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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
window_unresponsive_closure_.Cancel();
  Emit("responsive");
}
void BrowserWindow::WillCloseWindow(bool* prevent_default) {
  *prevent_default = Emit("close");
}
void BrowserWindow::RequestPreferredWidth(int* width) {
  *width = web_contents()->GetPreferredSize().width();
}
void BrowserWindow::OnCloseButtonClicked(bool* prevent_default) {
  
  
  
  *prevent_default = true;
  
  
  
  
  if (window_unresponsive_closure_.IsCancelled())
    ScheduleUnresponsiveEvent(5000);
  if (!web_contents())
    
    return;
  if (web_contents()->NeedToFireBeforeUnload())
    web_contents()->DispatchBeforeUnload();
  else
    web_contents()->Close();
}
void BrowserWindow::OnWindowClosed() {