Section content area is extended unexpectedly for a boundary item

Originator:skiinfobc
Number:rdar://FB8977366 Date Originated:21/01/2021
Status:Open Resolved:
Product:UIKit Product Version:iOS 14.3
Classification:Incorrect/Unexpected Behavior Reproducible:yes
 
Related to UICollectionView with Compositional Layout.

When adding a NSCollectionLayoutBoundarySupplementaryItem with extendsBoundary=false, the boundary of the section is extended unexpectedly.

I’ve included a reproduction. When showing the supplementary item with extendsBoundary=false, I’d expect the item to be added without changing the size of the section. However as can be seen in the reproduction, the following section is pushed down with some whitespace when this supplementary item is added.

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