[Android] Dimensions.get('screen').scale reports primary display density on secondary displays, breaking Fabric layout

[DouweBos]

Description

On Android, when a React Native activity runs on a secondary display (Samsung DeX, external monitor via desktop mode, ChromeOS desktop windowing, virtual/emulator display via am start --display N), the activity renders at the wrong scale. Content is laid out as if the surface were larger than the activity window, the visible region is a fraction of the window with the rest left black, and text rendering is sub-pixel-blurry.

Dimensions.get('window') and useWindowDimensions() report the activity's actual display correctly. Only Dimensions.get('screen').scale is wrong: it reports the device's primary-display density regardless of which display the activity is on.

window: { width: 1600, height: 720, scale: 1.5 }   ← matches the activity's display
screen: { width: 800,  height: 360, scale: 3   }   ← primary display's scale

Steps to reproduce

1. Boot a phone-form-factor AVD (verified on Pixel 9 Pro, API 36).
2. Attach a virtual secondary display at a different density than the primary (Pixel 9 Pro is 3.0×, 240dpi on the secondary is enough):

   adb emu multidisplay add 1 2400 1080 240 0

3. Install the reproducer, start Metro, launch onto the secondary display:

   cd ReproducerApp && npm install && npm run android && npm start &
   adb shell am force-stop com.reproducerapp
   adb shell am start -n com.reproducerapp/.MainActivity --display 3

Expected: screen.scale === window.scale when the activity is on a single display.

Actual: screen.scale is the primary's density; the React surface lays out at the primary's scale into a window that doesn't have that scale.

React Native Version

0.85.3

Affected Platforms

Runtime - Android

Areas

Fabric - The New Renderer, Bridgeless - The New Initialization Flow

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

System:
  OS: macOS 26.3.1
  CPU: (14) arm64 Apple M4 Pro
  Memory: 506.70 MB / 48.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 24.13.0
    path: /Users/douwe/.nvm/versions/node/v24.13.0/bin/node
  Yarn:
    version: 1.22.22
    path: /opt/homebrew/bin/yarn
  npm:
    version: 11.6.2
    path: /Users/douwe/.nvm/versions/node/v24.13.0/bin/npm
  Watchman:
    version: 2025.05.26.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.2
    path: /opt/homebrew/bin/pod
SDKs:
  Android SDK:
    API Levels:
      - "31"
      - "32"
      - "33"
      - "34"
      - "35"
      - "36"
    Build Tools:
      - 30.0.3
      - 34.0.0
      - 35.0.0
      - 36.0.0
      - 36.1.0
    Android NDK: Not Found
IDEs:
  Android Studio: 2025.3 AI-253.29346.138.2531.14876573
Languages:
  Java:
    version: 21.0.10
    path: /usr/bin/javac
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: 0.85.3
    wanted: 0.85.3
Android:
  hermesEnabled: true
  newArchEnabled: true

Stacktrace or Logs

adb logcat from the reproducer on the secondary display (2400x1080 @ 240dpi):

ReactNativeJS: '[repro] Dimensions.initial', '{"window":{"width":1600,"height":720,"scale":1.5,"fontScale":1},"screen":{"width":800,"height":360,"scale":3,"fontScale":1}}'

MANDATORY Reproducer

https://github.com/DouweBos/rn-secondary-display-repro

Screenshots and Videos

2400×1080 @ 240dpi secondary — cut off text, surface laid out at the wrong density:

Same with an app-side workaround that overwrites DisplayMetricsHolder with the activity's actual metrics — screen.scale matches window.scale, text renders crisply:

---

Related: #55659.