[Keyboard] Correctly convert down events that are immediately synthesized released (#99200)

packages/flutter/lib/src/services/hardware_keyboard.dart

@@ -887,6 +887,7 @@ class KeyEventManager {
     final PhysicalKeyboardKey physicalKey = rawEvent.physicalKey;
     final LogicalKeyboardKey logicalKey = rawEvent.logicalKey;
     final Set<PhysicalKeyboardKey> physicalKeysPressed = _hardwareKeyboard.physicalKeysPressed;
+    final List<KeyEvent> eventAfterwards = <KeyEvent>[];
     final KeyEvent? mainEvent;
     final LogicalKeyboardKey? recordedLogicalMain = _hardwareKeyboard.lookUpLayout(physicalKey);
     final Duration timeStamp = ServicesBinding.instance.currentSystemFrameTimeStamp;
@@ -923,6 +924,17 @@ class KeyEventManager {
       }
     }
     for (final PhysicalKeyboardKey key in physicalKeysPressed.difference(_rawKeyboard.physicalKeysPressed)) {
+      if (key == physicalKey) {
+        // Somehow, a down event is dispatched but the key is absent from
+        // keysPressed. Synthesize a up event for the key, but this event must
+        // be added after the main key down event.
+        eventAfterwards.add(KeyUpEvent(
+          physicalKey: key,
+          logicalKey: logicalKey,
+          timeStamp: timeStamp,
+          synthesized: true,
+        ));
+      } else {
         _keyEventsSinceLastMessage.add(KeyUpEvent(
           physicalKey: key,
           logicalKey: _hardwareKeyboard.lookUpLayout(key)!,
@@ -930,6 +942,7 @@ class KeyEventManager {
           synthesized: true,
         ));
       }
+    }
     for (final PhysicalKeyboardKey key in _rawKeyboard.physicalKeysPressed.difference(physicalKeysPressed)) {
       _keyEventsSinceLastMessage.add(KeyDownEvent(
         physicalKey: key,
@@ -938,9 +951,11 @@ class KeyEventManager {
         synthesized: true,
       ));
     }
-    if (mainEvent != null)
+    if (mainEvent != null) {
       _keyEventsSinceLastMessage.add(mainEvent);
     }
+    _keyEventsSinceLastMessage.addAll(eventAfterwards);
+  }
 
   /// Reset the inferred platform transit mode and related states.
   ///

packages/flutter/test/services/hardware_keyboard_test.dart

@@ -195,6 +195,56 @@ void main() {
     logs.clear();
   }, variant: KeySimulatorTransitModeVariant.all());
 
+  // Regression test for https://github.com/flutter/flutter/issues/99196 .
+  //
+  // In rawKeyData mode, if a key down event is dispatched but immediately
+  // synthesized to be released, the old logic would trigger a Null check
+  // _CastError on _hardwareKeyboard.lookUpLayout(key). The original scenario
+  // that this is triggered on Android is unknown. Here we make up a scenario
+  // where a ShiftLeft key down is dispatched but the modifier bit is not set.
+  testWidgets('Correctly convert down events that are synthesized released', (WidgetTester tester) async {
+    final FocusNode focusNode = FocusNode();
+    final List<KeyEvent> events = <KeyEvent>[];
+
+    await tester.pumpWidget(
+      KeyboardListener(
+        autofocus: true,
+        focusNode: focusNode,
+        child: Container(),
+        onKeyEvent: (KeyEvent event) {
+          events.add(event);
+        },
+      ),
+    );
+
+    // Dispatch an arbitrary event to bypass the pressedKeys check.
+    await simulateKeyDownEvent(LogicalKeyboardKey.keyA, platform: 'web');
+
+    // Dispatch an
+    final Map<String, dynamic> data2 = KeyEventSimulator.getKeyData(
+      LogicalKeyboardKey.shiftLeft,
+      platform: 'web',
+    )..['metaState'] = 0;
+    await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      SystemChannels.keyEvent.name,
+      SystemChannels.keyEvent.codec.encodeMessage(data2),
+      (ByteData? data) {},
+    );
+
+    expect(events, hasLength(3));
+    expect(events[1], isA<KeyDownEvent>());
+    expect(events[1].logicalKey, LogicalKeyboardKey.shiftLeft);
+    expect(events[1].synthesized, false);
+    expect(events[2], isA<KeyUpEvent>());
+    expect(events[2].logicalKey, LogicalKeyboardKey.shiftLeft);
+    expect(events[2].synthesized, true);
+    expect(ServicesBinding.instance.keyboard.physicalKeysPressed, equals(<PhysicalKeyboardKey>{
+      PhysicalKeyboardKey.keyA,
+    }));
+  }, variant: const KeySimulatorTransitModeVariant(<KeyDataTransitMode>{
+    KeyDataTransitMode.rawKeyData,
+  }));
+
   testWidgets('Instantly dispatch synthesized key events when the queue is empty', (WidgetTester tester) async {
     final FocusNode focusNode = FocusNode();
     final List<int> logs = <int>[];