NSCollectionLayoutBoundarySupplementaryItem background blur covering the entire layout section

My app has the following UI layout:

  • NSSplitViewController as the windows contentViewController
    • NSPageController in the content (right) split item
      • NSTabViewController as the root items of the NSPageController
        • NSViewController with a collection view in the first tab of that NSTabViewController

The collection view is using a NSCollectionViewCompositionalLayout in which the sections are set up to have a header using NSCollectionLayoutBoundarySupplementaryItem with pinToVisibleBounds=true and alignment=top

With macOS 26, the pinned supplementary item automatically gets a blurred/semi-transparent background that seamlessly integrates with the toolbar. When the window's title bar has a NSTitlebarAccessoryViewController added, the said semi-transparent background gets a bottom hard edge and a hairline to provide more visual separation from the main content.

During runtime, my NSPageController transitions from the NSTabViewController to another view controller. When transitioning back, the semi-transparent blur bleeds into the entire section. This happens no matter if there's a NSTitlebarAccessoryViewController added or not. It doesn't happen 100% of the cases, it seems to depend on section size, header visibility and/or scroll position. But it happens more often than not.

Most of the time, a second or so after the back transition - shortly after pageControllerDidEndLiveTransition: of the NSPageControllerDelegate is called - the view updates and the supplementary views are back to normal. Sometimes, the issue also appears not when transitioning using NSPageController, but simply by scrolling through the collection view.

Anyone has an idea what is happening here? Below are two screenshots of both the "ok" and "not ok" state

I'm on macOS 26.0.1 and I'm using XCode 26.0.1

NSCollectionLayoutBoundarySupplementaryItem background blur covering the entire layout section
 
 
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