Change FlutterViewController default opaque value to NO (flutter/engine#30664)

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -120,7 +120,7 @@ typedef enum UIAccessibilityContrast : NSInteger {
   NSAssert(engine != nil, @"Engine is required");
   self = [super initWithNibName:nibName bundle:nibBundle];
   if (self) {
-    _viewOpaque = YES;
+    _viewOpaque = NO;
     if (engine.viewController) {
       FML_LOG(ERROR) << "The supplied FlutterEngine " << [[engine description] UTF8String]
                      << " is already used with FlutterViewController instance "
@@ -203,7 +203,7 @@ typedef enum UIAccessibilityContrast : NSInteger {
     return;
   }
 
-  _viewOpaque = YES;
+  _viewOpaque = NO;
   _weakFactory = std::make_unique<fml::WeakPtrFactory<FlutterViewController>>(self);
   _engine = std::move(engine);
   _flutterView.reset([[FlutterView alloc] initWithDelegate:_engine opaque:self.isViewOpaque]);

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.mm

@@ -168,6 +168,16 @@ typedef enum UIAccessibilityContrast : NSInteger {
   self.messageSent = nil;
 }
 
+- (void)testFlutterViewControllerViewOpaue {
+  FlutterEngine* mockEngine = OCMPartialMock([[FlutterEngine alloc] init]);
+  [mockEngine createShell:@"" libraryURI:@"" initialRoute:nil];
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:mockEngine
+                                                                                nibName:nil
+                                                                                 bundle:nil];
+  // The FlutterViewController's FlutterViewLayer Opaque default is NO
+  XCTAssertFalse(viewController.isViewOpaque);
+}
+
 - (void)testkeyboardWillChangeFrameWillStartKeyboardAnimation {
   FlutterEngine* mockEngine = OCMPartialMock([[FlutterEngine alloc] init]);
   [mockEngine createShell:@"" libraryURI:@"" initialRoute:nil];