Fix Windows Enter key after focus loss (flutter#178523)

Fix issue: flutter#169447

Problem: On Windows, pressing Enter after refocusing the app could drop
the first keydown because the embedder still thought the key was
pressed, causing a double-press requirement and HardwareKeyboard
assertions.
Fix: In KeyboardKeyEmbedderHandler, synthesize a key-up whenever Win32
reports a “fresh” down for a key still tracked as pressed, so Flutter
sees a release before the new down. This keeps modifier/lock-key
synchronization intact.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Author: letrungdo

AUTHORS

@@ -136,3 +136,4 @@ Jin Jeongsu <[email protected]>
 Mairon Slusarz <[email protected]>
 Ricardo Dalarme <[email protected]>
 yiiim <[email protected]>
+letrungdo <[email protected]>

engine/src/flutter/shell/platform/windows/keyboard_key_embedder_handler.cc

@@ -225,22 +225,24 @@ void KeyboardKeyEmbedderHandler::KeyboardHookImpl(
   uint64_t result_logical_key;
 
   if (is_event_down) {
+    if (had_record && !was_down) {
+      // Windows delivered a new down event without a matching up event. This
+      // happens if the window lost focus before it could receive the key up.
+      // Synthesize an up event so the framework releases the key before
+      // processing the incoming down.
+      SendSynthesizeUpEvent(physical_key, last_logical_record);
+      last_logical_record_iter = pressingRecords_.find(physical_key);
+      had_record = last_logical_record_iter != pressingRecords_.end();
+      last_logical_record = had_record ? last_logical_record_iter->second : 0;
+    }
+
     if (had_record) {
-      if (was_down) {
-        // A normal repeated key.
-        type = kFlutterKeyEventTypeRepeat;
-        FML_DCHECK(had_record);
-        ConvertUtf32ToUtf8_(character_bytes, character);
-        eventual_logical_record = last_logical_record;
-        result_logical_key = last_logical_record;
-      } else {
-        // A non-repeated key has been pressed that has the exact physical key
-        // as a currently pressed one, usually indicating multiple keyboards are
-        // pressing keys with the same physical key, or the up event was lost
-        // during a loss of focus. The down event is ignored.
-        callback(true);
-        return;
-      }
+      // A normal repeated key.
+      type = kFlutterKeyEventTypeRepeat;
+      FML_DCHECK(had_record);
+      ConvertUtf32ToUtf8_(character_bytes, character);
+      eventual_logical_record = last_logical_record;
+      result_logical_key = last_logical_record;
     } else {
       // A normal down event (whether the system event is a repeat or not).
       type = kFlutterKeyEventTypeDown;

engine/src/flutter/shell/platform/windows/keyboard_key_embedder_handler_unittests.cc

@@ -747,17 +747,30 @@ TEST(KeyboardKeyEmbedderHandlerTest, RepeatedDownIsIgnored) {
 
   // KeyA's key up is missed.
 
-  // Press A again (should yield an empty event)
+  // Press A again (should synthesize an up event followed by a new down).
   last_handled = false;
   handler->KeyboardHook(
       kVirtualKeyA, kScanCodeKeyA, WM_KEYDOWN, 'a', false, false,
       [&last_handled](bool handled) { last_handled = handled; });
-  EXPECT_EQ(last_handled, true);
-  EXPECT_EQ(results.size(), 1);
+  EXPECT_EQ(last_handled, false);
+  ASSERT_EQ(results.size(), 2u);
+
   event = &results[0];
-  EXPECT_EQ(event->physical, 0);
-  EXPECT_EQ(event->logical, 0);
+  EXPECT_EQ(event->type, kFlutterKeyEventTypeUp);
+  EXPECT_EQ(event->physical, kPhysicalKeyA);
+  EXPECT_EQ(event->logical, kLogicalKeyA);
+  EXPECT_STREQ(event->character, "");
+  EXPECT_EQ(event->synthesized, true);
   EXPECT_EQ(event->callback, nullptr);
+
+  event = &results[1];
+  EXPECT_EQ(event->type, kFlutterKeyEventTypeDown);
+  EXPECT_EQ(event->physical, kPhysicalKeyA);
+  EXPECT_EQ(event->logical, kLogicalKeyA);
+  EXPECT_STREQ(event->character, "a");
+  EXPECT_EQ(event->synthesized, false);
+  event->callback(true, event->user_data);
+  EXPECT_EQ(last_handled, true);
   results.clear();
 }
 

engine/src/flutter/shell/platform/windows/keyboard_unittests.cc

@@ -1027,6 +1027,40 @@ TEST_F(KeyboardTest, RestartClearsKeyboardState) {
   EXPECT_EQ(tester.RedispatchedMessageCountAndClear(), 0);
 }
 
+// Press Enter, lose focus before the up event arrives, and press Enter again
+// once focus returns. The second down event should not be dropped.
+TEST_F(KeyboardTest, FreshKeyDownAfterMissedUpIsDelivered) {
+  KeyboardTester tester{GetContext()};
+  tester.Responding(true);
+
+  tester.InjectKeyboardChanges(std::vector<KeyboardChange>{
+      WmKeyDownInfo{VK_RETURN, kScanCodeEnter, kNotExtended, kWasUp}.Build(
+          kWmResultZero)});
+
+  tester.InjectKeyboardChanges(std::vector<KeyboardChange>{
+      WmKeyDownInfo{VK_RETURN, kScanCodeEnter, kNotExtended, kWasUp}.Build(
+          kWmResultZero)});
+
+  ASSERT_EQ(tester.key_calls.size(), 3u);
+  EXPECT_CALL_IS_EVENT(tester.key_calls[0], kFlutterKeyEventTypeDown,
+                       kPhysicalEnter, kLogicalEnter, "", kNotSynthesized);
+  EXPECT_CALL_IS_EVENT(tester.key_calls[1], kFlutterKeyEventTypeUp,
+                       kPhysicalEnter, kLogicalEnter, "", kSynthesized);
+  EXPECT_CALL_IS_EVENT(tester.key_calls[2], kFlutterKeyEventTypeDown,
+                       kPhysicalEnter, kLogicalEnter, "", kNotSynthesized);
+  tester.clear_key_calls();
+
+  tester.InjectKeyboardChanges(std::vector<KeyboardChange>{
+      WmKeyUpInfo{VK_RETURN, kScanCodeEnter, kNotExtended}.Build(
+          kWmResultZero)});
+
+  ASSERT_EQ(tester.key_calls.size(), 1u);
+  EXPECT_CALL_IS_EVENT(tester.key_calls[0], kFlutterKeyEventTypeUp,
+                       kPhysicalEnter, kLogicalEnter, "", kNotSynthesized);
+  tester.clear_key_calls();
+  EXPECT_EQ(tester.RedispatchedMessageCountAndClear(), 0u);
+}
+
 // Press Shift-A. This is special because Win32 gives 'A' as character for the
 // KeyA press.
 TEST_F(KeyboardTest, ShiftLeftKeyA) {