setNeedsRepaint();
     }
     m_location = localPoint;
@@ -845,7 +844,6 @@ bool PaintLayer::updateLayerPosition()
 #if ENABLE(ASSERT)
     m_needsPositionUpdate = false;
 #endif
-    return positionOrOffsetChanged;
 }
 
 TransformationMatrix PaintLayer::perspectiveTransform() const
@@ -1208,6 +1206,9 @@ 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())
@@ -1261,6 +1262,9 @@ PaintLayer* PaintLayer::removeChild(PaintLayer* oldChild)
     oldChild->setPreviousSibling(0);
     oldChild->setNextSibling(0);
     oldChild->m_parent = 0;
+    if (oldChild->ancestorOverflowLayer())

[chrishtr, 2016/03/24 17:36:03]

Do you have a layout test involving removal of paint layers, and testing whether
it correctly removes the pointer and avoids a stale pointer in the hash map?

[flackr, 2016/03/29 16:02:32]

sticky-overflow-changed.html does test layer removal (I've been using it to
ensure this case was handled correctly) but I could add a more explicit test.

+        oldChild->ancestorOverflowLayer()->getScrollableArea()->invalidateStickyConstraintsFor(oldChild);
+    oldChild->updateAncestorOverflowLayer(nullptr);
 
     dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();