void LayoutMultiColumnFlowThread::populate()
{
    LayoutBlockFlow* multicolContainer = multiColumnBlockFlow();
    ASSERT(!nextSibling());
    // Reparent children preceding the flow thread into the flow thread. It's multicol content
    // now. At this point there's obviously nothing after the flow thread, but layoutObjects (column
    // sets and spanners) will be inserted there as we insert elements into the flow thread.
    multicolContainer->moveChildrenTo(this, multicolContainer->firstChild(), this, true);
}
void PaintLayerStackingNode::rebuildZOrderLists() {
#if DCHECK_IS_ON()
  DCHECK(m_layerListMutationAllowed);
#endif
  DCHECK(isDirtyStackingContext());

  for (PaintLayer* child = layer()->firstChild(); child;
       child = child->nextSibling())
    child->stackingNode()->collectLayers(m_posZOrderList, m_negZOrderList);

  // Sort the two lists.
  if (m_posZOrderList)
    std::stable_sort(m_posZOrderList->begin(), m_posZOrderList->end(),
                     compareZIndex);

  if (m_negZOrderList)
    std::stable_sort(m_negZOrderList->begin(), m_negZOrderList->end(),
                     compareZIndex);

  // Append layers for top layer elements after normal layer collection, to
  // ensure they are on top regardless of z-indexes.  The layoutObjects of top
  // layer elements are children of the view, sorted in top layer stacking
  // order.
  if (layer()->isRootLayer()) {
    LayoutBlockFlow* rootBlock = layoutObject()->view();
    // If the viewport is paginated, everything (including "top-layer" elements)
    // gets redirected to the flow thread. So that's where we have to look, in
    // that case.
    if (LayoutBlockFlow* multiColumnFlowThread =
            rootBlock->multiColumnFlowThread())
      rootBlock = multiColumnFlowThread;
    for (LayoutObject* child = rootBlock->firstChild(); child;
         child = child->nextSibling()) {
      Element* childElement = (child->node() && child->node()->isElementNode())
                                  ? toElement(child->node())
                                  : 0;
      if (childElement && childElement->isInTopLayer()) {
        PaintLayer* layer = toLayoutBoxModelObject(child)->layer();
        // Create the buffer if it doesn't exist yet.
        if (!m_posZOrderList)
          m_posZOrderList = wrapUnique(new Vector<PaintLayerStackingNode*>);
        m_posZOrderList->append(layer->stackingNode());
      }
    }
  }

#if ENABLE(ASSERT)
  updateStackingParentForZOrderLists(this);
#endif

  m_zOrderListsDirty = false;
}