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* Electron version: 1.8.4
* Operating system: Windows

### Expected behavior

Off screen windows should respond to the `.focus()` method being called by making the content appear to be focused.

### Actual behavior

Currently the undocumented method `focusOnWebView()` method must be called in order to "focus" an offscreen window.

This behaviour should either be documented [in the off-screen rendering guide](https://electronjs.org/docs/tutorial/offscreen-rendering), or the `.focus()` method should fix this

### How to reproduce

* Create an offscreen window, and have its content output to some location (In my case an OpenGL texture)
* Go to a web page, such as google
* Observe no caret in focused text box, and selected text is the wrong colour

Not sure if this is just a documentation issue or a bug... Feel free to resolve as if it were working as intended and document this issue in the off-screen rendering guide.

atom/browser/api/atom_api_browser_window.cc
if (!window_->IsClosed())
    window_->CloseImmediately();
  api_web_contents_->RemoveObserver(this);
  
  
  base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, window_.release());
}
void BrowserWindow::OnInputEvent(const blink::WebInputEvent& event) {
  switch (event.GetType()) {
    case blink::WebInputEvent::kGestureScrollBegin:
    case blink::WebInputEvent::kGestureScrollUpdate:
    case blink::WebInputEvent::kGestureScrollEnd:
      Emit("scroll-touch-edge");
      break;
    default:
      break;
  }
}
void BrowserWindow::RenderViewHostChanged(content::RenderViewHost* old_host,
                                          content::RenderViewHost* new_host) {
  if (old_host)
    old_host->GetWidget()->RemoveInputEventObserver(this);
  if (new_host)
    new_host->GetWidget()->AddInputEventObserver(this);
}
void BrowserWindow::RenderViewCreated(