Flutter slideout container with handle

main.dart

import 'package:flutter/material.dart';

void main() => runApp(
      MaterialApp(
        debugShowCheckedModeBanner: false,
        home: Scaffold(
          body: Page(),
        ),
      ),
    );

class Page extends StatefulWidget {
  @override
  PageState createState() => PageState();
}

class PageState extends State<Page> {
  double left = 0;

  double leftStart;

  static final imgUrl = 'https://picsum.photos/id/10';
  static final double startW = 30;
  double initW = startW;
  double containerWidth = startW;

  @override
  Widget build(BuildContext context) {
    double screenW = MediaQuery.of(context).size.width;
    double screenH = MediaQuery.of(context).size.height;
    
    var networkImageUrl = '$imgUrl/${screenW.toInt()}/${screenH.toInt()}';

    print(screenW);
    print(screenH);

    return Stack(children: <Widget>[
      Image.network(
        networkImageUrl,
        width: screenW,
        height: screenH,
      ),
      Positioned(
        left: left,
        child: Container(
          width: containerWidth,
          height: MediaQuery.of(context).size.height,
          color: Colors.purple,
          child: Stack(
            children: <Widget>[
              Positioned(
                  left: 0,
                  child: Image.network(
                    networkImageUrl,
                    width: screenW,
                    height: screenH,
                    fit: BoxFit.none,
                  )),
              Align(
                alignment: Alignment.centerRight,
                child: GestureDetector(
                  onHorizontalDragStart: (details) {
                    setState(() {
                      leftStart = details.globalPosition.dx;
                    });
                  },
                  onHorizontalDragUpdate: (details) {
                    var offsetWidth = details.globalPosition.dx - leftStart;
                    var updatedWidth = (offsetWidth + initW).roundToDouble();
                    var screenWidth = MediaQuery.of(context).size.width;

                    setState(() {
                      if (updatedWidth < startW) {
                        containerWidth = startW;
                        return;
                      }

                      if (updatedWidth > screenWidth) {
                        containerWidth = screenWidth;
                        return;
                      }

                      containerWidth = updatedWidth;
                    });
                  },
                  onHorizontalDragEnd: (details) {
                    setState(() {
                      initW = containerWidth;
                    });
                  },
                  child: Container(
                    width: 30,
                    height: MediaQuery.of(context).size.height,
                    color: Colors.grey,
                    child: Center(
                      child: Icon(Icons.drag_handle),
                    ),
                  ),
                ),
              )
            ],
          ),
        ),
      )
    ]);
  }
}