- 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.cc
#if defined(USE_X11)
  bool resizable;
  if (options.Get(options::kResizable, &resizable)) {
    SetResizable(resizable);
  }
#endif
#if defined(OS_WIN) || defined(USE_X11)
  bool closable;
  if (options.Get(options::kClosable, &closable)) {
    SetClosable(closable);
  }
#endif
  bool movable;
  if (options.Get(options::kMovable, &movable)) {
    SetMovable(movable);
  }
  bool has_shadow;
  if (options.Get(options::kHasShadow, &has_shadow)) {
    SetHasShadow(has_shadow);
  }
  bool top;
  if (options.Get(options::kAlwaysOnTop, &top) && top) {
    SetAlwaysOnTop(true);
  }
  
  bool fullscreenable = true;
  bool fullscreen = false;
  if (options.Get(options::kFullscreen, &fullscreen) && !fullscreen)
    fullscreenable = false;