Instagram / IGListKit
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meta-codesync[bot] 
Add IGListExperimentSkipNilIndexPathFiltering to opt into nil-index-path-aware visible cell filtering

Summary:
There's a subtle bug in `-[IGListAdapter visibleCellsForSectionController:]` and `-[IGListAdapter fullyVisibleCellsForSectionController:]`... both methods filter `[collectionView visibleCells]` by `[collectionView indexPathForCell:cell].section == section`, but `indexPathForCell:` is allowed to return `nil` for cells in transitional states — mid-`prepareForReuse`, mid-detach during a `performBatchUpdates`, or in flight between dequeue and final attachment. When that happens, sending `-section` to a `nil` `NSIndexPath` returns `0` in Objective-C, so any visible cell with a cleared index path is silently treated as belonging to **section 0**.

For most consumers this is harmless or invisible. But callers that walk these results and then read layout / window-relative geometry from each returned cell can end up touching cells whose model has been swapped out. We've seen this surface as crashes when a consumer triggers a layout pass on a cell whose backing data is mid-replacement (for example, text typesetting on an `NSAttributedString` that's being swapped out as the cell prepares for reuse). It's also a logical correctness issue: section 0 is the only section that can ever observe transitional cells from other sections leaking in.

The fix itself is essentially one line: skip cells where `indexPathForCell:` returned `nil` before doing the section comparison. The interesting question is how to roll it out.

Given how far-reaching this could be for code depending on IGListKit, I've introduced this as an opt-in experiment so we can effectively test this in Instagram and Threads before iterating or making an informed decision.

The fix is strictly subtractive — it can only narrow the result set, never expand it. Most consumers will just get more correct results. But these methods are called from a wide variety of places across multiple apps that de... (continued)

10 of 10 new or added lines in 1 file covered. (100.0%)

11 existing lines in 1 file now uncovered.

4512 of 4569 relevant lines covered (98.75%)

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Builds Branch Commit Type Ran Committer Via Coverage
26610259200 main Add IGListExperimentSkipNilIndexPathFiltering to opt into nil-index-path-aware visible cell filtering Summary: There's a subtle bug in `-[IGListAdapter visibleCellsForSectionController:]` and `-[IGListAdapter fullyVisibleCellsForSectionController... push meta-codesync[bot] github
98.75
25713764463 main De-genericize IGListBindingSectionController for Swift migration Summary: ## Plan This module doesn't need A/B testing and will be migrated directly. ## This Diff Removes the `__covariant ObjectType : id<IGListDiffable>` lightweight generic type... push meta-codesync[bot] github
98.82
25202427585 main Fix heap corruption crash from concurrent array mutation during diff (#1578) Summary: Fixes https://github.com/Instagram/IGListKit/issues/1578 Callers of `IGListDiff`/`IGListDiffPaths` may pass `NSMutableArray` instances backed by collections th... push meta-codesync[bot] github
98.82
24539907142 main Replace IGFailAssert with NSLog for delegate recursion checks Summary: The delegate recursion protection code (for GitHub issue #1658) uses IGFailAssert to warn about invalid delegate configurations. However, IGFailAssert is defined as NSCAssert ... push meta-codesync[bot] github
98.82
22090533742 main Refine Demo Apps for iOS 26 (#1655) Summary: ## Changes in this pull request I went through and improved some of the demo apps UI to look a bit cleaner on iOS 26. ### Checklist - [x] All tests pass. Demo project builds and runs. - [x] I added ... push meta-codesync[bot] github
98.81
22090353914 main Updated the IGListKit docs to v5.2.0 (#1656) Summary: ## Changes in this pull request Updates the IGListKit docs to the latest API spec in v5.2.0 ### Checklist - [x] All tests pass. Demo project builds and runs. - [x] I added tests, an experim... push meta-codesync[bot] github
98.81
21429050053 main Bring Test Coverage back to > 99% (#1654) Summary: ## Changes in this pull request It's been a few years since I did an audit of our test suite. The majority of changes to IGListKit since then were adding additional error checking and hardening,... push meta-codesync[bot] github
98.77
21417204291 main Replaced FBReportMustFix with IGFailAssert (#1653) Summary: ## Changes in this pull request `FBReportMustFix` isn't recognized by the public version of IGListKit. It is wrapped by `IGFailAssert` to abstract error tracking between the public and ... push meta-codesync[bot] github
95.73
21085207139 main Run 'facebook-bad-const-placement-check fbtidy' linter on fbobjc via arc cast Reviewed By: Daij-Djan Differential Revision: D90872770 fbshipit-source-id: 7a13ea11c push meta-codesync[bot] github
95.79
20972428095 main improve assert details in IGListAdapter+UICollectionView Summary: These asserts could be very helpful to debug tricky crashes, but they don't tell us enough about which object/section-controller are returning invalid data. The backtrace itself do... push meta-codesync[bot] github
95.79
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