【Android&IOS】Percentage calculations ignore the parent container’s padding and are based on the content box instead of the border box.

Description

When the parent container has horizontal padding, using
left: '50%' + transform: [{ translateX: '-50%' }]
to center an absolutely positioned element results in the element being centered relative to the content box (excluding padding), rather than the full container width.

This behavior is inconsistent with web CSS, where left: 50% is calculated relative to the border box (including padding).

Additionally, testing shows that setting width: '50%' behaves consistently with left: '50%' — the percentage value is also calculated excluding padding, meaning it uses the content box width as the reference instead of the full container width.

As a result, both position and size percentages are computed relative to the content area, not the visual box including padding.

Both iOS and Android show the same issue

Steps to reproduce

1. clone the reproduction repo and install dependencies
2. npm run ios or npm run android

React Native Version

0.82.1、0.77.2

Affected Platforms

Runtime - iOS, Runtime - Android

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

System:
  OS: macOS 14.2.1
  CPU: (8) arm64 Apple M3
  Memory: 133.56 MB / 16.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 23.10.0
    path: /Users/didi/.nvm/versions/node/v23.10.0/bin/node
  Yarn:
    version: 1.22.22
    path: /Users/didi/.nvm/versions/node/v23.10.0/bin/yarn
  npm:
    version: 10.9.2
    path: /Users/didi/.nvm/versions/node/v23.10.0/bin/npm
  Watchman:
    version: 2024.10.21.00
    path: /usr/local/bin/watchman
Managers:
  CocoaPods:
    version: 1.15.2
    path: /usr/local/bin/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 23.4
      - iOS 17.4
      - macOS 14.4
      - tvOS 17.4
      - visionOS 1.1
      - watchOS 10.4
  Android SDK: Not Found
IDEs:
  Android Studio: 2023.3 AI-233.14808.21.2331.11709847
  Xcode:
    version: 15.3/15E204a
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.11
    path: /usr/bin/javac
  Ruby:
    version: 3.1.0
    path: /Users/didi/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.0.0
    wanted: 20.0.0
  react:
    installed: 19.1.1
    wanted: 19.1.1
  react-native:
    installed: 0.82.1
    wanted: 0.82.1
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: Not found
  newArchEnabled: false

Stacktrace or Logs

no log

MANDATORY Reproducer

https://github.com/yandadaFreedom/RN-bug-repo/tree/bug-left-percent

Screenshots and Videos

https://github.com/yandadaFreedom/RN-bug-repo/tree/bug-left-percent にある必須の再現コードから始め、npm run ios または npm run android を実行します。親に両プラットフォームで水平方向の padding がある場合の、パーセント指定による位置決めとサイズ設定を比較します。iOS と Android で、報告された border-box の期待どおりに動作が一貫していれば完了です。