- 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
return ContainsKey(messages_callback_map_, message);
}
void Window::UnhookAllWindowMessages() {
  messages_callback_map_.clear();
}
#endif
#if defined(OS_MACOSX)
void Window::ShowDefinitionForSelection() {
  window_->ShowDefinitionForSelection();
}
#endif
#if defined(TOOLKIT_VIEWS)
void Window::SetIcon(mate::Handle<NativeImage> icon) {
#if defined(OS_WIN)
  static_cast<NativeWindowViews*>(window_.get())->SetIcon(
      icon->GetHICON(GetSystemMetrics(SM_CXSMICON)), icon->GetHICON(256));
#elif defined(USE_X11)
  static_cast<NativeWindowViews*>(window_.get())->SetIcon(
      icon->image().AsImageSkia());
#endif
}
#endif
void Window::SetAspectRatio(double aspect_ratio, mate::Arguments* args) {
  gfx::Size extra_size;
  args->GetNext(&extra_size);
  window_->SetAspectRatio(aspect_ratio, extra_size);
}
v8::Local<v8::Value> Window::GetNativeWindowHandle() {