- 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 (!window_unresposive_closure_.IsCancelled())
    return;
  window_unresposive_closure_.Reset(
      base::Bind(&NativeWindow::NotifyWindowUnresponsive,
                 weak_factory_.GetWeakPtr()));
  base::MessageLoop::current()->PostDelayedTask(
      FROM_HERE,
      window_unresposive_closure_.callback(),
      base::TimeDelta::FromMilliseconds(ms));
}
void NativeWindow::NotifyWindowUnresponsive() {
  window_unresposive_closure_.Cancel();
  if (!is_closed_ && !HasModalDialog())
    FOR_EACH_OBSERVER(NativeWindowObserver,
                      observers_,
                      OnRendererUnresponsive());
}
void NativeWindow::OnCapturePageDone(const CapturePageCallback& callback,
                                     const SkBitmap& bitmap,
                                     content::ReadbackResponse response) {
  callback.Run(bitmap);
}
}