feat(android): implement preventNativeDismiss / onNativeDismissCancelled for formSheet

[wildseansy]

Problem

preventNativeDismiss and onNativeDismissCancelled were no-ops on Android. Because of this, navigation libraries (e.g. React Navigation) that rely on usePreventRemove + onNativeDismissCancelled to implement "unsaved changes" guards work correctly on iOS but are completely bypassed on Android formSheet — the sheet dismisses natively before the JS layer can intercept it.

Note: gestureEnabled / isDraggable support is tracked separately in #2937. This PR does not touch that prop.

Root cause

iOS exposes UIAdaptivePresentationControllerDelegate.presentationControllerShouldDismiss — a synchronous hook that can return NO before any visual motion occurs. Android's BottomSheetBehavior has no equivalent. By the time any callback fires, the sheet has already moved.

Solution

Implement the closest Android equivalent using BottomSheetBehavior.isHideable:

Step	What happens
preventNativeDismiss=true set	BottomSheetBehavior.isHideable = false → sheet cannot reach STATE_HIDDEN via gesture
User drags sheet downward	onSlide fires; slideOffset < -0.05f flags a dismiss attempt
User releases	Behavior snaps sheet back to last stable detent
Stable state fires	onNativeDismissCancelled is dispatched to JS
JS receives event	App responds — e.g. shows a "Discard changes?" confirmation

Runtime changes to preventNativeDismiss (e.g. a form going from clean → dirty) are reflected immediately via a property setter that updates BottomSheetBehavior.isHideable without requiring a re-mount.

The sheet moves slightly before bouncing back (unlike iOS where the gesture is cancelled before any visual motion), but the JS contract is identical on both platforms: preventNativeDismiss blocks the dismissal and onNativeDismissCancelled fires so the app can respond.

Example usage

The following pattern works on iOS today. After this PR, it also works on Android formSheet with no changes required in application code:

import { usePreventRemove } from '@react-navigation/native';
import { useState } from 'react';
import { Alert } from 'react-native';

function EditProfileScreen() {
  const [isDirty, setIsDirty] = useState(false);

  usePreventRemove(isDirty, ({ data }) => {
    // iOS:     fires because presentationControllerShouldDismiss returned NO
    // Android: fires via onNativeDismissCancelled after the sheet bounces back
    Alert.alert(
      'Discard changes?',
      'You have unsaved changes. Leave anyway?',
      [
        { text: 'Stay', style: 'cancel' },
        {
          text: 'Discard',
          style: 'destructive',
          onPress: () => navigation.dispatch(data.action),
        },
      ],
    );
  });

  return (
    <TextInput onChangeText={() => setIsDirty(true)} />
  );
}

Demo of fix in action

demo.mov

Files changed

File	Change
Screen.kt	Add isPreventNativeDismiss with a setter that keeps BottomSheetBehavior.isHideable in sync at runtime
ScreenViewManager.kt	Wire up setPreventNativeDismiss (was a no-op); register onNativeDismissCancelled event
SheetDelegate.kt	Apply isHideable = !screen.isPreventNativeDismiss during setup; SheetStateObserver tracks drag-toward-dismiss and dispatches event on bounce-back
ScreenEventEmitter.kt	Add dispatchOnNativeDismissCancelled()
ScreenNativeDismissCancelledEvent.kt	New event class (topNativeDismissCancelled)
src/types.tsx	Extend @platform annotations for preventNativeDismiss and onNativeDismissCancelled to include Android

Known differences from iOS

• The sheet moves slightly before bouncing back. iOS cancels the gesture before any visual motion via presentationControllerShouldDismiss. Android has no pre-gesture synchronous interception point.
• DISMISS_ATTEMPT_SLIDE_THRESHOLD = -0.05f is a heuristic. Open to feedback on whether this value needs tuning.
• Only formSheet presentation is affected. pageSheet falls back to modal on Android and is already JS-navigation-controlled.