On macOS 27.0 (26A428) with Xcode 27.0, Apple's unmodified sample Building a document-based app with SwiftUI stops responding permanently when another process replaces an open document's file through NSFileCoordinator.
AppKit reverts the document on the main thread, which then blocks in a semaphore wait inside SwiftUI:
-[NSDocument relinquishPresentedItemToWriter:]_block_invoke_8 (in AppKit)
-[NSDocument _revertToVersion:preservingFirst:error:] (in AppKit)
-[NSDocument revertToContentsOfURL:ofType:error:] (in AppKit)
URLPlatformDocument.read(from:ofType:) (in SwiftUI)
_dispatch_semaphore_wait_slow (in libdispatch.dylib)
No other thread is reading the document. In another app using Document, its DocumentReader was never called, and a document with unsaved changes hung the same way in URLPlatformDocument.write(to:ofType:for:originalContentsURL:).
Reproduction (the sample plus a short script): <https://github.com/DePasqualeOrg/swiftui-document-revert-hang>
Feedback report: FB24792850