I filed FB24577577 about the Dock disappearing after WKWebView element fullscreen in a macOS accessory app. I retested the original minimal reproduction on macOS 27.0 (26A428), and the symptom still occurs on the built-in display alone. Is this combination of public AppKit/WebKit APIs supported? Is there an application-side workaround that preserves both an accessory activation policy and a window that joins all Spaces? Reproduction The app uses: NSApplication.ActivationPolicy.accessory (no Dock tile). NSWindow.CollectionBehavior.canJoinAllSpaces on the window hosting the WKWebView. WKPreferences.isElementFullscreenEnabled = true. Element.requestFullscreen() to enter fullscreen. A normal titled window at normal window level and a local HTML page containing a single div are sufficient. No external website, video, networking, custom window subclass, or private API is involved. The key AppKit/WebKit configuration is: NSApplication.shared.setActivationPolicy(.accessory) window.collectionBehavior = [.canJoinAllSpaces] configuration.preferences.isElementFullscreenEnabled = true Load a local HTML page with one clickable div whose click handler calls requestFullscreen() on that element. Keep the application running and retain the window/web view. With Dock auto-hide disabled, enter element fullscreen, then exit and inspect the Dock. The macOS 27 retest entered by evaluating a click on the div through WKWebView.evaluateJavaScript, confirmed document.fullscreenElement was non-null, and exited with document.exitFullscreen(). The original macOS 26 testing also reproduced with Escape. Expected: the Dock resumes its normal always-visible behavior after fullscreen ends. Actual: the right-side Dock disappears despite auto-hide remaining disabled. macOS 27 retest (September 16, 2026) macOS 27.0, build 26A428. NSScreen enumerated exactly one built-in display at the start of this run. Dock on the right, auto-hide disabled throughout. Fullscreen entry was confirmed, and document.fullscreenElement was null after exit. Both NSApp.presentationOptions and NSApp.currentSystemPresentationOptions returned to 0. The Dock was visually absent after exit. Its on-screen window count changed from 1 to 0 and was still 0 six seconds later. Window counts were supporting evidence, not the sole visual criterion. An external display was reconnected later, so this retest does not establish indefinite persistence or that restarting Dock is the only recovery route. Earlier comparisons and product impact The original report was reproduced on macOS 26.6.2 (25G83). In those earlier comparisons, removing canJoinAllSpaces avoided the symptom; using native AppKit window fullscreen instead of WebKit element fullscreen also avoided it. These controls were not rerun on macOS 27. Restarting Dock restored it in the earlier tests. This affects Nifro, a desktop-wallpaper app using WKWebView. The current app keeps element fullscreen disabled. Joining all Spaces and having no Dock tile are useful parts of its wallpaper behavior, so changing either has a product cost. Could an AppKit/WebKit engineer advise whether this is an unsupported configuration or a system issue tracked by FB24577577, and whether a supported public-API workaround exists? I can provide further targeted diagnostics if needed.