Description

This PR updates the undo logic to fix two problems:

• Values with an invalid selection should not be saved in the history (this leads to the cursor disappearing).
• When an undo is received before the end of the throttling delay, the history should not be undo its last saved value because this will set the text field value to the penultimate value (the last one is not yet in the history).

Steps to Reproduce

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: const Text("Demo"),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              TextFormField(
                initialValue: "Flutter"
              ),
            ],
          ),
        ),
      ),
    );
  }
}

Problem 1: Tap ‘3’ and undo before the throttling delay ends (500ms).

Before this fix: the text is ‘Flutter’ and the cursor disappeared. Tapping a new key will insert it before ‘Flutter’.
After this fix: the text should be ‘Flutter’ and the cursor at the end.

Problem 2: Tap ‘3’, wait for the throttling delay, tap ‘3’ again and undo before the throttling delay ends.
Before this fix: the text value is ‘Flutter’ (penultimate state).
After this fix: the text value is ‘Flutter3’.

Related Issue

Fixes #120194

Tests

Adds 2 tests.