MultilineGrowRepro

Bare React Native 0.86.0 app (New Architecture, iOS) showing what happens when an auto-growing multiline TextInput inside a ScrollView gets a new line. Modeled on the alt text dialog of the Bluesky app.

Steps

1. npm install && (cd ios && pod install) (pnpm works too; after switching package managers restart Metro with --reset-cache, a running Metro keeps a stale module map and serves a bundle that fails with ReferenceError: Property 'window' doesn't exist)
2. npx react-native run-ios. If another Metro already owns 8081, run npx react-native start --port 8082 here and point the app at it once: xcrun simctl spawn booted defaults write org.reactjs.native.example.MultilineGrowRepro RCT_jsLocation localhost:8082 (run-ios --port alone did not reach the bundle URL provider on 0.86)
3. Tap the field, type three short lines, pressing Return after each.
4. Press Return again so the box has to grow past its minHeight (3 lines).
5. Type one character on the new line.

What you see on Fabric (measured, iPhone 17 Pro simulator, iOS 26.5)

On Return the carret is moved to the new line but the scroll view scrolls up only after another new line or new character is entered.

Expected (old architecture)

The box grows within the same keystroke and nothing else moves.