macOS: intermittent NSRangeException from _NSTouchBarFinderObservation removing a nextResponder observer

[h-rica]

An application built on gpui-kit intermittently aborts on macOS with an uncaught AppKit exception raised by the Touch Bar finder. No application frames are involved: the throw happens inside AppKit during a display-cycle flush on the main run loop.

Environment

• gpui-kit git revision 86f9c0fb5907549d9a5b0aff53b4b6865933a8e8 (GPUI gpui-pre =0.3.6)
• MacBookPro17,1 (M1, with Touch Bar), macOS 26.6.2
• Window opened with gpui_kit::open_window and TitleBar::window_options(), a TitleBar in the view, default key bindings and an app menu

Exception

*** Terminating app due to uncaught exception 'NSRangeException', reason: 'Cannot remove an observer <_NSTouchBarFinderObservation 0x…> for the key path "nextResponder" from <NSView 0x…> because it is not registered as an observer.'

Top of the exception backtrace:

Foundation  -[NSObject(NSKeyValueObserverRegistration) _removeObserver:forProperty:]
Foundation  -[NSObject(NSKeyValueObserverRegistration) removeObserver:forKeyPath:]
Foundation  -[NSObject(NSKeyValueObserverRegistration) removeObserver:forKeyPath:context:]
AppKit      -[_NSTouchBarFinderObservation invalidate]
AppKit      ___NSTouchBarFinderSetNeedsUpdateOnMain_block_invoke_2
AppKit      NSDisplayCycleObserverInvoke
AppKit      NSDisplayCycleFlush
QuartzCore  CA::Transaction::run_commit_handlers(CATransactionPhase)
QuartzCore  CA::Transaction::commit()
...
AppKit      -[NSApplication run]
<app>       gpui_macos::platform::MacPlatform::run

Reproduction

Rare and timing dependent. It occurred:

1. once during normal use of the app, about four minutes after launch, while focus was moving between two instances of the same app;
2. once in a single instance while programmatically switching the frontmost app between it and Finder (Accessibility AXFrontmost), within the first 25 switches.

A further ~320 switches did not reproduce it, neither in the app nor in a minimal gpui_kit::open_window window, so I cannot yet say whether anything specific to the app raises the rate.

Possibly related

• fix(editor): 关闭内置远程编辑器改为隐藏复用，规避 macOS Touch Bar 崩溃 (#262) feigeCode/navop#268 — a GPUI app hitting the same _NSTouchBarFinderObservation exception on an Intel Touch Bar Mac when closing a window; worked around by hiding windows with orderOut: instead of destroying them.
• GoogleUtilities swizzling causing crashes: Fatal Exception: NSRangeException Cannot remove an observer NSTouchBarFinderObservation firebase/firebase-ios-sdk#9371 — the same exception when a responder's class is modified at runtime. For context, gpui-base installs accessibilityHitTest: on the window's class via class_addMethod in macos_accessibility.rs; I have not shown that this is involved.