- 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/native_window.h
public content::WebContentsObserver {
 public:
  using CapturePageCallback = base::Callback<void(const SkBitmap& bitmap)>;
  class DialogScope {
   public:
    explicit DialogScope(NativeWindow* window)
        : window_(window) {
      if (window_ != NULL)
        window_->set_has_dialog_attached(true);
    }
    ~DialogScope() {
      if (window_ != NULL)
        window_->set_has_dialog_attached(false);
    }
   private:
    NativeWindow* window_;
    DISALLOW_COPY_AND_ASSIGN(DialogScope);
  };
  virtual ~NativeWindow();
  
  
  static NativeWindow* Create(
      brightray::InspectableWebContents* inspectable_web_contents,
      const mate::Dictionary& options);
  
  static NativeWindow* FromWebContents(content::WebContents* web_contents);
  void InitFromOptions(const mate::Dictionary& options);
  virtual void Close() = 0;
  virtual void CloseImmediately() = 0;
  virtual bool IsClosed() const { return is_closed_; }