height_picker.dart

class _HeightPickerState extends State<HeightPicker> {
  double startDragYOffset;
  int startDragHeight;
...
    return GestureDetector(
      behavior: HitTestBehavior.translucent,
      onTapDown: _onTapDown,
      onVerticalDragStart: _onDragStart,
      onVerticalDragUpdate: _onDragUpdate,
      ...
    );
...
  _onDragStart(DragStartDetails dragStartDetails) {
    int newHeight = _globalOffsetToHeight(dragStartDetails.globalPosition);
    widget.onChange(newHeight);
    setState(() {
      startDragYOffset = dragStartDetails.globalPosition.dy;
      startDragHeight = newHeight;
    });
  }

  _onDragUpdate(DragUpdateDetails dragUpdateDetails) {
    double currentYOffset = dragUpdateDetails.globalPosition.dy;
    double verticalDifference = startDragYOffset - currentYOffset;
    int diffHeight = verticalDifference ~/ _pixelsPerUnit;
    int height = _normalizeHeight(startDragHeight + diffHeight);
    setState(() => widget.onChange(height));
  }
...