The handler bound to ready-to-show is not running, so the webContents.setZoomFactor is never getting called. The handler isn't running because your window is immediately shown, so the event isn't emitted.

Relevant bit of docs, for reference:

While loading the page, the ready-to-show event will be emitted when the renderer process has rendered the page for the first time if the window has not been shown yet.

So, if you add a show: false to your BrowserWindow options and an event.newGuest.show() after ready-to-show, it should work as you expect.

One reason that it's not setting the zoomFactor when you initialize the BrowserWindow is that zoomFactor is a property of webPreferences, not of the top-level options type.

That being said, with zoomFactor correctly situated, you're still right, it doesn't work at BrowserWindow instantiation. It looks like this is because the native-window-opened webContents is already created by the time you intercept the new-window event, and Electron's zoom controller isn't updated when the webPreferences are merged at BrowserWindow creation.