Comment by krackers

Neat, good to know! And it does seem my mental model of event loop was broken. Accessibility related interactions don't have any related NSEvent.

They are handled as part of the "conceptual" run loop, but they seem to be dispatched internally by AXRuntime library from a callback off some mach port. And because of this, the call to nextEventMatchingEventMask in the main -[NSApplication run] loop never even sees any such NSEvent.

    -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:]  (in AppKit)
        _DPSNextEvent  (in AppKit)
          _BlockUntilNextEventMatchingListInModeWithFilter  (in HIToolbox)
            ReceiveNextEventCommon  (in HIToolbox)
              RunCurrentEventLoopInMode  (in HIToolbox)
                CFRunLoopRunSpecific  (in CoreFoundation)
                  __CFRunLoopRun  (in CoreFoundation)
                    __CFRunLoopDoSource1  (in CoreFoundation)
                      __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__  (in CoreFoundation)
                        mshMIGPerform  (in HIServices)
                          _XPerformAction  (in HIServices)
                            _AXXMIGPerformAction  (in HIServices)

In some sense this is sort of similar to apple events, which are also "hidden" from the caller of nextEventMatchingEventMask. From what I can see those are handled by DPSNextEvent, which sorts based on the raw carbon EventRef. aevt types have `AEProcessAppleEvent` called on them, then the event is just consumed silently. Others get converted to a CGEvent and returned back to caller for it to handle. But of course accessibility events didn't exist in Classic mac, so they can't be handled at this layer so they were pushed further down. You can almost see the historical legacy here..

[1] https://www.cocoawithlove.com/2009/01/demystifying-nsapplica...