- 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.cc
return;
  
  if (!g_custom_schemes.empty())
    command_line->AppendSwitchASCII(switches::kRegisterStandardSchemes,
                                    g_custom_schemes);
  
  if (!g_custom_service_worker_schemes.empty())
    command_line->AppendSwitchASCII(switches::kRegisterServiceWorkerSchemes,
                                    g_custom_service_worker_schemes);
#if defined(OS_WIN)
  
  PWSTR current_app_id;
  if (SUCCEEDED(GetCurrentProcessExplicitAppUserModelID(&current_app_id))) {
    command_line->AppendSwitchNative(switches::kAppUserModelId, current_app_id);
    CoTaskMemFree(current_app_id);
  }
#endif
  
  if (ContainsKey(pending_processes_, process_id))
    process_id = pending_processes_[process_id];
  
  
  content::WebContents* web_contents =
      WebContentsPreferences::GetWebContentsFromProcessID(process_id);
  if (!web_contents)
    return;
  WebContentsPreferences::AppendExtraCommandLineSwitches(
      web_contents, command_line);
}
void AtomBrowserClient::DidCreatePpapiPlugin(