[web] Semantics node map inconsistent after GlobalKey reparent that removes a later sibling #194420

Steps to reproduce

In Flutter web debug mode, a valid same-frame GlobalKey reparent that also removes a later sibling throws Bad state: Semantics node map was inconsistent after update. The reproduction uses only Flutter widgets and the SDK integration_test binding (no app code, custom RenderObjects, manually assigned node IDs or animations).

1. In an ordinary Flutter project with flutter_test and integration_test SDK dev dependencies, save the code sample below as integration_test/repro_test.dart.
2. Add test_driver/integration_test.dart:

   import 'package:integration_test/integration_test_driver.dart';
   Future<void> main() => integrationDriver();

3. Start a ChromeDriver matching your Chrome on port 4444 (we used Chrome for Testing / ChromeDriver 155.0.8059.79), then run:

   flutter drive -d web-server --headless --browser-name=chrome \
     --driver-port=4444 --target=integration_test/repro_test.dart \
     --driver=test_driver/integration_test.dart

4. Controls: repeat with --dart-define=REPRO_MODE=sibling-first and --dart-define=REPRO_MODE=fresh-child. Add --profile to run without debug assertions.

Expected results

Moving the GlobalKey child to its new parent and removing its old sibling leaves a consistent engine semantics node map.

Actual results

Debug fails every time (2/2 fresh Chrome sessions); both controls pass; --profile passes (the consistency check is inside an assert). The framework tree changes from 0 → 1 → 2 → [3 retained, 4 removed] to 0 → 5 → 6 → 3, but the engine map keeps orphan node 4.

Suspected cause (source reading, not a verified bisect): in the web engine, SemanticsObject._visitDepthFirstInTraversalOrderCanSkipSubtree calls child!._visitDepthFirstInTraversalOrder(searchSubtree) (lib/_engine/engine/semantics/semantics.dart, around line 2651 in 3.47.5). That helper stops the whole traversal when the callback returns false, instead of skipping only that subtree. _finalizeTree returns false for a retained/reparented node, so later removed siblings at sufficient depth are never visited and stay in the map. That fits the sibling-first control passing. The pruning runs outside the assert, so profile/release builds likely keep the stale node too (only the check is debug-only). The code appears to come from #177069 (commit 16ae36ce20bbbe0543e70b2be5564f2dd595f06d, related issue #175180); this is a candidate, not a bisect result.

Code sample

import 'package:flutter/widgets.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:integration_test/integration_test.dart';
// No Nebrivo imports, database, PDF, Riverpod, routes, or animation.
void main() {
  IntegrationTestWidgetsFlutterBinding.ensureInitialized();
  testWidgets('reparent a child while removing its old siblings', (tester) async {
    final handle = tester.ensureSemantics();
    final retainedKey = GlobalKey();
    const mode = String.fromEnvironment('REPRO_MODE', defaultValue: 'reparent');
    final changed = ValueNotifier(false);
    Widget node(String label, Widget child, {Key? key}) => Semantics(
      key: key, container: true, explicitChildNodes: true,
      label: label, child: child,
    );
    try {
      await tester.pumpWidget(Directionality(
        textDirection: TextDirection.ltr,
        child: ValueListenableBuilder<bool>(
          valueListenable: changed,
          builder: (context, ready, _) {
            final retained = node('retained', const SizedBox(width: 20, height: 20),
              key: mode == 'fresh-child' ? const ValueKey('retained') : retainedKey);
            final removed = node('removed sibling', const SizedBox(width: 20, height: 20));
            return ready
              ? node('new outer', node('new middle', retained), key: const ValueKey('new'))
              : node('old outer', node('old middle', Column(children: [
                  if (mode == 'sibling-first') removed,
                  retained,
                  if (mode != 'sibling-first') removed,
                ])), key: const ValueKey('old'));
          },
        ),
      ));
      debugPrint('REPRO: before reparent');
      debugDumpSemanticsTree(DebugSemanticsDumpOrder.traversalOrder);
      changed.value = true;
      await tester.pump();
      debugPrint('REPRO: after reparent');
      debugDumpSemanticsTree(DebugSemanticsDumpOrder.traversalOrder);
      // Leave any framework error with the binding so the driver records the
      // original engine stack and fails. Never suppress a semantics failure.
      expect(find.bySemanticsLabel('retained'), findsOneWidget);
    } finally {
      handle.dispose();
      changed.dispose();
    }
  });
}

Logs

══╡ EXCEPTION CAUGHT BY SCHEDULER LIBRARY ╞══════════════════════
The following StateError was thrown during a scheduler callback:
Bad state: Semantics node map was inconsistent after update:

BEFORE: The semantics node map is consistent:
  Nodes in tree:
    0: [1]
    1: [2]
    2: [3, 4]
    3: []
    4: []
  Nodes in map: [0, 1, 2, 3, 4]


AFTER: The semantics node map is inconsistent:
  Nodes in tree:
    0: [5]
    5: [6]
    6: [3]
    3: []
  Nodes in map: [0, 3, 4, 5, 6]

When the exception was thrown, this was the stack:
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 274:3       throw_
lib/_engine/engine/semantics/semantics.dart 3281:9                                <fn>
lib/_engine/engine/semantics/semantics.dart 3330:14                               updateSemantics
lib/_engine/engine/window.dart 109:5                                              updateSemantics
package:flutter_test/src/window.dart 1276:5                                       updateSemantics
package:flutter/src/rendering/view.dart 384:5                                     updateSemantics
package:flutter/src/rendering/binding.dart 268:48                                 <fn>
package:flutter/src/semantics/semantics.dart 5078:22                              sendSemanticsUpdate
package:flutter/src/rendering/object.dart 1651:7                                  flushSemantics
package:flutter/src/rendering/object.dart 1653:14                                 flushSemantics
package:flutter/src/rendering/binding.dart 699:7                                  drawFrame
package:flutter/src/widgets/binding.dart 1573:13                                  drawFrame
package:flutter/src/rendering/binding.dart 558:5                                  [_handlePersistentFrameCallback]

Flutter Doctor output