height_picker.dart

...
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      behavior: HitTestBehavior.translucent,
      onTapDown: _onTapDown,
      child: Stack(
        children: <Widget>[
          _drawPersonImage(),
          _drawSlider(),
          _drawLabels(),
        ],
      ),
    );
  }

  _onTapDown(TapDownDetails tapDownDetails) {
    int height = _globalOffsetToHeight(tapDownDetails.globalPosition);
    widget.onChange(_normalizeHeight(height));
  }

  int _normalizeHeight(int height) {
    return math.max(widget.minHeight, math.min(widget.maxHeight, height));
  }

  int _globalOffsetToHeight(Offset globalOffset) {
    RenderBox getBox = context.findRenderObject();
    Offset localPosition = getBox.globalToLocal(globalOffset);
    double dy = localPosition.dy;
    dy = dy - marginTopAdapted(context) - labelsFontSize / 2;
    int height = widget.maxHeight - (dy ~/ _pixelsPerUnit);
    return height;
  }
...