[iPhone Duo] MediaQuery.sizeOf reports the whole screen inside a dialog confined to one half of a foldable #192667

Steps to reproduce

DisplayFeatureSubScreen confines a dialog, bottom sheet or popup to one sub-screen of a foldable and wraps it in MediaQuery(data: mediaQuery.removeDisplayFeatures(closestSubScreen)). removeDisplayFeatures adjusts padding, viewInsets and viewPadding, and filters displayFeatures. It does not adjust size.

So a widget inside the dialog reads the width of the whole screen while occupying half of it. This runs as a widget test and passes today:

import 'dart:ui';

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

void main() {
  testWidgets('MediaQuery.size is not reduced to the sub-screen', (
    WidgetTester tester,
  ) async {
    tester.view.devicePixelRatio = 1.0;
    tester.view.physicalSize = const Size(800, 900);
    // A 40px fold down the middle: the sub-screens are 0..380 and 420..800.
    tester.view.displayFeatures = <DisplayFeature>[
      const DisplayFeature(
        bounds: Rect.fromLTRB(380, 0, 420, 900),
        type: DisplayFeatureType.fold,
        state: DisplayFeatureState.postureHalfOpened,
      ),
    ];
    addTearDown(tester.view.reset);

    late Size reported;
    await tester.pumpWidget(
      MaterialApp(
        home: Builder(
          builder: (BuildContext context) => ElevatedButton(
            onPressed: () => showDialog<void>(
              context: context,
              anchorPoint: Offset.zero,
              builder: (BuildContext inner) {
                reported = MediaQuery.sizeOf(inner);
                return const SizedBox.expand(key: Key('body'));
              },
            ),
            child: const Text('open'),
          ),
        ),
      ),
    );
    await tester.tap(find.text('open'));
    await tester.pumpAndSettle();

    final Rect body = tester.getRect(find.byKey(const Key('body')));
    print('the dialog occupies    -> ${body.width} x ${body.height}');
    print('MediaQuery.sizeOf says -> ${reported.width} x ${reported.height}');

    expect(body.width, 380);
    expect(reported.width, 800); // passes today; 380 is what a caller expects
  });
}

Expected results

MediaQuery.sizeOf inside the sub-screen reports the size the child actually has — 380×900 here — or the documentation says that it deliberately does not, so callers know to reach for LayoutBuilder instead.

Actual results

the dialog occupies    -> 380.0 x 900.0
MediaQuery.sizeOf says -> 800.0 x 900.0

Anything derived from it is off by the width of the other half plus the fold. A box asking for MediaQuery.sizeOf(context).width / 2 asks for 400 and is silently clamped to 380 — there is no overflow banner, just a layout that is quietly wrong.

Additional context

The doc comment on removeDisplayFeatures lists what it adjusts and does not mention size either way:

Creates a copy of this media query data by removing [displayFeatures] that are completely outside the given sub-screen and adjusting the [padding], [viewInsets] and [viewPadding] to be zero on the sides that are not included in the sub-screen.

Whether the fix is to narrow size or to document that it is not narrowed is a call for the team — the two have very different blast radii for apps already shipping on Android foldables.

Two things this is not, which I checked while measuring:

• The asymmetric insets are handled correctly. With padding of l=20 t=59 r=5 b=34 and the same fold, a dialog on the left half lands at 20..380 and one on the right at 420..795, so each side is honoured independently.
• It is not iOS-specific. It reproduces on Android foldables today. It is worth raising now because the same code path is what MediaQuery.displayFeatures would drive on iPhone Duo ([iPhone Duo] Proposal: populate MediaQuery.displayFeatures on iOS for iPhone Duo (foldable) #192515), which ships on 23 October, so the number of apps hitting it is about to grow.

Reproduced on 3.44.6 stable and on a master checkout (3.48.0-1.0.pre-702); removeDisplayFeatures is unchanged between them.

[✓] Flutter (Channel stable, 3.44.6, on macOS 26.5.2 25F84 darwin-arm64, locale es-CO)
[✓] Android toolchain - develop for Android devices (Android SDK version 36.1.0)
[✓] Xcode - develop for iOS and macOS (Xcode 26.1.1)
[✓] Chrome - develop for the web
[✓] Connected device (4 available)
[✓] Network resources