Fabric Unmount Crash (-[RCTViewComponentView unmountChildComponentView:index:]) when using unstable-native-tabs on iOS 27 Beta

[darshanraashmi]

Description

After resolving the initial launch crash, the app successfully booted, but crashed with a fatal EXC_CRASH (SIGABRT) whenever the app attempted to mount or unmount Expo Router's Native Tabs (expo-router/unstable-native-tabs).

Crash Stack Trace Highlight:

text

Thread 0 Crashed
-[NSAssertionHandler handleFailureInFunction:file:lineNumber:description:] + 268
-[RCTViewComponentView unmountChildComponentView:index:] + 2476
RCTPerformMountInstructions

Steps to reproduce

This is a crash occurring deep inside React Native's New Architecture (Fabric) rendering engine. unstable-native-tabs uses react-native-screens under the hood to interface with the native iOS UITabBarController.

On iOS 27 Beta, when navigating or conditionally rendering these native tabs, the native view hierarchy managed by react-native-screens falls out of sync with Fabric's shadow tree. When Fabric issues the RCTPerformMountInstructions to unmount a child view, the native RCTViewComponentView fails an assertion because the view it is trying to unmount is in an unexpected state, triggering the SIGABRT.

How We Solved It (The Workaround)
Since this is a deep native bug between Fabric and react-native-screens on the iOS 27 Beta SDK, we bypassed the buggy native code entirely at the JavaScript level.

We added a platform and version check in our router configuration (app-navigator.tsx and _layout.tsx). If the device is running iOS 27, we dynamically skip rendering the (which triggers NativeTabs) and instead fall back to our custom JavaScript-based @react-navigation/bottom-tabs.

Our Code Solution:

javascript

const IS_IOS = Platform.OS === 'ios';
const iosVersion = parseInt(String(Platform.Version), 10);
// iOS 27 Beta causes react-native-screens to crash in Fabric on unmount.
const IOS_27_BETA_BROKEN = iosVersion === 27;
// iOS 18+ uses NativeTabs. iOS < 18 (and iOS 27 Beta) use the custom JS tab bar fallback.
if (IS_IOS && iosVersion >= 26 && !IOS_27_BETA_BROKEN) {
return ;
}
This avoids invoking the native UITabBarController bridging on iOS 27, keeping the app stable while still giving iOS 18+ users the premium native tabs experience.

Snack or a link to a repository

Since this involves a deep native Fabric crash on a beta iOS version, a Snack cannot reproduce it (Snack does not support New Architecture on iOS 27 beta). However, any standard Expo project using Expo Router's unstable-native-tabs on iOS 27 beta with the New Architecture enabled will reproduce this.

Screens version

4.23.0

React Native version

0.83.6

Platforms

iOS

JavaScript runtime

Hermes

Workflow

Expo managed workflow

Architecture

Fabric (New Architecture)

Build type

Debug mode

Device

Real device

Device model

iPhone 17 Pro Max, iOS 27 beta, Xcode 27 beta

Acknowledgements

Yes

[github-actions]

Hey! 👋

The issue doesn't seem to contain a minimal reproduction.

Could you provide a snack or a link to a GitHub repository under your username that reproduces the problem?

[LKuchno]

Hi, @darshanraashmi darshanraashmi I tried reproducing the issue. I created a simple expo-router project using the template, and modified it to have tabs:

Setup

• Started from the default expo-router template — Expo SDK 56.0.12, expo-router 56.2.11, React Native 0.85.3, react-native-screens 4.25.2, New Architecture / Fabric enabled.
• Added a nested route group (tabs) that renders NativeTabs from expo-router/unstable-native-tabs.
• Kept the tab group behind a plain Stack screen so the native tab bar is mounted on push and unmounted on router.back() - to exercise exactly the mount/unmount path from the report.

Build issues on the Xcode beta / iOS 27 SDK (separate from the reported crash)

Before I could run anything, the build failed with Swift compiler/SDK mismatch errors coming from Expo's precompiled Swift modules - the JS runtime module (ExpoModulesJSI / JavaScriptRuntime.swift) and other modules wired into the app/UIScene lifecycle (ExpoModulesWorklets, ExpoUI). Expo issues reports: expo/expo#46664 and expo/expo#46876

I worked around it by forcing the Expo modules to build from source (while keeping prebuilt React Native) via expo-build-properties:

["expo-build-properties", { "ios": { "usePrecompiledModules": false } }]

After that, the app built and ran successfully using Xcode beta2 on an iPhone 17 Pro real device and also simulator with iOS 27.

Result: I could not reproduce the crash.
Mounting and unmounting the NativeTabs repeatedly did not trigger the EXC_CRASH (SIGABRT) / -[RCTViewComponentView unmountChildComponentView:index:] abort. The app stayed alive across the mount/unmount cycles.

ScreenRecording_06-25-2026.12-53-33_1.MP4

To investigate further, we would need a minimal reproduction example. We would also appreciate a video recording of the bug.

Thanks!

[darshanraashmi]

Hi @LKuchno LKuchno,

Thank you for looking into this and sharing your reproduction steps.

I have just published a minimal reproduction repository at [native-tabs-bug-repro](https://github.com/darshanraashmi/native-tabs-bug-repro). It is configured to match the environment where I am encountering the EXC_CRASH (SIGABRT) during the mount/unmount cycle of NativeTabs.

Regarding the build issues you encountered, I also wanted to point out [Issue #47119](expo/expo#47119), which I opened recently regarding unstable-native-tabs behavior on iOS 27. It seems there may be broader regressions or breaking changes in how NativeTabs interfaces with the updated UIKit APIs on the iOS 27 SDK.

Please let me know if you are able to replicate the crash using the new repo, or if you need any further logs or information from my end.

[LKuchno]

Hi @darshanraashmi,

Thanks for putting together the minimal reproduction!
I cloned native-tabs-bug-repro and ran it after running npx expo prebuild and applying the modifications needed to work around the Expo build issues mentioned in my previous comment.

After this I installed the build via Xcode beta 2 onto an iPhone 17 Pro running iOS 27 beta 2.
Result: I still can't reproduce the crash.
Repeatedly mounting and unmounting the NativeTabs (I used toggle on the screen) did not trigger the EXC_CRASH (SIGABRT) / -[RCTViewComponentView unmountChildComponentView:index:] abort. The app stayed alive across every mount/unmount cycle, same as with my own earlier project.

ScreenRecording_06-26-2026.08-36-17_1.MP4

To help narrow this down, could you share:

• The exact iOS build number on your device.
• The precise steps for preparing the app to build, and the interaction sequence on device that triggers the crash.
• A video recording of the bug.

I did see your related #47119 on unstable-native-tabs behavior on iOS 27 - agreed there may be broader UIKit API changes in play on the new SDK, so it's possible this is environment specific.

Thanks!