The "session" event is emitted by handleRedirect, which is never called in the main window with the popup workflow. The callback provided to onSession is therefore never called.

https://github.com/solid/solid-auth-fetcher/blob/877dd07e8a0ecd00121414b21d1aeffbb72660d0/src/AuthFetcher.ts#L121

One option would be to emit a "session" event after the session is created in PopUpLoginHandler
https://github.com/solid/solid-auth-fetcher/blob/877dd07e8a0ecd00121414b21d1aeffbb72660d0/src/login/popUp/PopUpLoginHandler.ts#L73

As a workaround, when using the popup workflow, the onSession callback can be called on completion of login, e.g.

solidAuthFetcher.login({
      oidcIssuer: "https://identityProvider.com",
      popUp: true,
      popUpRedirectPath: "/popup.html"
    }).then( session => console.log(session.webId)  )