title: supplementalTargetForAction:sender: doesn't get called
body: <p>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 <code>supplementalTargetForAction:sender:</code> on the splitViewController to provide the child view controller to the responder chain (<a href="https://stackoverflow.com/a/30938725/921573">see this post</a>). </p>
  
  <p>However, <code>supplementalTargetForAction:sender:</code>never gets called. </p>
  
  <p>The documentation states:</p>
  
  <blockquote>
    <p>If this NSResponder instance does not itself respondsToSelector:, then
    supplementalTargetForAction:sender: is called.</p>
  </blockquote>
  
  <p>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. </p>
  
  <p>Here's my code:</p>
  
  <pre><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
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/D9uDU.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/D9uDU.png" alt="storyboard sccreenshot"></a></p>
  
  <p>What am I doing wrong?</p>