Allow setting a custom mouse cursor for the scrollbar inside TextField #185811

Use case

TextField has a mouseCursor parameter, but it applies to the entire widget including the scrollbar area. When a multiline TextField displays a scrollbar, the mouse cursor stays as an I-beam when hovering over the scrollbar thumb and track. Users expect the cursor to change to a default arrow pointer, matching native platform behavior.

There is currently no way to set a different cursor for the scrollbar only — TextField.mouseCursor changes both the text area and scrollbar cursors together.

Screen recording

https://github.com/user-attachments/assets/placeholder

Minimal reproduction

import 'package:flutter/material.dart';

void main() => runApp(const App());

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

  @override
  Widget build(BuildContext context) {
    final text = List.generate(30, (i) => 'Line $i: Text content.').join('\n');
    return MaterialApp(
      home: Scaffold(
        body: Padding(
          padding: const EdgeInsets.all(32),
          child: SizedBox(
            width: 400,
            height: 300,
            child: TextField(
              maxLines: null,
              expands: true,
              controller: TextEditingController(text: text),
              decoration: const InputDecoration(border: OutlineInputBorder()),
            ),
          ),
        ),
      ),
    );
  }
}

Steps:

1. Run the code above on desktop or web
2. Hover over the text area — cursor shows I-beam (correct)
3. Move cursor to the scrollbar — cursor stays I-beam (expected: arrow)

Flutter version: 3.35.7 (Dart 3.9.2)

Proposal

Two possible approaches:

a) Add a scrollbarMouseCursor parameter to TextField, allowing consumers to set the cursor specifically for the scrollbar area independently from the text area cursor.

b) Add a mouseCursor parameter to RawScrollbar, so any scrollbar (not just inside TextField) can have a custom cursor when hovered.

Duplicate of #185733 (closed in favor of this simplified issue).