MediaQuery.alwaysUse24HourFormat is not refreshed when the platform's 24-hour setting changes at runtime #194637

Steps to reproduce

1. Run the sample below on iOS or Android with the device set to 12-hour time.
2. With the app still running, switch the device to 24-hour time in Settings.
3. Return to the app.

Expected results

MediaQuery.alwaysUse24HourFormatOf(context) reflects the platform's current setting (as PlatformDispatcher.alwaysUse24HourFormat does), so the sample reads 24-hour, and TimeOfDay.format / showTimePicker switch to the 24-hour clock.

Actual results

MediaQuery keeps the previous value until one of metrics, text scale, platform brightness or accessibility features changes, or the app restarts, so every MediaQuery-driven time format stays on the old clock.

I have reproduced this deterministically with the widget test below, which drives the same framework path the engine does on this change; I have not captured it on a physical device.

Code sample

import 'package:flutter/material.dart';

void main() => runApp(const MaterialApp(home: Probe()));

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

  @override
  Widget build(BuildContext context) => Scaffold(
        body: Center(
          child: Text(MediaQuery.alwaysUse24HourFormatOf(context)
              ? '24-hour'
              : '12-hour'),
        ),
      );
}

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('MediaQuery sees a 24-hour change', (tester) async {
    addTearDown(tester.platformDispatcher.clearAlwaysUse24HourTestValue);
    await tester.pumpWidget(MaterialApp(
      home: Builder(
        builder: (context) =>
            Text('${MediaQuery.alwaysUse24HourFormatOf(context)}'),
      ),
    ));
    expect(find.text('false'), findsOneWidget);

    // What the engine does when only this setting changes: update the value
    // and invoke onPlatformConfigurationChanged.
    tester.platformDispatcher.alwaysUse24HourFormatTestValue = true;
    tester.platformDispatcher.onPlatformConfigurationChanged?.call();
    await tester.pump();

    expect(find.text('true'), findsOneWidget); // fails: still 'false'
  });
}

Analysis (3.44.6)

• dart:ui PlatformDispatcher._updateUserSettingsData: when only alwaysUse24HourFormat changes, it updates the configuration and invokes onPlatformConfigurationChanged; onTextScaleFactorChanged, onPlatformBrightnessChanged and onSystemFontFamilyChanged are each gated on their own value changing.
• Nothing in packages/flutter/lib listens to onPlatformConfigurationChanged.
• _MediaQueryFromViewState (widgets/media_query.dart) re-reads MediaQueryData.fromView only from didChangeDependencies, didChangeMetrics, didChangeTextScaleFactor, didChangePlatformBrightness and didChangeAccessibilityFeatures.
• Related, in flutter_test: the TestPlatformDispatcher.alwaysUse24HourFormatTestValue setter only stores the value (unlike textScaleFactorTestValue, which calls onTextScaleFactorChanged), so a test cannot observe a change without invoking a callback itself.

Workaround

Re-provide MediaQuery from a widget below MaterialApp (its builder:) that listens to onPlatformConfigurationChanged, chaining any existing handler, and overrides alwaysUse24HourFormat with PlatformDispatcher.alwaysUse24HourFormat.

Flutter Doctor output

Flutter 3.44.6 • channel stable