<!--
Thanks for opening an issue! A few things to keep in mind:

- The issue tracker is only for bugs and feature requests.
- Before reporting a bug, please try reproducing your issue against
  the latest version of Electron.
- If you need general advice, join our Slack: http://atom-slack.herokuapp.com
-->

* 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
return window_->SetContentProtection(enable);
}
void BrowserWindow::SetFocusable(bool focusable) {
  return window_->SetFocusable(focusable);
}
void BrowserWindow::SetProgressBar(double progress, mate::Arguments* args) {
  mate::Dictionary options;
  std::string mode;
  NativeWindow::ProgressState state = NativeWindow::PROGRESS_NORMAL;
  args->GetNext(&options) && options.Get("mode", &mode);
  if (mode == "error") {
    state = NativeWindow::PROGRESS_ERROR;
  } else if (mode == "paused") {
    state = NativeWindow::PROGRESS_PAUSED;
  } else if (mode == "indeterminate") {
    state = NativeWindow::PROGRESS_INDETERMINATE;
  } else if (mode == "none") {
    state = NativeWindow::PROGRESS_NONE;
  }
  window_->SetProgressBar(progress, state);
}
void BrowserWindow::SetOverlayIcon(const gfx::Image& overlay,
                            const std::string& description) {
  window_->SetOverlayIcon(overlay, description);
}
bool BrowserWindow::SetThumbarButtons(mate::Arguments* args) {