在滚动SingleChildScrollView的同时，摆动更改堆栈中可拖动小部件的偏移量

import 'package:flutter/material.dart';

class DraggableWidget extends StatefulWidget {
  @override
  _DraggableWidgetState createState() => _DraggableWidgetState();
}

class _DraggableWidgetState extends State<DraggableWidget> {
  Offset offset = Offset(0, 0);

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      child: Stack(
        children: [
          // 可滚动的内容
          Container(
            height: 1000,
            color: Colors.grey,
          ),
          // 可拖动的小部件
          Positioned(
            left: offset.dx,
            top: offset.dy,
            child: GestureDetector(
              onPanUpdate: (details) {
                setState(() {
                  offset = Offset(
                    offset.dx + details.delta.dx,
                    offset.dy + details.delta.dy,
                  );
                });
              },
              child: Container(
                width: 100,
                height: 100,
                color: Colors.blue,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: Scaffold(
      body: DraggableWidget(),
    ),
  ));
}