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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_->Show();
}
void BrowserWindow::ShowInactive() {
  
  if (IsModal())
    return;
  window_->ShowInactive();
}
void BrowserWindow::Hide() {
  window_->Hide();
}
bool BrowserWindow::IsVisible() {
  return window_->IsVisible();
}
bool BrowserWindow::IsEnabled() {
  return window_->IsEnabled();
}
void BrowserWindow::SetEnabled(bool enable) {
  window_->SetEnabled(enable);
}
void BrowserWindow::Maximize() {
  window_->Maximize();
}
void BrowserWindow::Unmaximize() {
  window_->Unmaximize();
}
bool BrowserWindow::IsMaximized() {
  return window_->IsMaximized();
}
void BrowserWindow::Minimize() {