On Desktop and Web, Flutter maintains keyboard state based on key events received when the window has focus.
If a keyboard event happens when the window has lost focus, Flutter keyboard state is not updated.
This leads to visible bugs, especially ones related to modifier keys. See #112488

Steps to Reproduce

1. Launch the below code sample.
2. Enter some test in the text field.
3. Click somewhere on the text.
4. Hold shift key pressed.
5. While holding the shift key, click somewhere else on the text, the text between both click positions is selected.
6. While holding the shift key, click on another window (Flutter window loses focus).
7. Release the shift key.
8. Click inside the text field.

Expected results: At step 8, the click should just move the cursor and no text should be selected

Actual results: At step 8, the click updates the selection (as the shift key was still pressed).

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(
      home: Scaffold(
        body: Container(
          alignment: Alignment.center,
          padding: const EdgeInsets.all(16),
          child: const TextField(),
        ),
      ),
    );
  }
}

As per this discussion, the modifier keys states should be also updated based on pointer events (which contain a modifier state property).

This should be implemented on the engine side for:

• Web : fixed by [Web] Synthesize key events for modifier keys on pointer events. engine#36724
• Linux : fixed by [Linux] Synthesize modifier keys events on pointer events engine#37491
• MacOS : [macos] Synthesize modifier keys events on pointer events engine#37870
• Windows : fixed by [Windows] Synthesize modifier keys events on pointer events engine#38138