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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
.SetMethod("mergeAllWindows", &BrowserWindow::MergeAllWindows)
      .SetMethod("selectPreviousTab", &BrowserWindow::SelectPreviousTab)
      .SetMethod("selectNextTab", &BrowserWindow::SelectNextTab)
      .SetMethod("moveTabToNewWindow", &BrowserWindow::MoveTabToNewWindow)
      .SetMethod("toggleTabBar", &BrowserWindow::ToggleTabBar)
      .SetMethod("addTabbedWindow", &BrowserWindow::AddTabbedWindow)
#endif
      .SetMethod("setVibrancy", &BrowserWindow::SetVibrancy)
      .SetMethod("_setTouchBarItems", &BrowserWindow::SetTouchBar)
      .SetMethod("_refreshTouchBarItem", &BrowserWindow::RefreshTouchBarItem)
      .SetMethod("_setEscapeTouchBarItem",
                 &BrowserWindow::SetEscapeTouchBarItem)
#if defined(OS_WIN)
      .SetMethod("hookWindowMessage", &BrowserWindow::HookWindowMessage)
      .SetMethod("isWindowMessageHooked",
                 &BrowserWindow::IsWindowMessageHooked)
      .SetMethod("unhookWindowMessage", &BrowserWindow::UnhookWindowMessage)
      .SetMethod("unhookAllWindowMessages",
                 &BrowserWindow::UnhookAllWindowMessages)
      .SetMethod("setThumbnailClip", &BrowserWindow::SetThumbnailClip)
      .SetMethod("setThumbnailToolTip", &BrowserWindow::SetThumbnailToolTip)
      .SetMethod("setAppDetails", &BrowserWindow::SetAppDetails)
#endif
#if defined(TOOLKIT_VIEWS)
      .SetMethod("setIcon", &BrowserWindow::SetIcon)
#endif
      .SetProperty("id", &BrowserWindow::ID)
      .SetProperty("webContents", &BrowserWindow::WebContents);
}
v8::Local<v8::Value> BrowserWindow::From(v8::Isolate* isolate,