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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
mate::Arguments* args) {
  std::string display_name;
  if (!args->GetNext(&display_name))
    display_name = path;
  window_->PreviewFile(path, display_name);
}
void BrowserWindow::CloseFilePreview() {
  window_->CloseFilePreview();
}
void BrowserWindow::SetParentWindow(v8::Local<v8::Value> value,
                             mate::Arguments* args) {
  if (IsModal()) {
    args->ThrowError("Can not be called for modal window");
    return;
  }
  mate::Handle<BrowserWindow> parent;
  if (value->IsNull() || value->IsUndefined()) {
    RemoveFromParentChildWindows();
    parent_window_.Reset();
    window_->SetParentWindow(nullptr);
  } else if (mate::ConvertFromV8(isolate(), value, &parent)) {
    parent_window_.Reset(isolate(), value);
    window_->SetParentWindow(parent->window_.get());
    parent->child_windows_.Set(isolate(), ID(), GetWrapper());
  } else {
    args->ThrowError("Must pass BrowserWindow instance or null");
  }
}
v8::Local<v8::Value> BrowserWindow::GetParentWindow() const {