您可以使用软件包https://pub.dev/packages/popup_menu
并修改此软件包的文件https://github.com/chinabrant/popup_menu/blob/master/lib/triangle_painter.dart 按照以下代码将三角形移至左侧位置

else {
      path.moveTo(size.width / 2.0 - 20,0.0);
      path.lineTo(-20.0,size.height + 1);
      path.lineTo(size.width - 20,size.height + 1);

代码段

Container(
          color: Colors.blue,child: MaterialButton(
            height: 45.0,key: btnKey,onPressed: maxColumn,child: Text('Show Menu'),),...

void maxColumn() {
PopupMenu menu = PopupMenu(
    // backgroundColor: Colors.teal,// lineColor: Colors.tealAccent,maxColumn: 1,items: [
      MenuItem(title: 'Copy',image: Image.asset('assets/copy.png')),],onClickMenu: onClickMenu,stateChanged: stateChanged,onDismiss: onDismiss);
menu.show(widgetKey: btnKey);
}

演示

完整的示例代码

import 'package:flutter/material.dart';
import 'package:popup_menu/popup_menu.dart';
import 'package:popup_menu_example/gesture_demo.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',home: MyHomePage(title: 'Popup Menu Example'),);
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  PopupMenu menu;
  GlobalKey btnKey = GlobalKey();
  GlobalKey btnKey2 = GlobalKey();
  GlobalKey btnKey3 = GlobalKey();
  GlobalKey btnKey4 = GlobalKey();

  @override
  void initState() {
    super.initState();

    menu = PopupMenu(items: [
      // MenuItem(title: 'Copy',// MenuItem(title: 'Home',image: Icon(Icons.home,color: Colors.white,)),MenuItem(
          title: 'Mail',image: Icon(
            Icons.mail,MenuItem(
          title: 'Power',image: Icon(
            Icons.power,MenuItem(
          title: 'Setting',image: Icon(
            Icons.settings,MenuItem(
          title: 'PopupMenu',image: Icon(
            Icons.menu,))
    ],onDismiss: onDismiss,maxColumn: 4);
  }

  void stateChanged(bool isShow) {
    print('menu is ${isShow ? 'showing' : 'closed'}');
  }

  void onClickMenu(MenuItemProvider item) {
    print('Click menu -> ${item.menuTitle}');
  }

  void onDismiss() {
    print('Menu is dismiss');
  }

  @override
  Widget build(BuildContext context) {
    PopupMenu.context = context;

    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),body: Container(
        alignment: Alignment.centerRight,child: Column(
          mainAxisAlignment: MainAxisAlignment.start,children: <Widget>[
            Container(
              color: Colors.blue,child: MaterialButton(
                height: 45.0,Container(
              child: MaterialButton(
                key: btnKey2,height: 45.0,onPressed: customBackground,Container(
              child: MaterialButton(
                key: btnKey3,onPressed: onDismissOnlyBeCalledOnce,Container(
              child: MaterialButton(
                height: 30.0,child: Text('Gestures Demo'),onPressed: onGesturesDemo,);
  }

  void onDismissOnlyBeCalledOnce() {
    menu.show(widgetKey: btnKey3);
  }

  void onGesturesDemo() {
    menu.dismiss();
    return;
    Navigator.push(
      context,MaterialPageRoute(builder: (context) => GestureDemo()),);
  }

  void checkState(BuildContext context) {
    final snackBar = new SnackBar(content: new Text('这是一个SnackBar!'));

    Scaffold.of(context).showSnackBar(snackBar);
  }


  void maxColumn() {
    PopupMenu menu = PopupMenu(
        // backgroundColor: Colors.teal,items: [
          MenuItem(title: 'Copy',onDismiss: onDismiss);
    menu.show(widgetKey: btnKey);
  }

  //
  void customBackground() {
    PopupMenu menu = PopupMenu(
        // backgroundColor: Colors.teal,// maxColumn: 2,MenuItem(
              title: 'Home',// textStyle: TextStyle(fontSize: 10.0,color: Colors.tealAccent),image: Icon(
                Icons.home,MenuItem(
              title: 'Mail',image: Icon(
                Icons.mail,MenuItem(
              title: 'Power',image: Icon(
                Icons.power,MenuItem(
              title: 'Setting',image: Icon(
                Icons.settings,MenuItem(
              title: 'PopupMenu',image: Icon(
                Icons.menu,))
        ],onDismiss: onDismiss);
    menu.show(widgetKey: btnKey2);
  }
}