Fix Tooltip child state loss when cursor enters/leaves window

[Copilot]

Tooltip rebuilds its child with a new State whenever the mouse cursor enters or leaves the window. Root cause: _handleMouseTrackerChange calls setState on _mouseIsConnected, which conditionally adds/removes a MouseRegion wrapper in build(), changing the widget tree depth and destroying child state.

Same class of bug fixed in Flutter material: flutter/flutter#119199

Changes

• Remove _handleMouseTrackerChange listener and its registration/teardown — eliminates the setState that triggered tree restructuring
• Replace _mouseIsConnected state field with a getter reading directly from RendererBinding.instance.mouseTracker.mouseIsConnected
• Always include MouseRegion in the widget tree — tree structure stays stable regardless of mouse connection state, preserving child state
• Add regression test verifying child initState is called exactly once across mouse pointer add/remove cycles

// Before: conditional wrapping changes tree depth → child state lost
if (_mouseIsConnected) {
  result = MouseRegion(..., child: result);
}

// After: always present → stable tree → child state preserved
result = MouseRegion(..., child: GestureDetector(..., child: result));

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>🐛 Tooltip widget replaces child when cursor enter/leaves window</issue_title>
<issue_description>This seems to have been an issue in flutter material too, fixed here:

When the mouse leaves/enters the window, the child of Tooltip is created with an entirely new state. Repro:

import 'package:fluent_ui/fluent_ui.dart';

void main() {
  runApp(FluentApp(
    home: ColoredBox(
      color: Colors.white,
      child: Center(
        child: Tooltip(
          message: 'if this string is empty, issue does not occur',
          child: StatefulText(),
        ),
      ),
    ),
  ));
}

class StatefulText extends StatefulWidget {
  const StatefulText({super.key});

  @override
  State<StatefulText> createState() => _StatefulTextState();
}

class _StatefulTextState extends State<StatefulText> {
  static int count = 1;

  @override
  void initState() {
    super.initState();
    count++;
  }

  @override
  Widget build(BuildContext context) {
    return Text(
      'this initState ran $count times (on window cursor enter/exit)',
    );
  }
}
```</issue_description>

<agent_instructions>Check the fix made in the material library: https://github.com/flutter/flutter/pull/119199 and apply a similar fix in this project.

Additionally, make feature-parity with the Flutter material built in tooltip, while maintaining the extra functionality of this fluent_ui package.

Make sure performance is maintained.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes 🐛 Tooltip widget replaces child when cursor enter/leaves window #1125

---

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[bdlukaa]

/gemini review

[gemini-code-assist]

Code Review

This pull request addresses a bug causing state loss in a Tooltip's child widget when the mouse enters or leaves the window. The changes remove the stateful tracking of mouse connection and instead make the MouseRegion a permanent part of the widget tree, which stabilizes the tree structure and preserves the child's state. This is achieved by removing the _mouseIsConnected state field and its update logic, replacing it with a getter, and refactoring the build method to always include the MouseRegion. A regression test is also added to verify that the child's initState is called only once during mouse connection changes.