[PixPMusic]

Description

On the iOS 27.1 RC1 Duo simulator, the split host reports oneBesideSecondary → secondaryOnly while expanding, before the primary column appears and before onExpand. At that point UIKit still reports isCollapsed = YES, although the host's current horizontal size class is regular. A consumer tracking only collapse/expand completion can therefore mistake expansion for a compact Back action and clear the selected thread.

Expose the host's current horizontalSizeClass on the existing onDisplayModeWillChange event. This is a prerequisite for the T3 navigation fix; the library change alone does not change navigation or toolbar placement. It adds no new event or delegate and does not change completion-event semantics.

Changes

Pass the current host trait into the existing Fabric event as compact, regular, or unspecified, and expose the same union in the public event type. No package-version bump or release archive is included.

Test plan

Library TypeScript, scoped ESLint/Prettier, native formatting and Fabric code generation passed. A local test archive applied only this source change to the T3 app's existing t3.17 dependency, and the SDK 27.1 simulator native build passed.

LLDB confirmed regular traits with stale isCollapsed = YES during expansion, and compact traits during a real folded Back. A consumer using the payload passed two cycles of folded Back → select another thread → unfold → fold, with the selected thread visible throughout. The event trace shows horizontalSizeClass: regular before primary appearance during expansion; the router no longer pops there.

Consumer evidence: main reproduces the hidden second selection; the companion consumer opens both selections. This is the same folded Duo, fixture and theme. The recordings demonstrate the native prerequisite together with the app router reconciliation, not a standalone library UI change.

Minimal consumer probe:

import React from 'react';
import { Button, Text, View } from 'react-native';
import { Split, type SplitHostCommands } from 'react-native-screens';

export default function SplitTraitProbe() {
  const host = React.useRef<SplitHostCommands>(null);
  return (
    <Split.Host
      ref={host}
      preferredDisplayMode="oneBesideSecondary"
      onDisplayModeWillChange={({ nativeEvent }) => console.log(nativeEvent)}
      onCollapse={() => console.log('collapse')}
      onExpand={() => console.log('expand')}>
      <Split.Column>
        <View style={{ flex: 1 }}>
          <Text>Primary</Text>
          <Button title="Open detail" onPress={() => host.current?.show('secondary')} />
        </View>
      </Split.Column>
      <Split.Column>
        <View style={{ flex: 1 }}>
          <Text>Detail</Text>
          <Button title="Back to primary" onPress={() => host.current?.show('primary')} />
        </View>
      </Split.Column>
    </Split.Host>
  );
}

Open a thread while unfolded, fold, press Back, select another thread, and unfold. Inspect the expansion event before primary appearance; its size class should be regular. Android does not implement this iOS split host. The final app dependency update must wait for a maintainer release/archive from t3/main.

Model: GPT-6.1 Sol | Harness: Codex in T3 Code

[PixPMusic]

@juliusmarminge This is the native prerequisite for the reproducible Duo navigation bug: expansion currently looks like a compact Back to the consumer until the late onExpand callback. The current trait payload was verified in a rebuilt SDK 27.1 client and two full folded Back/select/unfold/fold cycles. If accepted, the T3 app needs a released archive containing it before its dependency update can be ready. This PR does not move controls or change the UI layout.