SelectableRegion web: right-click during a route transition crashes with 'getTransformTo cannot be called before SelectionContainer is laid out' #189575

Steps to reproduce

1. Run the code sample below on the web (flutter run -d chrome).
2. Click "Open detail" to start a route transition.
3. Right-click during the transition window (roughly the ~300 ms while the page animation is running).

The crash is timing-dependent: the right-click has to land while the outgoing route's SelectableRegion is detached from layout, so it may take a few attempts to hit locally. We hit it regularly in a production web app (both dart2js and wasm builds). It is not reproducible via flutter test because tester.pump() is synchronous and can't race the JS event loop — a live-engine repro needs a synthesized dispatchEvent('mousedown', {button: 2}) against the HtmlElementView element during the transition frame.

Expected results

Right-clicking during a route transition either shows the browser/Flutter context menu or does nothing.

Actual results

Assertion failure (debug) / DartError (release):

Assertion failed: .../widgets/selection_container.dart:303:7
_selectionContainerContext?.findRenderObject() != null
"getTransformTo cannot be called before SelectionContainer is laid out."

Analysis (updated after digging into the lifecycle): the stale _activeClient is caused by three concrete defects in the framework, not just an unguarded geometry read:

1. PlatformSelectableRegionContextMenu.detach has an inverted condition — if (_activeClient != client) { _activeClient = null; }. Detaching the active client leaves it registered, while detaching an unrelated client wipes another region's registration.
2. SelectableRegionState._handleFocusChanged calls attach unconditionally at the end of the method, so a region that just lost focus is immediately re-registered.
3. SelectableRegionState.dispose never detaches, so the static can keep pointing at a disposed delegate.

The mousedown listener then calls client.getTransformTo(null) on that stale delegate (_platform_selectable_region_context_menu_web.dart, line 96 on current master) and hits the assertion above. Same bug family as #151536 / PR #184900 ("geometry read on an unlaid-out SelectionContainer"), different read path.

Fixing the three lifecycle defects is the primary fix — done in #189587, including hardening for BrowserContextMenu.enabled changing at runtime. A guard at the geometry read could stay as defense-in-depth, but note it is incomplete on its own: with assertions compiled out (release mode), a disposed delegate fails the _selectionContainerContext! null-check with a TypeError, which an on AssertionError / on StateError catch would not cover.

Code sample

import 'package:flutter/material.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Builder(
        builder: (BuildContext context) => SelectionArea(
          child: Scaffold(
            body: Center(
              child: TextButton(
                onPressed: () => Navigator.of(context).push(
                  MaterialPageRoute<void>(
                    builder: (_) => const Scaffold(
                      body: Center(child: Text('Detail page')),
                    ),
                  ),
                ),
                child: const Text('Open detail'),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Screenshots or Video

Logs

DartError: Assertion failed: .../widgets/selection_container.dart:303:7
_selectionContainerContext?.findRenderObject() != null
"getTransformTo cannot be called before SelectionContainer is laid out."
package:flutter/src/widgets/selection_container.dart 303:35                           getTransformTo
package:flutter/src/widgets/_platform_selectable_region_context_menu_web.dart 96:41   <fn>
package:flutter/src/widgets/_platform_selectable_region_context_menu_web.dart 136:11  <fn>

Flutter Doctor output

[✓] Flutter (Channel stable, 3.44.4, on macOS 26.5.2 25F84 darwin-arm64, locale en-US) [609ms]
    • Flutter version 3.44.4 on channel stable at ~/repos/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision ad70ec4617 (3 weeks ago), 2026-06-24 11:07:06 -0700
    • Engine revision a10d8ac38d
    • Dart version 3.12.2
    • DevTools version 2.57.0
    • Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations, enable-native-assets, enable-swift-package-manager, omit-legacy-version-file, enable-lldb-debugging, enable-uiscene-migration

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    • Android SDK at ~/Library/Android/sdk
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    • Platform android-36, build-tools 36.0.0
    • ANDROID_HOME = ~/Library/Android/sdk
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      This is the JDK bundled with the latest Android Studio installation on this machine.
      To manually set the JDK path, use: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment (build 21.0.6+-13391695-b895.109)
    • All Android licenses accepted.

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    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 17F113
    • CocoaPods version 1.16.2

[✓] Chrome - develop for the web [6ms]
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Connected device (2 available) [6.8s]
    • macOS (desktop) • macos  • darwin-arm64   • macOS 26.5.2 25F84 darwin-arm64
    • Chrome (web)    • chrome • web-javascript • Google Chrome 150.0.7871.116

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• No issues found!