Prevent iOS Alert and Modal from rendering in the top-left corner (#57251)

Summary:
On iOS, both `Alert` and `Modal` could render in the top-left corner of the screen instead of filling it.

- `RCTAlertController`: when the alert window is created from a `UIWindowScene`, it was initialized without a frame and defaulted to a zero-sized frame. We now set its frame to the scene's coordinate space bounds.
- `ModalHostViewUtils`: `ModalHostViewScreenSize` used `RCTScreenSize()`, whose value is cached once via `dispatch_once`. If that first read happens before the screen is ready, it caches `0` and stays `0`, so the modal renders zero-sized in the top-left corner. We now read `RCTViewportSize()` on the main queue, which reads the key window's bounds live.

Fixes #45453

## Changelog:

[IOS] [FIXED] - Prevent Alert and Modal from rendering in the top-left corner

Pull Request resolved: #57251

Test Plan:
- Open an `Alert` on iOS and confirm it is centered and correctly sized.
- Open a `Modal` on iOS and confirm it fills the screen instead of appearing in the top-left corner.

## Screenshots:

<img width="284" height="542" alt="Alert" src="https://github.com/user-attachments/assets/e98ae159-9752-4b6e-8a97-0276984af3ba" />

<img width="284" height="542" alt="Modal" src="https://github.com/user-attachments/assets/1fff4217-5f28-4017-8699-01b757e7fb8a" />

Reviewed By: javache

Differential Revision: D109310703

Pulled By: cortinico

fbshipit-source-id: db3d66a704921c524faed62d82695528d75698eb

Author: zoontek

packages/react-native/React/Base/RCTUtils.mm

@@ -12,6 +12,7 @@
 #import <objc/message.h>
 #import <objc/runtime.h>
 #import <zlib.h>
+#import <atomic>
 
 #import <UIKit/UIKit.h>
 
@@ -383,24 +384,26 @@ UIDeviceOrientation RCTDeviceOrientation(void)
 
 CGSize RCTScreenSize(void)
 {
-  static CGSize portraitSize;
-  static dispatch_once_t onceToken;
-  dispatch_once(&onceToken, ^{
+  static std::atomic<CGSize> cachedSize;
+  __block CGSize size = cachedSize.load(std::memory_order_relaxed);
+
+  if (CGSizeEqualToSize(size, CGSizeZero)) {
     RCTUnsafeExecuteOnMainQueueSync(^{
       CGSize screenSize = [UIScreen mainScreen].bounds.size;
-      portraitSize = CGSizeMake(MIN(screenSize.width, screenSize.height), MAX(screenSize.width, screenSize.height));
+      size = CGSizeMake(MIN(screenSize.width, screenSize.height), MAX(screenSize.width, screenSize.height));
+      cachedSize.store(size, std::memory_order_relaxed);
     });
-  });
+  }
 
 #if !TARGET_OS_TV
   if (UIDeviceOrientationIsLandscape(RCTDeviceOrientation())) {
-    return CGSizeMake(portraitSize.height, portraitSize.width);
+    return CGSizeMake(size.height, size.width);
   } else {
-    return CGSizeMake(portraitSize.width, portraitSize.height);
+    return CGSizeMake(size.width, size.height);
   }
 #else
   // tvOS doesn't have device orientation, always return landscape size
-  return CGSizeMake(portraitSize.height, portraitSize.width);
+  return CGSizeMake(size.height, size.width);
 #endif
 }
 

packages/react-native/React/CoreModules/RCTAlertController.mm

@@ -23,6 +23,7 @@ - (UIWindow *)alertWindow
     UIWindowScene *scene = RCTKeyWindow().windowScene;
     if (scene != nil) {
       _alertWindow = [[UIWindow alloc] initWithWindowScene:scene];
+      _alertWindow.frame = scene.coordinateSpace.bounds;
     } else {
       _alertWindow = [[UIWindow alloc] initWithFrame:UIScreen.mainScreen.bounds];
     }