supplementalTargetForAction:sender: doesn't get called

Question

I am trying to send an action from a menu item to a child view controller of a NSSplitViewController. As I understand I am supposed to override supplementalTargetForAction:sender: on the splitViewController to provide the child view controller to the responder chain (see this post).

However, supplementalTargetForAction:sender:never gets called.

The documentation states:

If this NSResponder instance does not itself respondsToSelector:, then supplementalTargetForAction:sender: is called.

In a debugging effort I have created a fresh Xcode project with just a normal viewController, overridden the method there to see if it gets called - it doesn't.

Here's my code:

@implementation ViewController

- (BOOL)respondsToSelector:(SEL)aSelector {
    NSString *s  = NSStringFromSelector(aSelector);
    if ([s hasPrefix:@"foo"]) {
        // let's pretend we do not respond here.
        // I expect `supplementalTargetForAction:sender:` to get called.
        return NO;
    }
    return [super respondsToSelector:aSelector];
}

- (id)supplementalTargetForAction:(SEL)action sender:(id)sender {
    // nobody calls me ever :(
    return [super supplementalTargetForAction:action sender:sender];
}

- (IBAction)foo:(id)sender {
    NSLog(@"Foo");
}

@end

What am I doing wrong?