- 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.h
void DidGetRedirectForResourceRequest(
      content::RenderFrameHost* render_frame_host,
      const content::ResourceRedirectDetails& details) override;
  void DidNavigateMainFrame(
      const content::LoadCommittedDetails& details,
      const content::FrameNavigateParams& params) override;
  bool OnMessageReceived(const IPC::Message& message) override;
  void WebContentsDestroyed() override;
  void NavigationEntryCommitted(
      const content::LoadCommittedDetails& load_details) override;
  void TitleWasSet(content::NavigationEntry* entry, bool explicit_set) override;
  void DidUpdateFaviconURL(
      const std::vector<content::FaviconURL>& urls) override;
  void PluginCrashed(const base::FilePath& plugin_path,
                     base::ProcessId plugin_pid) override;
  void MediaStartedPlaying(const MediaPlayerId& id) override;
  void MediaStoppedPlaying(const MediaPlayerId& id) override;
  void DidChangeThemeColor(SkColor theme_color) override;
  
  void DevToolsFocused() override;
  void DevToolsOpened() override;
  void DevToolsClosed() override;
 private:
  enum Type {
    BROWSER_WINDOW,  
    WEB_VIEW,  
    REMOTE,  
  };
  AtomBrowserContext* GetBrowserContext() const;
  uint32_t GetNextRequestId() {