Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/paint/PaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 296 matching lines @@
         // in this case, there is no need to dirty our ancestors further.
         if (layer->isSelfPaintingLayer()) {
             ASSERT(!parent() || parent()->m_hasSelfPaintingLayerDescendantDirty || parent()->m_hasSelfPaintingLayerDescendant);
             break;
         }
     }
 }
 
 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
 {
     if (scrollsWithViewport() != other->scrollsWithViewport())
         return true;
     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();
     updateLayerPositionsAfterScrollRecursive(scrollDelta, isPaintInvalidationContainer());
 }
 
 void PaintLayer::updateLayerPositionsAfterScrollRecursive(const DoubleSize& scrollDelta, bool paintInvalidationContainerWasScrolled)
 {
     updateLayerPosition();
     if (paintInvalidationContainerWasScrolled && !isPaintInvalidationContainer()) {
@@ skipping 2443 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif