- 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
#include "atom/browser/native_window.h"
#include <string>
#include <utility>
#include <vector>
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/atom_browser_main_parts.h"
#include "atom/browser/window_list.h"
#include "atom/common/api/api_messages.h"
#include "atom/common/native_mate_converters/file_path_converter.h"
#include "atom/common/options_switches.h"
#include "base/files/file_util.h"
#include "base/json/json_writer.h"
#include "components/prefs/pref_service.h"
#include "base/message_loop/message_loop.h"
#include "base/strings/utf_string_conversions.h"
#include "brightray/browser/inspectable_web_contents.h"
#include "brightray/browser/inspectable_web_contents_view.h"
#include "content/browser/renderer_host/render_widget_host_impl.h"
#include "content/public/browser/navigation_entry.h"
#include "content/public/browser/plugin_service.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/render_widget_host.h"
#include "content/public/browser/render_widget_host_view.h"
#include "content/public/common/content_switches.h"
#include "ipc/ipc_message_macros.h"
#include "native_mate/dictionary.h"
#include "ui/gfx/codec/png_codec.h"