[DropdownButton] Fix highlight behavior

[TahaTesser]

DropdownButtonFormField highlights and unhighlight quickly when tapping

Expected behavior

Widget should be once and should stay highlighted until focus is moved to another widget

minimal code sample

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,
      ),
      home: const MyHomePage(),
    );
  }
}

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

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

class _State extends State<MyHomePage> {
  int? dropdownValue = 1;
  final List<int> hugeMenuItems = List<int>.generate(100, (int index) => index);
  late FocusNode focusNode;

  @override
  void initState() {
    super.initState();
    focusNode = FocusNode();
  }

  @override
  void dispose() {
    focusNode.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Title'),
      ),
      body: Column(
        children: <Widget>[
          const TextField(),
          DropdownButton(
              focusNode: focusNode,
              value: dropdownValue,
              onTap: () {
                print('FocusNode has focus: ${focusNode.hasPrimaryFocus}');
              },
              items: hugeMenuItems.map<DropdownMenuItem<int>>((int item) {
                return DropdownMenuItem<int>(
                  key: ValueKey<int>(item),
                  value: item,
                  child: Text(item.toString(),
                      key: ValueKey<String>('Text $item')),
                );
              }).toList(),
              onChanged: (int? value) {
                setState(() => dropdownValue = value);
              }),
        ],
      ),
    );
  }
}

Also address #93716 (comment)

[bernaferrari]

Ideally it would support TextInputAction.go or something like that.

[TahaTesser]

Highlight is working as intended as it seems, it should stay highlighted, see

The main issue is the highlight is not smooth or persistent

[TahaTesser]

Looks like focus moves away whenever there is new route (here when popup shows up in a route)

flutter/packages/flutter/lib/src/material/dropdown.dart

Lines 1312 to 1317 in 1c673b2

	navigator.push(_dropdownRoute!).then<void>((_DropdownRouteResult<T>? newValue) {
	_removeDropdownRoute();
	if (!mounted || newValue == null)
	return;
	widget.onChanged?.call(newValue.result);
	});

I think it is core framework issue than this widget

I don't think this is a regression since this behavior can be observed before the dropdowbutton PR landed

[flutter-triage-bot]

This issue is missing a priority label. Please set a priority label when adding the triaged-design label.