https://stackoverflow.com/questions/53273191/custom-carbon-key-event-handler-fails-after-mouse-events

FilterableMenu.swift

import AppKit
import Carbon

// Custom NSMenu implementation allowing to filter key inputs.
class Menu: NSMenu {
  required init(coder decoder: NSCoder) {
    super.init(coder: decoder)
  }

  init() {
    super.init(title: "Test")

    // We add header with custom view which will intercept Carbon events.
    let headerItem = NSMenuItem(title: "filter", action: nil, keyEquivalent: "")
    headerItem.view = FilterMenuItemView()
    addItem(headerItem)

    // Ignore, this is just to fill menu with items.
    addItem(NSMenuItem(title: "test1", action: nil, keyEquivalent: ""))
    addItem(NSMenuItem(title: "test2", action: nil, keyEquivalent: ""))
  }
}

// Custom menu which listens to Carbon events allowing to filter input.
class FilterMenuItemView: NSView {
  private var eventHandler: EventHandlerRef?

  required init?(coder decoder: NSCoder) {
    super.init(coder: decoder)
  }

  init() {
    super.init(frame: NSRect(x: 0, y: 0, width: 20, height: 21))
  }

  // The most important part where event interception happens.
  override func viewDidMoveToWindow() {
    super.viewDidMoveToWindow()

    if window != nil {
      // 1/6: Window is loaded and we can attach our event listener
      if let dispatcher = GetEventDispatcherTarget() {
        // 2/6: Add handler callback
        let eventHandlerCallback: EventHandlerUPP = { eventHandlerCallRef, eventRef, userData in
          // 3/6: This callback simply propagates it downwards and prints response
          let response = CallNextEventHandler(eventHandlerCallRef, eventRef!)
          print("Response \(response)")
          return response
        }

        // 4/6: We need to listen only for key down and repeat events.
        let eventSpecs = [
          EventTypeSpec(eventClass: OSType(kEventClassKeyboard), eventKind: UInt32(kEventRawKeyDown)),
          EventTypeSpec(eventClass: OSType(kEventClassKeyboard), eventKind: UInt32(kEventRawKeyRepeat))
        ]

        // 5/6: Finally, add our event handler
        InstallEventHandler(dispatcher, eventHandlerCallback, 2, eventSpecs, nil, &eventHandler)
      }
    } else {
      // 6/6: Window is unloaded, so we need to remove our handler.
      RemoveEventHandler(eventHandler)
    }
  }
}

Menu().popUp(positioning: nil, at: NSEvent.mouseLocation, in: nil)