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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 (options.Get("parent", &parent) && !parent.IsEmpty())
    parent_window_.Reset(isolate, parent.ToV8());
  
  window_.reset(NativeWindow::Create(
      web_contents->managed_web_contents(),
      options,
      parent.IsEmpty() ? nullptr : parent->window_.get()));
  web_contents->SetOwnerWindow(window_.get());
  
  
  ui::GpuSwitchingManager::SetTransparent(window_->transparent());
#if defined(TOOLKIT_VIEWS)
  
  mate::Handle<NativeImage> icon;
  if (options.Get(options::kIcon, &icon) && !icon.IsEmpty())
    SetIcon(icon);
#endif
  window_->InitFromOptions(options);
  window_->AddObserver(this);
  InitWith(isolate, wrapper);
  AttachAsUserData(window_.get());
  
  
  if (!parent.IsEmpty())
    parent->child_windows_.Set(isolate, ID(), wrapper);
  auto* host = web_contents->web_contents()->GetRenderViewHost();
  if (host)
    host->GetWidget()->AddInputEventObserver(this);
}
BrowserWindow::~BrowserWindow() {