Nested horizontal FlatList reports items as viewable while its parent row is outside the screen

Description

Description
System Info
React Native: 0.86.2
React: 19.2.3
React Native Gradle Plugin: 0.86.2
Node: 22.22.3
Project Node version (.nvmrc): 22.22.3
npm: 10.9.8
Yarn: 3.5.0
OS: Linux x86_64, kernel 6.8.0-136-generic
Android compile SDK: 36
Android target SDK: 36
Android min SDK: 24 (Android 7.0+)
Java: 17.0.19
Hermes: enabled
New Architecture: enabled

Expected behavior
For a horizontal FlatList nested inside a vertically scrolling FlatList, the inner list’s onViewableItemsChanged should not report an item as viewable until
the parent row is visible in the device viewport.

An item should satisfy both conditions:

Visible within the inner horizontal FlatList
AND
The parent row containing that FlatList is visible on screen

Actual behavior
onViewableItemsChanged for a nested horizontal FlatList fires when its parent row mounts, even if the parent row is below the device viewport.

The inner FlatList appears to calculate viewability relative to its own local scroll viewport only. It does not account for the outer vertical FlatList’s
clipping or whether the parent row is actually visible on the screen.

This produces false impression events for analytics: items are marked viewable before the user can see them.

Details
Our application has a vertical FlatList of sections. Several sections contain horizontal FlatLists.

On the initial screen:

Sections 1–4: visible in the device viewport
Sections 5–8: mounted by the outer FlatList but below the device viewport

The nested horizontal FlatLists in Sections 5–8 invoke onViewableItemsChanged immediately, reporting their initial items as viewable before the user scrolls
the outer list.

`import React, { useRef } from 'react';
import { FlatList, StyleSheet, Text, View } from 'react-native';

const SECTION_HEIGHT = 220;
const ITEM_WIDTH = 140;

const sections = Array.from({ length: 8 }, (_, index) => ({
id: section-${index + 1},
title: Section ${index + 1},
}));

const items = Array.from({ length: 8 }, (_, index) => ({
id: item-${index + 1},
title: Item ${index + 1},
}));

function NestedHorizontalList({ sectionId }) {
// These objects must retain their identity for the lifetime of a FlatList.
const viewabilityConfig = useRef({
itemVisiblePercentThreshold: 50,
minimumViewTime: 0,
waitForInteraction: false,
}).current;

const onViewableItemsChanged = useRef(({ viewableItems }) => {
console.log(
[${sectionId}] reported viewable before outer list is scrolled:,
viewableItems.map(({ item }) => item.id).join(', ')
);
}).current;

return (
<FlatList
horizontal
data={items}
keyExtractor={(item) => item.id}
renderItem={({ item }) => (

{item.title}

)}
viewabilityConfig={viewabilityConfig}
onViewableItemsChanged={onViewableItemsChanged}
showsHorizontalScrollIndicator={false}
/>
);
}

export default function App() {
return (
<FlatList
data={sections}
getItemLayout={(_, index) => ({
length: SECTION_HEIGHT,
offset: SECTION_HEIGHT * index,
index,
})}
keyExtractor={(item) => item.id}
renderItem={({ item }) => (

{item.title}

)}
/>
);
}

const styles = StyleSheet.create({
section: {
height: SECTION_HEIGHT,
paddingTop: 16,
borderBottomWidth: StyleSheet.hairlineWidth,
borderBottomColor: '#9ca3af',
},
sectionTitle: {
marginHorizontal: 16,
marginBottom: 12,
fontSize: 18,
fontWeight: '600',
},
item: {
width: ITEM_WIDTH,
height: 120,
marginLeft: 12,
alignItems: 'center',
justifyContent: 'center',
borderRadius: 8,
backgroundColor: '#dbeafe',
},
});`

For analytics, this is incorrect: “viewable” should represent visibility to the user, not only visibility within an off-screen nested list container.

Steps to reproduce

Reproduction steps

1. Render a vertical FlatList with multiple fixed-height rows.
2. Place a horizontal FlatList inside every row.
3. Add onViewableItemsChanged and viewabilityConfig to each inner horizontal FlatList.
4. Launch the screen without scrolling.
5. Observe that inner lists belonging to below-the-fold parent rows report items as viewable.

React Native Version

0.86.2

Affected Platforms

Runtime - Android, Runtime - iOS

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

System:
  OS: Linux 6.8 Ubuntu 22.04.5 LTS 22.04.5 LTS (Jammy Jellyfish)
  CPU: (16) x64 12th Gen Intel(R) Core(TM) i7-1270P
  Memory: 13.94 GB / 30.59 GB
  Shell:
    version: 5.1.16
    path: /bin/bash
Binaries:
  Node:
    version: 22.22.3
    path: /home/user/.nvm/versions/node/v22.22.3/bin/node
  Yarn:
    version: 3.5.0
    path: /home/user/.nvm/versions/node/v22.22.3/bin/yarn
  npm:
    version: 10.9.8
    path: /home/user/.nvm/versions/node/v22.22.3/bin/npm

SDKs:
  Android SDK:
    API Levels: 28, 30, 31, 32, 33, 34, 35, 36
    Build Tools: 36.1.0
    System Images: android-36 | Google Play Intel x86_64 Atom
  Android NDK: Not required for standard Expo development

Languages:
  Java:
    version: 17.0.19

npmPackages:
  react: 19.2.3
  react-native: 0.86.2
  expo: ~57.0.7

Android:
  hermesEnabled: true
  newArchEnabled: true

Stacktrace or Logs

NA

MANDATORY Reproducer

https://snack.expo.dev/@testophobia/-nested-flatlist-viewability-repro

Screenshots and Videos

No response

Bắt đầu bằng cách chạy trình tái hiện Snack được liên kết và theo dõi các callback FlatList-onViewableItemsChanged lồng nhau trong quá trình mount ban đầu và khi cuộn danh sách bên ngoài. Kiểm tra hành vi viewability của FlatList đối với danh sách ngang bên trong và xác minh kết quả trên Android và iOS. Được xem là hoàn tất khi các hàng parent nằm bên dưới vùng hiển thị không báo cáo các item cho đến khi parent hiển thị, trong khi các item bên trong đang hiển thị vẫn tiếp tục được báo cáo.