RawKeyboard synthesizes key pressing state for modifier (#93897)

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

@@ -772,15 +772,22 @@ class RawKeyboard {
       ..._keysPressed.keys,
       if (event is RawKeyDownEvent) event.physicalKey,
     };
-    for (final ModifierKey key in modifiersPressed.keys) {
+    ModifierKey? thisKeyModifier;
+    for (final ModifierKey key in ModifierKey.values) {
+      final Set<PhysicalKeyboardKey>? thisModifierKeys = _modifierKeyMap[_ModifierSidePair(key, KeyboardSide.all)];
+      if (thisModifierKeys == null)
+        continue;
+      if (thisModifierKeys.contains(event.physicalKey)) {
+        thisKeyModifier = key;
+      }
       if (modifiersPressed[key] == KeyboardSide.any) {
-        final Set<PhysicalKeyboardKey>? thisModifierKeys = _modifierKeyMap[_ModifierSidePair(key, KeyboardSide.all)];
-        anySideKeys.addAll(thisModifierKeys!);
+        anySideKeys.addAll(thisModifierKeys);
         if (thisModifierKeys.any(keysPressedAfterEvent.contains)) {
           continue;
         }
       }
-      final Set<PhysicalKeyboardKey>? mappedKeys = _modifierKeyMap[_ModifierSidePair(key, modifiersPressed[key])];
+      final Set<PhysicalKeyboardKey>? mappedKeys = modifiersPressed[key] == null ?
+        <PhysicalKeyboardKey>{} : _modifierKeyMap[_ModifierSidePair(key, modifiersPressed[key])];
       assert(() {
         if (mappedKeys == null) {
           debugPrint(
@@ -809,6 +816,20 @@ class RawKeyboard {
       _keysPressed.remove(PhysicalKeyboardKey.fn);
     }
     _keysPressed.addAll(modifierKeys);
+    // In rare cases, the event presses a modifier key but the key does not
+    // exist in the modifier list. Enforce the pressing state.
+    if (event is RawKeyDownEvent && thisKeyModifier != null
+        && !_keysPressed.containsKey(event.physicalKey)) {
+      // So far this inconsistancy is only found on Linux GTK for AltRight in a
+      // rare case. (See https://github.com/flutter/flutter/issues/93278 .) In
+      // other cases, this inconsistancy will be caught by an assertion later.
+      if (event.data is RawKeyEventDataLinux && event.physicalKey == PhysicalKeyboardKey.altRight) {
+        final LogicalKeyboardKey? logicalKey = _allModifiersExceptFn[event.physicalKey];
+        if (logicalKey != null) {
+          _keysPressed[event.physicalKey] = logicalKey;
+        }
+      }
+    }
   }
 
   final Map<PhysicalKeyboardKey, LogicalKeyboardKey> _keysPressed = <PhysicalKeyboardKey, LogicalKeyboardKey>{};

packages/flutter/test/services/raw_keyboard_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' as ui;
+
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
@@ -348,6 +350,55 @@ void main() {
       );
     }, skip: isBrowser); // [intended] This is a GLFW-specific test.
 
+
+    // Regression test for https://github.com/flutter/flutter/issues/93278 .
+    //
+    // GTK has some weird behavior where the tested key event sequence will
+    // result in a AltRight down event without Alt bitmask.
+    testWidgets('keysPressed modifiers are synchronized with key events on Linux GTK (down events)', (WidgetTester tester) async {
+      expect(RawKeyboard.instance.keysPressed, isEmpty);
+      Future<void> simulate(bool keyDown, int scancode, int keycode, int modifiers) async {
+        final Map<String, dynamic> data = <String, dynamic>{
+            'type': keyDown ? 'keydown' : 'keyup',
+            'keymap': 'linux',
+            'toolkit': 'gtk',
+            'scanCode': scancode,
+            'keyCode': keycode,
+            'modifiers': modifiers,
+          };
+        // Dispatch an empty key data to disable HardwareKeyboard sanity check,
+        // since we're only testing if the raw keyboard can handle the message.
+        // In real application the embedder responder will send correct key data
+        // (which is tested in the engine.)
+        TestDefaultBinaryMessengerBinding.instance!.keyEventManager.handleKeyData(const ui.KeyData(
+          type: ui.KeyEventType.down,
+          timeStamp: Duration.zero,
+          logical: 0,
+          physical: 0,
+          character: null,
+          synthesized: false,
+        ));
+        await TestDefaultBinaryMessengerBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
+          SystemChannels.keyEvent.name,
+          SystemChannels.keyEvent.codec.encodeMessage(data),
+          (ByteData? data) {},
+        );
+      }
+
+      await simulate(true,  0x6c/*AltRight*/,  0xffea/*AltRight*/,  0x2000000);
+      await simulate(true,  0x32/*ShiftLeft*/, 0xfe08/*NextGroup*/, 0x2000008/*MOD3*/);
+      await simulate(false, 0x6c/*AltRight*/,  0xfe03/*AltRight*/,  0x2002008/*MOD3|Reserve14*/);
+      await simulate(true,  0x6c/*AltRight*/,  0xfe03/*AltRight*/,  0x2002000/*Reserve14*/);
+      expect(
+        RawKeyboard.instance.keysPressed,
+        equals(
+          <LogicalKeyboardKey>{
+            LogicalKeyboardKey.altRight,
+          },
+        ),
+      );
+    }, skip: isBrowser); // [intended] This is a GTK-specific test.
+
     testWidgets('keysPressed modifiers are synchronized with key events on web', (WidgetTester tester) async {
       expect(RawKeyboard.instance.keysPressed, isEmpty);
       // Generate the data for a regular key down event. Change the modifiers so