- 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
NativeWindow* native_window) {
  auto existing = TrackableObject::FromWrappedClass(isolate, native_window);
  if (existing)
    return existing->GetWrapper();
  else
    return v8::Null(isolate);
}
}  
}  
namespace {
using atom::api::Window;
void Initialize(v8::Local<v8::Object> exports, v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context, void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  v8::Local<v8::Function> constructor = mate::CreateConstructor<Window>(
      isolate, "BrowserWindow", base::Bind(&Window::New));
  mate::Dictionary browser_window(isolate, constructor);
  browser_window.SetMethod("fromId",
                           &mate::TrackableObject<Window>::FromWeakMapID);
  browser_window.SetMethod("getAllWindows",
                           &mate::TrackableObject<Window>::GetAll);
  mate::Dictionary dict(isolate, exports);
  dict.Set("BrowserWindow", browser_window);
}
}  
NODE_MODULE_CONTEXT_AWARE_BUILTIN(atom_browser_window, Initialize)