How to extend an element between two boxes in Flutter without exceeding their size?

If found another solution,

I use the GestureDetector like this :

import 'package:flutter/material.dart';
import 'package:planning/models/appointment.dart';

class AppointmentWidget extends StatefulWidget {
  final Appointment appointment;
  final double cellWidth;

  const AppointmentWidget({Key? key, required this.appointment, required this.cellWidth}) : super(key: key);

  @override
  _AppointmentWidgetState createState() => _AppointmentWidgetState();
}

class _AppointmentWidgetState extends State<AppointmentWidget> {
  late double _width = 0.0;
  double _delta = 0.0;

  @override
  void initState() {
    super.initState();
    _width = calculateWidth();
  }

  @override
  Widget build(BuildContext context) {
    return Positioned(
      left: (widget.appointment.startDay.weekday - 1) * widget.cellWidth,
      child: Row(
        children: [
          Container(
            width: _width,
            height: 50,
            color: widget.appointment.color,
          ),
          Align(
              alignment: Alignment.centerRight,
              child: GestureDetector(
                onHorizontalDragUpdate: (details) {
                  setState(() {
                    _width += details.delta.dx;
                    _delta = details.localPosition.dx;
                  });
                },
                onHorizontalDragEnd: (details) {
                  setState(() {
                    print(_delta);
                    print(widget.cellWidth / 2);
                    if(_delta > widget.cellWidth / 2) {
                        widget.appointment.endDay = widget.appointment.endDay.add(Duration(days: 1));
                    } else if (_delta < widget.cellWidth / 2) {
                        widget.appointment.endDay = widget.appointment.endDay.subtract(Duration(days: 1));
                    }
                    _width = calculateWidth();
                  });
                },
                child: Container(
                    height: 50,
                    width: 30,
                    color: Colors.green,
                    child: Icon(Icons.chevron_right, color: Colors.black)
                ),
              )
          )
        ],
      ),
    );
  }

  double calculateWidth() {
    print(widget.appointment.endDay.difference(widget.appointment.startDay).inDays);
    return ((widget.appointment.endDay.difference(widget.appointment.startDay).inDays + 1) * widget.cellWidth) - 30;
  }
}