private void offsetHeaderBy(int dy) {
   final int prevTop = mContainer.getTop();
   final int clampedDy =
       MathUtils.clamp(prevTop + dy, getHeaderTopMinimum(), getHeaderTopMaximum()) - prevTop;
   mContainer.offsetTopAndBottom(clampedDy);
 }
 public int getHeaderTop() {
   return mContainer.getTop();
 }