function setup (parentWindow) {
  var view = views[parentWindow.id] = new BrowserView({
    webPreferences: {
      defaultEncoding: 'utf-8'
    }
  })
  view.webContents.loadFile(path.join(__dirname, 'fg', 'overlay', 'index.html'))
}
Overlay

NOTES
- There can only ever be one overlay for a given browser window
- Overlay views are created with each browser window and then shown/hidden as needed