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

从 React/Fabric/Surface/RCTFabricSurface.mm 中的 RCTFontSizeMultiplier() 开始，然后运行链接的 react-native-font-scaling-repro 设置，并在其 share extension 中重现问题。使用自定义字体的 Text 组件、系统字体以及 app extension 中的 Dynamic Type 行为验证此更改。当自定义字体文本能够以正确的缩放比例显示，且不需要 allowFontScaling={false} 时，即表示完成。