- 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
const CapturePageCallback& callback) {
  const auto view = web_contents()->GetRenderWidgetHostView();
  const auto host = view ? view->GetRenderWidgetHost() : nullptr;
  if (!view || !host) {
    callback.Run(SkBitmap());
    return;
  }
  
  const gfx::Size view_size = rect.IsEmpty() ? view->GetViewBounds().size() :
                                               rect.size();
  
  
  
  gfx::Size bitmap_size = view_size;
  const gfx::NativeView native_view = view->GetNativeView();
  const float scale =
      gfx::Screen::GetScreen()->GetDisplayNearestWindow(native_view)
      .device_scale_factor();
  if (scale > 1.0f)
    bitmap_size = gfx::ScaleToCeiledSize(view_size, scale);
  host->CopyFromBackingStore(
      gfx::Rect(rect.origin(), view_size),
      bitmap_size,
      base::Bind(&NativeWindow::OnCapturePageDone,
                 weak_factory_.GetWeakPtr(),
                 callback),
      kBGRA_8888_SkColorType);
}
void NativeWindow::ShowDefinitionForSelection() {