nsIFrame* childFrame; aRootFrame->FirstChild(aPresContext, nsnull, &childFrame);
nsIFrame* childFrame = aRootFrame->GetFirstChild(nsnull);
GetRootScrollableFrame(nsIPresContext* aPresContext, nsIFrame* aRootFrame){  nsIScrollableFrame* scrollableFrame = nsnull;  if (nsLayoutAtoms::viewportFrame == aRootFrame->GetType()) {    nsIFrame* childFrame;    aRootFrame->FirstChild(aPresContext, nsnull, &childFrame);    if (childFrame) {      if (nsLayoutAtoms::scrollFrame == childFrame->GetType()) {        // Use this frame, even if we are using GFX frames for the        // viewport, which contains another scroll frame below this        // frame, since the GFX scrollport frame does not implement        // nsIScrollableFrame.        CallQueryInterface(childFrame, &scrollableFrame);      }    }  }#ifdef DEBUG  else {    NS_WARNING("aRootFrame is not a viewport frame");  }#endif // DEBUG  return scrollableFrame;}