Text with custom fonts is invisible in iOS App Extensions (Fabric / New Architecture)

When using React Native 0.84+ with the new architecture in an iOS App Extension (Share Extension, Action Extension, etc.), all <Text> components that use a custom fontFamily are invisible. Text using the system font renders fine, but with the wrong size.

Setting allowFontScaling={false} on individual <Text> components works around the issue.

Root Cause

RCTFontSizeMultiplier() in React/Fabric/Surface/RCTFabricSurface.mm calls:

return mapping[RCTSharedApplication().preferredContentSizeCategory].floatValue;

In App Extensions, RCTSharedApplication() returns nil (because UIApplication.sharedApplication is unavailable in extensions). This causes:

1. nil.preferredContentSizeCategory → nil
2. mapping[nil] → nil
3. nil.floatValue → 0.0

This 0.0 is stored as layoutContext.fontSizeMultiplier, which propagates to all text measurement:

// In RCTFontWithFontProperties (prebuilt binary):
CGFloat effectiveFontSize = fontProperties.sizeMultiplier * fontProperties.size;
// effectiveFontSize = 0.0 * 14.0 = 0.0

System fonts survive because [UIFont systemFontOfSize:0] returns a 12pt default font. But custom fonts fail because [UIFont fontWithName:@"CustomFont" size:0] returns nil, falling through to a Helvetica fallback with incorrect metrics — resulting in zero-sized text layout.

Proposed Fix

Replace RCTSharedApplication().preferredContentSizeCategory with UITraitCollection.currentTraitCollection.preferredContentSizeCategory, which works in both apps and extensions:

// Before (broken in extensions):
return mapping[RCTSharedApplication().preferredContentSizeCategory].floatValue;

// After (works everywhere):
NSString *category;
if (!RCTRunningInAppExtension()) {
    category = RCTSharedApplication().preferredContentSizeCategory;
} else {
    category = UITraitCollection.currentTraitCollection.preferredContentSizeCategory;
}
return mapping[category ?: UIContentSizeCategoryLarge].floatValue;

This also correctly handles Dynamic Type accessibility settings in extensions, which the current code silently ignores.

Workarounds

Per-component (JS)

<Text style={{ fontFamily: 'Inter-Regular' }} allowFontScaling={false}>
  This text will be visible
</Text>

Native swizzle (ObjC++)

Swizzle RCTFabricSurface._updateLayoutContext to correct fontSizeMultiplier from 0 to 1.0 after the original implementation sets it. See ShareExtensionReactHelper.mm in the main project for a working implementation.

Steps to reproduce

1. Clone this https://github.com/erkie/react-native-font-scaling-repro
2. npm install && cd ios && bundle exec pod install && cd ..
3. npm run ios
4. Open Safari, navigate to any page, tap Share → select "FontScalingReproShareExtension"

React Native Version

0.84.1

Affected Platforms

Runtime - iOS

Areas

Fabric - The New Renderer

Output of npx @react-native-community/cli info

System:
  OS: macOS 26.3.1
  CPU: (14) arm64 Apple M4 Pro
  Memory: 172.81 MB / 48.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 24.13.0
    path: /Users/thingtesting/.local/state/fnm_multishells/75626_1775109555064/bin/node
  Yarn: Not Found
  npm:
    version: 11.6.2
    path: /Users/thingtesting/.local/state/fnm_multishells/75626_1775109555064/bin/npm
  Watchman:
    version: 2025.05.26.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.2
    path: /Users/thingtesting/.rbenv/shims/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.0
      - iOS 26.0
      - macOS 26.0
      - tvOS 26.0
      - visionOS 26.0
      - watchOS 26.0
  Android SDK:
    API Levels:
      - "34"
      - "35"
      - "36"
    Build Tools:
      - 34.0.0
      - 35.0.0
      - 36.0.0
    System Images:
      - android-34 | Google APIs ARM 64 v8a
      - android-34 | Google APIs Intel x86_64 Atom
    Android NDK: Not Found
IDEs:
  Android Studio: 2025.2 AI-252.27397.103.2522.14617522
  Xcode:
    version: 26.0/17A324
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.16
    path: /opt/homebrew/opt/openjdk@17/libexec/openjdk.jdk/Contents/Home/bin/javac
  Ruby:
    version: 3.4.5
    path: /Users/thingtesting/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.1.0
    wanted: 20.1.0
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.84.1
    wanted: 0.84.1
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: true

Stacktrace or Logs

Please review screenshots

MANDATORY Reproducer

https://github.com/erkie/react-native-font-scaling-repro

Screenshots and Videos

Inside app, works with custom fonts

Inside share extension, invisible texts

Bắt đầu trong React/Fabric/Surface/RCTFabricSurface.mm tại RCTFontSizeMultiplier(), sau đó chạy setup react-native-font-scaling-repro được liên kết và tái hiện vấn đề trong share extension của nó. Xác minh thay đổi với các component Text sử dụng font tùy chỉnh, font hệ thống và hành vi của Dynamic Type trong một app extension. Hoàn thành khi văn bản sử dụng font tùy chỉnh hiển thị với tỷ lệ chính xác mà không yêu cầu allowFontScaling={false}.