[macos] [NSViewController viewWillAppear] can be called multiple times (flutter#34072)

Fixes flutter#105963

Author: knopp

shell/platform/darwin/macos/framework/Source/FlutterViewController.mm

@@ -429,7 +429,11 @@ - (BOOL)launchEngine {
 // ViewController as a listener for a keyUp event before it's handled by NSApplication, and should
 // NOT modify the event to avoid any unexpected behavior.
 - (void)listenForMetaModifiedKeyUpEvents {
-  NSAssert(_keyUpMonitor == nil, @"_keyUpMonitor was already created");
+  if (_keyUpMonitor != nil) {
+    // It is possible for [NSViewController viewWillAppear] to be invoked multiple times
+    // in a row. https://github.com/flutter/flutter/issues/105963
+    return;
+  }
   FlutterViewController* __weak weakSelf = self;
   _keyUpMonitor = [NSEvent
       addLocalMonitorForEventsMatchingMask:NSEventMaskKeyUp

shell/platform/darwin/macos/framework/Source/FlutterViewControllerTest.mm

@@ -22,6 +22,7 @@ - (bool)testKeyEventsAreNotPropagatedIfHandled;
 - (bool)testFlagsChangedEventsArePropagatedIfNotHandled;
 - (bool)testKeyboardIsRestartedOnEngineRestart;
 - (bool)testTrackpadGesturesAreSentToFramework;
+- (bool)testViewWillAppearCalledMultipleTimes;
 
 + (void)respondFalseForSendEvent:(const FlutterKeyEvent&)event
                         callback:(nullable FlutterKeyEventCallback)callback
@@ -134,6 +135,10 @@ + (void)respondFalseForSendEvent:(const FlutterKeyEvent&)event
   ASSERT_TRUE([[FlutterViewControllerTestObjC alloc] testTrackpadGesturesAreSentToFramework]);
 }
 
+TEST(FlutterViewControllerTest, testViewWillAppearCalledMultipleTimes) {
+  ASSERT_TRUE([[FlutterViewControllerTestObjC alloc] testViewWillAppearCalledMultipleTimes]);
+}
+
 }  // namespace flutter::testing
 
 @implementation FlutterViewControllerTestObjC
@@ -518,4 +523,14 @@ - (bool)testTrackpadGesturesAreSentToFramework {
   return true;
 }
 
+- (bool)testViewWillAppearCalledMultipleTimes {
+  id engineMock = OCMClassMock([FlutterEngine class]);
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engineMock
+                                                                                nibName:@""
+                                                                                 bundle:nil];
+  [viewController viewWillAppear];
+  [viewController viewWillAppear];
+  return true;
+}
+
 @end