Adjust Width from the Left Side of Container – Flutter

I checked out your dartpad code. To achieve what you want, I suggest you put two empty Containers on either side of your handles and decrease their size when the handles are dragged (your center Container should also be inside an Expanded widget to take up all the allowed space). here is the example code:

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // Application name
      title: 'Flutter Stateful Clicker Counter',
      theme: ThemeData(
        // Application theme data, you can set the colors for the application as
        // you want
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Clicker Counter Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({Key? key, required this.title}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  double? rightContainerWidth, leftContainerWidth;

  @override
  Widget build(BuildContext context) {
    rightContainerWidth ??= MediaQuery.of(context).size.width / 2 - 20;
    leftContainerWidth ??= MediaQuery.of(context).size.width / 2 - 20;
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Wrap(
          children: <Widget>[
            Row(mainAxisAlignment: MainAxisAlignment.center, children: [
              // left handle
              Container(
                width: leftContainerWidth,
              ),
              GestureDetector(
                  onHorizontalDragUpdate: (DragUpdateDetails details) {
                    setState(() {
                      leftContainerWidth = details.globalPosition.dx;
                    });
                  },
                  child: Container(
                      decoration: BoxDecoration(
                          color: Colors.red,
                          borderRadius: BorderRadius.only(
                            topLeft: Radius.circular(20),
                            bottomLeft: Radius.circular(20),
                          )),
                      width: 10,
                      height: 200)),

              Expanded(
                child: Container(
                    //  padding: const EdgeInsets.fromLTRB(20, 0, 20, 0),

                    child: ClipRect(
                        child: Container(
                  //   width: _counter+0.2,
                  height: 200,

                  color: Colors.green,
                ))),
              ),

              GestureDetector(
                  onHorizontalDragStart: (DragStartDetails details) {
                    print("st: ${details.localPosition.dx}");
                    // dx for start is the x offset of the mouse relative to the container
                    //   changeX = (_counter as double) - details.localPosition.dx.floor();
                  },
                  onHorizontalDragUpdate: (DragUpdateDetails details) {
                    setState(() {
                      // print(details.localPosition.dx);
                      rightContainerWidth = MediaQuery.of(context).size.width -
                          details.globalPosition.dx;
                    });
                  },
                  child: Container(
                      width: 10,
                      height: 200,
                      decoration: BoxDecoration(
                          color: Colors.blue,
                          borderRadius: BorderRadius.only(
                            topRight: Radius.circular(20),
                            bottomRight: Radius.circular(20),
                          )))),
              Container(
                width: rightContainerWidth,
              ),
            ])
          ],
        ),
      ),
    );
  }
}

Caution: I did not add conditional statements to prevent overflows, make sure you also add them!