[Strict TS API] NativeSyntheticEvent doesn't match BaseSyntheticEvent

Description

My team uses unified codebase that share web and native code, so that is why a BaseSyntheticEvent event was used here.

import type {BaseSyntheticEvent} from 'react';

<TextInput onSubmitEditing={
  (event: BaseSyntheticEvent) => {
        event.bubbles; // boolean | undefined 
         // error: BaseSyntheticEvent require `boolean` only type
  }} 
/>

this issue doesn't not exist in prev. RN versions

Steps to reproduce

see above

React Native Version

0.87.0

Affected Platforms

Runtime - Android, Runtime - iOS

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

System:
  OS: macOS 26.5
  CPU: (11) arm64 Apple M3 Pro
  Memory: 1.27 GB / 36.00 GB
  Shell:
    version: 4.0.1
    path: /opt/homebrew/bin/fish
Binaries:
  Node:
    version: 24.13.1
    path: /usr/local/bin/node
  Yarn:
    version: 3.6.4
    path: /opt/homebrew/bin/yarn
  npm:
    version: 11.8.0
    path: /usr/local/bin/npm
  Watchman:
    version: 2025.09.01.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.2
    path: /Users/xdxd/.rbenv/shims/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.2
      - iOS 26.2
      - macOS 26.2
      - tvOS 26.2
      - visionOS 26.2
      - watchOS 26.2
  Android SDK:
    API Levels:
      - "34"
      - "35"
      - "36"
    Build Tools:
      - 33.0.1
      - 34.0.0
      - 35.0.0
      - 36.0.0
      - 37.0.0
    System Images:
      - android-34 | Android TV ARM 64 v8a
      - android-34 | Google TV ARM 64 v8a
      - android-35 | Google APIs ARM 64 v8a
      - android-35 | Google Play ARM 64 v8a
      - android-36 | Android TV ARM 64 v8a
      - android-36 | Google TV ARM 64 v8a
    Android NDK: Not Found
IDEs:
  Android Studio: 2026.1 AI-261.23567.138.2611.15646644
  Xcode:
    version: 26.2/17C52
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.13
    path: /usr/bin/javac
  Ruby:
    version: 2.6.10
    path: /Users/xdxd/.rbenv/shims/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.2.0
    wanted: 20.2.0
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.87.0
    wanted: 0.87.0
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: Not found
  newArchEnabled: false

Stacktrace or Logs

error TS2322: Type '(event: BaseSyntheticEvent) => void' is not assignable to type '(e: Readonly<{ bubbles: boolean | undefined; cancelable: boolean; currentTarget: number | ReactNativeElement; defaultPrevented: boolean; dispatchConfig: Readonly<{ registrationName: string; }>; ... 10 more ...; type: string; }>) => unknown'.
  Types of parameters 'event' and 'e' are incompatible.
    Type 'Readonly<{ bubbles: boolean | undefined; cancelable: boolean; currentTarget: number | ReactNativeElement; defaultPrevented: boolean; dispatchConfig: Readonly<{ registrationName: string; }>; ... 10 more ...; type: string; }>' is not assignable to type 'BaseSyntheticEvent<object, any, any>'.
      Types of property 'bubbles' are incompatible.
        Type 'boolean | undefined' is not assignable to type 'boolean'.
          Type 'undefined' is not assignable to type 'boolean'.

27       <TextInput onSubmitEditing={(event: BaseSyntheticEvent) => {

MANDATORY Reproducer

https://github.com/retyui/repo_with_virus_to_steal_corporate_data_so_you_get_fired_and_spend_the_rest_of_your_life_in_prison

Screenshots and Videos

...

TextInput onSubmitEditing 型と、React Native 0.87 で使用されている NativeSyntheticEvent の定義から始めます。Issue にある厳密な TypeScript の代入を再現し、その後 React の BaseSyntheticEvent の型付けと比較します。関連するイベント型を弱めることなく報告されたハンドラーが互換になること、および示された例に対するリグレッションチェックがあることを完了条件とします。