fix(scroll): use widget.scrollBehavior and add trackpad support

[sichang824]

Problem

On macOS (and desktop platforms in general), mouse wheel scroll events (PointerScrollEvent) are only delivered to the widget that wins the hit test. In ChatList, the topmost widgets are message bubbles (containing MouseRegion, SelectableText, etc.), not the internal Scrollable.

The current ScrollConfiguration in _renderList() only configures dragDevices (for drag-to-scroll), but this does not affect which widget receives wheel events. As a result, mouse wheel scrolling does not work properly on desktop platforms.

Additionally, the widget.scrollBehavior parameter is passed to FlutterListView but is ignored by the outer ScrollConfiguration, making it impossible for users to customize scroll behavior.

Solution

1. Use widget.scrollBehavior if provided - This allows users to pass a custom ScrollBehavior that will be applied to the outer ScrollConfiguration, giving full control over scroll behavior.

2. Add PointerDeviceKind.trackpad to default dragDevices - This improves trackpad scrolling support on desktop platforms.

Changes

// Before
_renderList() {
  return ScrollConfiguration(
    behavior: ScrollConfiguration.of(context).copyWith(dragDevices: {
      PointerDeviceKind.touch,
      PointerDeviceKind.mouse,
    }),
    child: SmartRefresher(...),
  );
}

// After
_renderList() {
  final scrollBehavior = widget.scrollBehavior ??
      ScrollConfiguration.of(context).copyWith(dragDevices: {
        PointerDeviceKind.touch,
        PointerDeviceKind.mouse,
        PointerDeviceKind.trackpad,
      });
  return ScrollConfiguration(
    behavior: scrollBehavior,
    child: SmartRefresher(...),
  );
}

Testing

• Added unit tests in test/scroll_behavior_test.dart
• Verified mouse wheel scrolling works on macOS with this fix