Disappearing items in Masonry layout

[j0sip]

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:

flashlist-issue.mp4

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

[j0sip]

I have opened an PR (#2104) with a potential fix. I'm happy to make any necessary adjustments or discuss a different approach. Looking forward to your feedback.