Repository: flutter
Issue: `MouseRegion`: Property for tracking mouse movements regardless of whether buttons are pressed or not
Labels: c: new feature,engine,c: proposal,a: mouse,P3,team-engine,triaged-engine

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## Use case

I want to track mouse movements regardless of whether mouse buttons are pressed or not.

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## Proposal

The [`MouseRegion`](https://api.flutter.dev/flutter/widgets/MouseRegion-class.html) class provides the [`onHover`](https://api.flutter.dev/flutter/widgets/MouseRegion/onHover.html) property that is triggered when a pointer moves into a position within this widget **without** buttons pressed.

Unfortunately, there is no property that is triggered when a pointer moves into a position within this widget **with** buttons pressed.

### Minimal mouse tracker example:
_(only works without buttons pressed)_
```
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Mouse Tracker Demo',
      home: MyMouseTracker(),
    );
  }
}

class MyMouseTracker extends StatefulWidget {
  MyMouseTracker({Key? key}) : super(key: key);

  @override
  _MyMouseTrackerState createState() => _MyMouseTrackerState();
}

class _MyMouseTrackerState extends State<MyMouseTracker> {
  double x = 0.0;
  double y = 0.0;

  void _updateLocation(PointerEvent details) {
    setState(() {
      x = details.position.dx;
      y = details.position.dy;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MouseRegion(
      onHover: _updateLocation,
      child: Scaffold(
        body: Center(
          child: Text(
            'Mouse pointer position: (${x.toStringAsFixed(2)}, ${y.toStringAsFixed(2)})',
          ),
        ),
      ),
    );
  }
}
```

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