https://stackoverflow.com/questions/73270235/flutter-how-do-i-give-constraints-to-this-gesturedetector/73270312#73270312

clamp_object_position.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      debugShowCheckedModeBanner: false,
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return const Scaffold(
      body: Sample(),
    );
  }
}

class Sample extends StatefulWidget {
  const Sample({Key? key}) : super(key: key);

  @override
  State<Sample> createState() => _SampleState();
}

class _SampleState extends State<Sample> {
  Offset offset = Offset.zero;

  static const _kObjectSize = Size.square(100);

  double get screenWidth => MediaQuery.of(context).size.width;
  double get screenHeight => MediaQuery.of(context).size.height;

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        Positioned(
          left: offset.dx,
          top: offset.dy,
          child: Container(
            color: Colors.red,
            width: _kObjectSize.width,
            height: _kObjectSize.height,
          ),
        ),
        Positioned.fill(
          child: GestureDetector(
            onPanUpdate: (details) {
              double limitX = (offset.dx + details.delta.dx)
                  .clamp(0, screenWidth - _kObjectSize.width)
                  .toDouble();

              double limitY = (offset.dy + details.delta.dy)
                  .clamp(0, screenHeight - _kObjectSize.height)
                  .toDouble();

              offset = Offset(limitX, limitY);

              setState(() {});
            },
          ),
        ),
      ],
    );
  }
}