- 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
const mate::Dictionary& options)
    : embedder_(nullptr),
      request_id_(0) {
  
  bool is_guest = false;
  options.Get("isGuest", &is_guest);
  type_ = is_guest ? WEB_VIEW : BROWSER_WINDOW;
  
  std::string partition;
  mate::Handle<api::Session> session;
  if (options.Get("session", &session)) {
  } else if (options.Get("partition", &partition) && !partition.empty()) {
    bool in_memory = true;
    if (base::StartsWith(partition, "persist:", base::CompareCase::SENSITIVE)) {
      in_memory = false;
      partition = partition.substr(8);
    }
    session = Session::FromPartition(isolate, partition, in_memory);
  } else {
    
    session = Session::FromPartition(isolate, "", false);
  }
  session_.Reset(isolate, session.ToV8());
  content::WebContents* web_contents;
  if (is_guest) {
    content::SiteInstance* site_instance = content::SiteInstance::CreateForURL(
        session->browser_context(), GURL("chrome-guest://fake-host"));
    content::WebContents::CreateParams params(
        session->browser_context(), site_instance);
    guest_delegate_.reset(new WebViewGuestDelegate);