- 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/atom_browser_client.h
static void SuppressRendererProcessRestartForOnce();
  
  static void SetCustomSchemes(const std::vector<std::string>& schemes);
  
  static void SetCustomServiceWorkerSchemes(
      const std::vector<std::string>& schemes);
 protected:
  
  void RenderProcessWillLaunch(content::RenderProcessHost* host) override;
  content::SpeechRecognitionManagerDelegate*
      CreateSpeechRecognitionManagerDelegate() override;
  content::AccessTokenStore* CreateAccessTokenStore() override;
  void OverrideWebkitPrefs(content::RenderViewHost* render_view_host,
                           content::WebPreferences* prefs) override;
  std::string GetApplicationLocale() override;
  void OverrideSiteInstanceForNavigation(
      content::BrowserContext* browser_context,
      content::SiteInstance* current_instance,
      const GURL& dest_url,
      content::SiteInstance** new_instance) override;
  void AppendExtraCommandLineSwitches(base::CommandLine* command_line,
                                      int child_process_id) override;
  void DidCreatePpapiPlugin(content::BrowserPpapiHost* browser_host) override;
  content::QuotaPermissionContext* CreateQuotaPermissionContext() override;
  void AllowCertificateError(
      content::WebContents* web_contents,
      int cert_error,
      const net::SSLInfo& ssl_info,
      const GURL& request_url,