[BUG]: genui's Slider is a two-position switch over its default range, and crashes below it #2737

• I have searched the existing issues to make sure this bug has not already been reported.

Describe the Bug

slider.dart derives the number of stops from the range itself:

divisions: (sliderData.max - sliderData.min).toInt(),

The schema documents min as defaulting to 0.0 and max to 1.0. That range gives divisions: 1, which means the thumb can only rest at the two ends. Every value between them snaps, and a 0.5 is announced by assistive technology as 100%. A brightness, volume or opacity slider built on the documented defaults is a switch.

A range narrower than 1 is worse: min: 0, max: 0.5 gives divisions: 0 and Flutter asserts.

'package:flutter/src/material/slider.dart': Failed assertion: line 209 pos 15:
'divisions == null || divisions > 0': is not true.

So an agent can take the surface down with a message the catalog schema accepts.

Two smaller things in the same widget:

• The number shown next to the thumb is value?.toStringAsFixed(0), read from the data model only. With a literal value it shows the minimum instead, and toStringAsFixed(0) means a 0-to-1 slider can only ever read 0 or 1.
• Because of the snapping, the value the user leaves behind is not the value the agent sees: the write goes through onChanged, so the data model receives the snapped number.

Steps to Reproduce

import 'package:a2ui_core/a2ui_core.dart' as core;
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:genui/genui.dart';

Future<SurfaceController> show(
    WidgetTester tester, List<Map<String, dynamic>> components) async {
  final catalog = BasicCatalogItems.asCatalog().copyWith(catalogId: 'repro');
  final controller = SurfaceController(catalogs: [catalog]);
  controller.handleMessage(
      core.UpdateComponentsMessage(surfaceId: 's', components: components));
  controller.handleMessage(
      core.CreateSurfaceMessage(surfaceId: 's', catalogId: 'repro'));
  await tester.pumpWidget(MaterialApp(
      home: Scaffold(body: Surface(surfaceContext: controller.contextFor('s')))));
  await tester.pumpAndSettle();
  return controller;
}

void main() {
  testWidgets('the default range is not a two-position switch', (tester) async {
    await show(tester, [
      {'id': 'root', 'component': 'Slider', 'value': 0.5, 'min': 0, 'max': 1},
    ]);

    expect(tester.widget<Slider>(find.byType(Slider)).divisions, isNot(1));
  });

  testWidgets('a sub-unit range does not crash', (tester) async {
    await show(tester, [
      {'id': 'root', 'component': 'Slider', 'value': 0.2, 'min': 0, 'max': 0.5},
    ]);

    expect(tester.takeException(), isNull);
  });
}

The first fails with Expected: not <1> / Actual: <1>; the second with the assertion above.

Expected Behavior

A slider is continuous unless something says otherwise: divisions: null by default, and a declared step — a step property on the component, or divisions derived from it — when the catalog wants discrete stops. Whatever the rule ends up being, it cannot produce 0, and it should not turn the documented default range into two positions.

Environment Details

• OS: macOS 26.5.2
• Browser/Platform: Flutter 3.44.6 stable, Dart 3.10
• SDK/Package Name & Version: genui 0.10.3 (main, d88aa76), a2ui_core 0.1.1
• Protocol Version: v0.9.1

Additional Context

Found while prototyping a rendering conformance suite that states what a surface must expose as semantics — role, name, value, state, actions — so the same cases can run against Flutter, SwiftUI, Compose and the web. The case declared value: "0.5" and this renderer announced 100%, which is what led back to the divisions.

Worth saying that a golden or screenshot test would not have caught this one: the slider looks right. What is wrong is where the thumb is allowed to stop and what it tells a screen reader.

Happy to send the PR.