- 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.h
#ifndef ATOM_BROWSER_API_ATOM_API_WINDOW_H_
#define ATOM_BROWSER_API_ATOM_API_WINDOW_H_
#include <map>
#include <string>
#include <vector>
#include "base/memory/scoped_ptr.h"
#include "ui/gfx/image/image.h"
#include "atom/browser/api/trackable_object.h"
#include "atom/browser/native_window.h"
#include "atom/browser/native_window_observer.h"
#include "atom/common/api/atom_api_native_image.h"
#include "native_mate/handle.h"
class GURL;
namespace gfx {
class Rect;
}
namespace mate {
class Arguments;
class Dictionary;
}
namespace atom {
class NativeWindow;
namespace api {
class WebContents;