_eventFirstResponderChainDescription

Stephan Casas:

AppKit includes a private category on NSApplication that adds _eventFirstResponderChainDescription — a string describing the current responder chain. This can be a really useful debugging tool!

When your views aren’t handling input events in the way you’d expect, consider dropping this extension into your project to see what’s what[…]

Update (2024-03-25): You can also set the _NS_4445425547 user default to see a Cocoa debug menu. I tend to just leave this enabled in my apps.

Update (2024-04-11): Antoine:

Today’s Darwin crazy hidden debugging tool of the day: iOS has a built in HUD for showing performance statistics like FPS, frame duration etc. […] This HUD can be activated by calling the private CARenderServerSetDebugOption function

See also: Marcin Krzyzanowski.