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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
Emit("sheet-end");
}
void BrowserWindow::OnWindowEnterHtmlFullScreen() {
  Emit("enter-html-full-screen");
}
void BrowserWindow::OnWindowLeaveHtmlFullScreen() {
  Emit("leave-html-full-screen");
}
void BrowserWindow::OnExecuteWindowsCommand(const std::string& command_name) {
  Emit("app-command", command_name);
}
void BrowserWindow::OnTouchBarItemResult(const std::string& item_id,
                                  const base::DictionaryValue& details) {
  Emit("-touch-bar-interaction", item_id, details);
}
void BrowserWindow::OnNewWindowForTab() {
  Emit("new-window-for-tab");
}
#if defined(OS_WIN)
void BrowserWindow::OnWindowMessage(UINT message,
                                    WPARAM w_param,
                                    LPARAM l_param) {
  if (IsWindowMessageHooked(message)) {
    messages_callback_map_[message].Run(
        ToBuffer(isolate(), static_cast<void*>(&w_param), sizeof(WPARAM)),
        ToBuffer(isolate(), static_cast<void*>(&l_param), sizeof(LPARAM)));
  }
}
#endif
mate::WrappableBase* BrowserWindow::New(mate::Arguments* args) {