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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_->MergeAllWindows();
}
void BrowserWindow::MoveTabToNewWindow() {
  window_->MoveTabToNewWindow();
}
void BrowserWindow::ToggleTabBar() {
  window_->ToggleTabBar();
}
void BrowserWindow::AddTabbedWindow(NativeWindow* window,
                                    mate::Arguments* args) {
  const bool windowAdded = window_->AddTabbedWindow(window);
  if (!windowAdded)
    args->ThrowError("AddTabbedWindow cannot be called by a window on itself.");
}
void BrowserWindow::SetVibrancy(mate::Arguments* args) {
  std::string type;
  args->GetNext(&type);
  auto* render_view_host = web_contents()->GetRenderViewHost();
  if (render_view_host) {
    auto* impl = content::RenderWidgetHostImpl::FromID(
        render_view_host->GetProcess()->GetID(),
        render_view_host->GetRoutingID());
    if (impl)
      impl->SetBackgroundOpaque(type.empty() ? !window_->transparent() : false);
  }
  window_->SetVibrancy(type);
}
void BrowserWindow::SetTouchBar(