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 71c699920159b8cfb6b664a842bb261c24c6ff37..ac0bd10ab0671bdd44b2ec5cab3e485744b91333 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -314,7 +314,8 @@ 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
@@ -324,6 +325,11 @@ bool PaintLayer::scrollsWithRespectTo(const PaintLayer* other) const
     return ancestorScrollingLayer() != other->ancestorScrollingLayer();
 }
 
+void PaintLayer::updateLayerPositionAfterFrameScroll(const DoubleSize& scrollDelta)

[chrishtr, 2015/12/09 00:37:38]

What's the point of declaring a new method with an argument that does nothing
but call another method?

[flackr, 2015/12/10 23:43:15]

I was trying to be consistent with the other methods identifying the purpose or
time for updating the layer position, but I guess this is silly, made
updateLayerPosition public instead.

+{
+    updateLayerPosition();
+}
+
 void PaintLayer::updateLayerPositionsAfterOverflowScroll(const DoubleSize& scrollDelta)
 {
     m_clipper.clearClipRectsIncludingDescendants();