Fixing synthesizing keys for multiple keys pressed down on flutter web (#19632)

Author: erickzanardo

lib/web_ui/lib/src/engine/keyboard.dart

@@ -91,8 +91,7 @@ class Keyboard {
 
     final String timerKey = keyboardEvent.code!;
 
-    // Don't synthesize a keyup event for modifier keys because the browser always
-    // sends a keyup event for those.
+    // Don't handle synthesizing a keyup event for modifier keys
     if (!_isModifierKey(event)) {
       // When the user enters a browser/system shortcut (e.g. `cmd+alt+i`) the
       // browser doesn't send a keyup for it. This puts the framework in a
@@ -103,7 +102,10 @@ class Keyboard {
       // event within a specific duration ([_keydownCancelDuration]) we assume
       // the user has released the key and we synthesize a keyup event.
       _keydownTimers[timerKey]?.cancel();
-      if (event.type == 'keydown') {
+
+      // Only keys affected by modifiers, require synthesizing
+      // because the browser always sends a keyup event otherwise
+      if (event.type == 'keydown' && _isAffectedByModifiers(event)) {
         _keydownTimers[timerKey] = Timer(_keydownCancelDuration, () {
           _keydownTimers.remove(timerKey);
           _synthesizeKeyup(event);
@@ -185,4 +187,11 @@ bool _isModifierKey(html.KeyboardEvent event) {
   return key == 'Meta' || key == 'Shift' || key == 'Alt' || key == 'Control';
 }
 
+/// Returns true if the [event] is been affects by any of the modifiers key
+///
+/// This is a strong indication that this key is been used for a shortcut
+bool _isAffectedByModifiers(html.KeyboardEvent event) {
+  return event.ctrlKey || event.shiftKey || event.altKey || event.metaKey;
+}
+
 void _noopCallback(ByteData? data) {}

lib/web_ui/test/keyboard_test.dart

@@ -473,18 +473,37 @@ void testMain() {
         }
         messages.clear();
 
-        // When repeat events stop for a long-enough period of time, a keyup
-        // should be synthesized.
+        Keyboard.instance.dispose();
+      },
+    );
+
+    testFakeAsync(
+      'do not synthesize keyup when keys are not affected by meta modifiers',
+      (FakeAsync async) {
+        Keyboard.initialize();
+
+        List<Map<String, dynamic>> messages = <Map<String, dynamic>>[];
+        ui.window.onPlatformMessage = (String channel, ByteData data,
+            ui.PlatformMessageResponseCallback callback) {
+          messages.add(const JSONMessageCodec().decodeMessage(data));
+        };
+
+        dispatchKeyboardEvent(
+          'keydown',
+          key: 'i',
+          code: 'KeyI',
+        );
+        dispatchKeyboardEvent(
+          'keydown',
+          key: 'o',
+          code: 'KeyO',
+        );
+        messages.clear();
+
+        // Wait for a long-enough period of time and no events
+        // should be synthesized
         async.elapse(Duration(seconds: 3));
-        expect(messages, <Map<String, dynamic>>[
-          <String, dynamic>{
-            'type': 'keyup',
-            'keymap': 'web',
-            'key': 'i',
-            'code': 'KeyI',
-            'metaState': 0x0,
-          }
-        ]);
+        expect(messages, hasLength(0));
 
         Keyboard.instance.dispose();
       },