Adapt and reland old position sticky implementation

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 981 matching lines @@
 #endif
 
 bool ScrollingCoordinator::hasVisibleSlowRepaintViewportConstrainedObjects(FrameView* frameView) const
 {
     const FrameView::ViewportConstrainedObjectSet* viewportConstrainedObjects = frameView->viewportConstrainedObjects();
     if (!viewportConstrainedObjects)
         return false;
 
     for (const LayoutObject* layoutObject : *viewportConstrainedObjects) {
         ASSERT(layoutObject->isBoxModelObject() && layoutObject->hasLayer());
-        ASSERT(layoutObject->style()->position() == FixedPosition);
+        ASSERT(layoutObject->style()->position() == FixedPosition
+            || layoutObject->style()->position() == StickyPosition);
         DeprecatedPaintLayer* layer = toLayoutBoxModelObject(layoutObject)->layer();
 
         // Whether the Layer scrolls with the viewport is a tree-depenent
         // property and our viewportConstrainedObjects collection is maintained
         // with only LayoutObject-level information.
         if (!layer->scrollsWithViewport())
             continue;
 
         // If the whole subtree is invisible, there's no reason to scroll on
         // the main thread because we don't need to generate invalidations
@@ skipping 71 matching lines @@
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : nullptr)
         return WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink