Flutter - WebViewController 스크롤 이동

import 'package:flutter/foundation.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';
 
class WebViewApp extends StatefulWidget {
 
  @override
  State<WebViewApp> createState() => _WebViewAppState();
}
 
class _WebViewAppState extends State<WebViewApp>  {
  late WebViewController _controller;
  bool buttonshow = false;
 
 
 
 
  void scrollToTop(){
    _controller.runJavascript("window.scrollTo({top: 0, behavior: 'smooth'});");
    floatingButtonVisibility();
  }
 
  void floatingButtonVisibility() async {
    int y = await  _controller.getScrollY();
    if(y>50){
      setState(() {
        buttonshow = true;
      });
    }else {
      setState(() {
        buttonshow = false;
      });
    }
  }
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Flutter WebView'),
      ),
      body:  WebView(
          initialUrl: 'https://flutter.dev',
          javascriptMode: JavascriptMode.unrestricted,
          onWebViewCreated: (WebViewController webViewController) {
            _controller = webViewController;
          },
          gestureRecognizers: Set()
            ..add(
                Factory<VerticalDragGestureRecognizer>(() => VerticalDragGestureRecognizer()
                  ..onDown = (tap) {
                    floatingButtonVisibility();
                  }))
 
      ),
      floatingActionButton: Visibility(
        visible: buttonshow,
        child: FloatingActionButton(
          onPressed: () {
            scrollToTop();
          },
          backgroundColor: Colors.blue,
          child: const Icon(Icons.navigation),
        ),
      ),
    );
  }
}