Disappearing items in Masonry layout

Description

When using FlashList with masonry enabled and optimizeItemArrangement set to false, items can sometimes disappear during scrolling. The issue is more likely to happen where there is a difference in column heights.

Current behavior

When there is a significant discrepancy of visible items' indices, cards sometimes disappear unexpectedly while scrolling. The root cause seems to be that the layout uses binary search for calculating visible items; and because binary search expects sorted items, the algorithm fails when there is a difference in indices between columns

Video:

https://github.com/user-attachments/assets/68e9a65a-4981-4675-b705-4985e300f491

Expected behavior

Cards should not disappear or flicker as user scrolls.

Reproduction

The issue can be reproduced in fixture app.

1. Disable optimizeItemArrangement in ComplexMasonry example:

        optimizeItemArrangement={false}

2. Open the ComplexMasonry screen and scroll down (see video above)

Platform

• iOS
• Android
• Web (if applicable)

Environment

FlashList version: "2.2.2"

Additional context

Checklist

• I've searched existing issues and couldn't find a duplicate
• I've provided a minimal reproduction (Expo Snack preferred)
• I'm using the latest version of @shopify/flash-list
• I've included all required information above

Start with fixture/react-native/src/ComplexMasonry.tsx at the optimizeItemArrangement setting, then reproduce the disappearance by opening ComplexMasonry and scrolling with it disabled. Trace the visible-item calculation described in the issue, including its binary-search assumption across columns. Done means cards no longer disappear or flicker on both iOS and Android in this reproduction.