title: Drop-down menu in NSToolbar like Mail.app
body: <p>I'd like a toolbar button with an attached dropdown menu, like the "Flag" button in the toolbar in Mail.app:</p>
  
  <p><a href="https://i.stack.imgur.com/icKov.png" rel="noreferrer"><img src="https://i.stack.imgur.com/icKov.png" alt="Flag menu from Mail.app"></a></p>
  
  <p>I'd hoped that making a normal <code>NSMenuItem</code> and adding a menu as the <code>menuFormRepresentation</code> would do the trick, but that menu only appears when the button goes into overflow mode.</p>
  
  <p>I had also hoped that adding an <code>NSPopupButton</code> as a custom view would work, but that makes the whole view a menu, whereas I want the left part of the component to behave like a normal button, and the right dropdown part bring up the menu.</p>
  
  <p>Is there some trick to making the NSToolbarItem show a component like this, or is this two custom views stuck together?</p>