README

API	supports modifiers-only shortcuts	event propagation	works through Secure Input	can work from background thread	Requires Input Monitoring permission
CGEvent.tapCreate	Yes	Can propagate or not	No (modifiers work)	Yes	No
RegisterEventHotKey/InstallEventHandler	No	Can't propagate; but not needed	Yes	No	No
NSEvent.addGlobalMonitorForEvents	Yes	Can't propagate	No (modifiers work)	No	No
CGSSetHotModifierWithExclusion + CGSSetHotKeyWithExclusion	Yes	Can't propagate	Yes	No	No
IOHIDManagerRegisterInputValueCallback	Yes	?	No (modifiers work)	Yes	Yes

Also to consider:

• macOS may send keyboard events disordered
• macOS may not send some keyboard events (e.g. when under heavy load)
• when macOS sends an event, other APIs may disagree (e.g. receiving a keyDown event for shift. Calling NSEvent.modifierFlags can return that shift is down, or that it's not down. It's data-racy.