- 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
gfx::AcceleratedWidget handle = window_->GetAcceleratedWidget();
  return ToBuffer(
      isolate(), static_cast<void*>(&handle), sizeof(gfx::AcceleratedWidget));
}
void Window::SetVisibleOnAllWorkspaces(bool visible) {
  return window_->SetVisibleOnAllWorkspaces(visible);
}
bool Window::IsVisibleOnAllWorkspaces() {
  return window_->IsVisibleOnAllWorkspaces();
}
int32_t Window::ID() const {
  return weak_map_id();
}
v8::Local<v8::Value> Window::WebContents(v8::Isolate* isolate) {
  if (web_contents_.IsEmpty())
    return v8::Null(isolate);
  else
    return v8::Local<v8::Value>::New(isolate, web_contents_);
}
void Window::BuildPrototype(v8::Isolate* isolate,
                            v8::Local<v8::ObjectTemplate> prototype) {
  mate::ObjectTemplateBuilder(isolate, prototype)
      .MakeDestroyable()
      .SetMethod("close", &Window::Close)
      .SetMethod("focus", &Window::Focus)
      .SetMethod("blur", &Window::Blur)