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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
content::RenderViewHost* render_view_host) {
  if (!window_->transparent())
    return;
  content::RenderWidgetHostImpl* impl = content::RenderWidgetHostImpl::FromID(
      render_view_host->GetProcess()->GetID(),
      render_view_host->GetRoutingID());
  if (impl)
    impl->SetBackgroundOpaque(false);
}
void BrowserWindow::DidFirstVisuallyNonEmptyPaint() {
  if (window_->IsVisible())
    return;
  
  
  const auto view = web_contents()->GetRenderWidgetHostView();
  view->Show();
  view->SetSize(window_->GetContentSize());
  
  base::ThreadTaskRunnerHandle::Get()->PostTask(
      FROM_HERE,
      base::Bind([](base::WeakPtr<BrowserWindow> self) {
        if (self)
          self->Emit("ready-to-show");
      }, GetWeakPtr()));
}
void BrowserWindow::BeforeUnloadDialogCancelled() {