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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
NativeWindow* native_window) {
  auto existing = TrackableObject::FromWrappedClass(isolate, native_window);
  if (existing)
    return existing->GetWrapper();
  else
    return v8::Null(isolate);
}
}  
}  
namespace {
using atom::api::BrowserWindow;
void Initialize(v8::Local<v8::Object> exports, v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context, void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  BrowserWindow::SetConstructor(isolate, base::Bind(&BrowserWindow::New));
  mate::Dictionary browser_window(
      isolate, BrowserWindow::GetConstructor(isolate)->GetFunction());
  browser_window.SetMethod(
      "fromId",
      &mate::TrackableObject<BrowserWindow>::FromWeakMapID);
  browser_window.SetMethod(
      "getAllWindows",
      &mate::TrackableObject<BrowserWindow>::GetAll);
  mate::Dictionary dict(isolate, exports);
  dict.Set("BrowserWindow", browser_window);
}
}  
NODE_BUILTIN_MODULE_CONTEXT_AWARE(atom_browser_window, Initialize)