FEATURE REQUEST: wrap _showFixedGroupHeader in Position

[N00nDay]

Wrap in Postion like so:

Widget _showFixedGroupHeader(int index) {
    if (widget.elements.length > 0) {
      _groupHeaderKey = GlobalKey();
      return Positioned(
        left: 0,
          right: 0,
          top: 55.0,
          child: Container(
        key: _groupHeaderKey,
        color:
            widget.floatingHeader ? null : widget.stickyHeaderBackgroundColor,
        width: widget.floatingHeader ? null : MediaQuery.of(context).size.width,
        child: widget.groupSeparatorBuilder(_sortedElements[index]),
      ),
      );
    }
    return Container();
  }

Then allow us to pass top or bottom so we can set the height of the top group header which is crucial for transparent appbar ui.