docs: public int findHeaderPositionUnder(int x, int y) {
      for (int i = 0; i < mHeaderRects.size(); i++) {
        Rect rect = mHeaderRects.get(mHeaderRects.keyAt(i));
        if (rect.contains(x, y)) {
          int position = mHeaderRects.keyAt(i);
          if (mVisibilityAdapter == null || mVisibilityAdapter.isPositionVisible(position)) {
            return position;
          }
        }
      }
      return -1;
    }