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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 defined(OS_WIN)
  std::vector<TaskbarHost::ThumbarButton> buttons;
  if (!args->GetNext(&buttons)) {
    args->ThrowError();
    return false;
  }
  auto window = static_cast<NativeWindowViews*>(window_.get());
  return window->taskbar_host().SetThumbarButtons(
      window_->GetAcceleratedWidget(), buttons);
#else
  return false;
#endif
}
void BrowserWindow::SetMenu(v8::Isolate* isolate, v8::Local<v8::Value> value) {
  mate::Handle<Menu> menu;
  if (value->IsObject() &&
      mate::V8ToString(value->ToObject()->GetConstructorName()) == "Menu" &&
      mate::ConvertFromV8(isolate, value, &menu) && !menu.IsEmpty()) {
    menu_.Reset(isolate, menu.ToV8());
    window_->SetMenu(menu->model());
  } else if (value->IsNull()) {
    menu_.Reset();
    window_->SetMenu(nullptr);
  } else {
    isolate->ThrowException(v8::Exception::TypeError(
        mate::StringToV8(isolate, "Invalid Menu")));
  }
}
void BrowserWindow::SetAutoHideMenuBar(bool auto_hide) {