- Electron version: v0.36.9
- Operating system: Windows 10/OS X

In #779 a `disposition` was added to the `new-window` event in [webContents](https://github.com/atom/electron/blob/master/docs/api/web-contents.md#event-new-window) but it always seems to be set to `new-window` when clicking on target="_blank" links in our web app wrapper: https://github.com/irccloud/irccloud-desktop/blob/v0.1.3/app/main.js#L109

If I Ctrl (Win) or Cmd (OS X) or middle click on links, I would expect to see a `background-tab` disposition, as in Chrome that will open links in a background tab, while keeping focus on the current tab. I'd like to catch this behaviour to set `activate` to false in `shell.openExternal`.

Is this possible?

atom/browser/api/atom_api_web_contents.cc
params.guest_delegate = guest_delegate_.get();
    web_contents = content::WebContents::Create(params);
  } else {
    content::WebContents::CreateParams params(session->browser_context());
    web_contents = content::WebContents::Create(params);
  }
  Observe(web_contents);
  AttachAsUserData(web_contents);
  InitWithWebContents(web_contents);
  managed_web_contents()->GetView()->SetDelegate(this);
  
  new WebContentsPreferences(web_contents, options);
  
  WebContentsPermissionHelper::CreateForWebContents(web_contents);
  web_contents->SetUserAgentOverride(GetBrowserContext()->GetUserAgent());
  if (is_guest) {
    guest_delegate_->Initialize(this);
    NativeWindow* owner_window = nullptr;
    if (options.Get("embedder", &embedder_) && embedder_) {
      
      auto relay =
          NativeWindowRelay::FromWebContents(embedder_->web_contents());
      if (relay)
        owner_window = relay->window.get();
    }
    if (owner_window)
      SetOwnerWindow(owner_window);
  }
}
WebContents::~WebContents() {