- Electron version: All
- Operating system: OS X

Thanks @bengotow  for the y-offset, would it be possible to do a x-offset also? It may seem like an odd request but if you have a full-height sidebar menu then the sheet should be centered on the content pane. It's best understood with a screenshot (the dialog should not overlap the sidebar)...

<img width="902" alt="screen shot 2016-05-08 at 12 33 58-2" src="https://cloud.githubusercontent.com/assets/26971/15097682/39fb09ea-151b-11e6-812a-815e800b0cdf.png">

Probably would have been easier if I identified this potential issue in #4679 (sorry!).

Perhaps the best backwards compatible way to introduce this functionality is to keep the existing API but also accept an object with `x` and `y` values:

``` js
var toolbarRect = document.getElementById('toolbar').getBoundingClientRect();
var sidebarRect = document.getElementById('sidebar').getBoundingClientRect();

win.setSheetOffset({
  x: sidebarRect.width / 2,
  y: toolbarRect.height
});
```

I'm happy to have a go implementing this myself but I haven't hacked on the codebase before and since @bengotow implemented the original feature then perhaps he is better placed to add this functionality?

atom/browser/api/atom_api_window.cc
Emit("app-command", command_name);
}
#if defined(OS_WIN)
void Window::OnWindowMessage(UINT message, WPARAM w_param, LPARAM l_param) {
  if (IsWindowMessageHooked(message)) {
    messages_callback_map_[message].Run(
        ToBuffer(isolate(), static_cast<void*>(&w_param), sizeof(WPARAM)),
        ToBuffer(isolate(), static_cast<void*>(&l_param), sizeof(LPARAM)));
  }
}
#endif
mate::WrappableBase* Window::New(v8::Isolate* isolate, mate::Arguments* args) {
  if (!Browser::Get()->is_ready()) {
    isolate->ThrowException(v8::Exception::Error(mate::StringToV8(
        isolate, "Cannot create BrowserWindow before app is ready")));
    return nullptr;
  }
  if (args->Length() > 1) {
    args->ThrowError();
    return nullptr;
  }
  mate::Dictionary options;
  if (!(args->Length() == 1 && args->GetNext(&options))) {
    options = mate::Dictionary::CreateEmpty(isolate);
  }
  return new Window(isolate, options);
}
void Window::Close() {