SetScrollbarVisibility(mHScrollbarBox, aAdd);
if (!aAdd) SetScrollbarVisibility(mHScrollbarBox, aAdd);
nsGfxScrollFrameInner::AddRemoveScrollbar(nsBoxLayoutState& aState, nsRect& aScrollAreaSize, PRBool aOnTop, PRBool aHorizontal, PRBool aAdd){  if (aHorizontal) {     if (!mHScrollbarBox)       return PR_FALSE;     nsSize hSize;     mHScrollbarBox->GetPrefSize(aState, hSize);     nsBox::AddMargin(mHScrollbarBox, hSize);     SetScrollbarVisibility(mHScrollbarBox, aAdd);     PRBool hasHorizontalScrollbar;     PRBool fit = AddRemoveScrollbar(hasHorizontalScrollbar, aScrollAreaSize.y, aScrollAreaSize.height, hSize.height, aOnTop, aAdd);     mHasHorizontalScrollbar = hasHorizontalScrollbar;    // because mHasHorizontalScrollbar is a PRPackedBool     if (!fit)        SetScrollbarVisibility(mHScrollbarBox, !aAdd);     return fit;  } else {     if (!mVScrollbarBox)       return PR_FALSE;     nsSize vSize;     mVScrollbarBox->GetPrefSize(aState, vSize);     nsBox::AddMargin(mVScrollbarBox, vSize);     SetScrollbarVisibility(mVScrollbarBox, aAdd);     PRBool hasVerticalScrollbar;     PRBool fit = AddRemoveScrollbar(hasVerticalScrollbar, aScrollAreaSize.x, aScrollAreaSize.width, vSize.width, aOnTop, aAdd);     mHasVerticalScrollbar = hasVerticalScrollbar;    // because mHasVerticalScrollbar is a PRPackedBool     if (!fit)        SetScrollbarVisibility(mVScrollbarBox, !aAdd);     return fit;  }}