Slideout container

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;
  
  final double startW = 100;
  double initW = 100;
  double containerWidth = 100;
  
  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        Positioned(
          left: left,
          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.roundToDouble();
              });
            },
            child: Container(
              width: containerWidth,
              height: MediaQuery.of(context).size.height,
              color: Colors.purple,
              child: Text(containerWidth.toString())
            )
          )
        )
      ]
    );
  }
}