TypeError: can't redefine non-configurable property "$router"

[ChristophWurst]

Steps to reproduce

1. Install Mail v4.1.0-beta1
2. ???

Expected behavior

App loads

Actual behavior

08:06:27.883 TypeError: can't redefine non-configurable property "$router"
    NextJS 3
sentry.js:2:62546
    NextJS 23

Mail app version

v4.1.0-beta1

Nextcloud version

No response

Mailserver or service

No response

Operating system

No response

PHP engine version

None

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

No response

[ChristophWurst]

Fixed by #10403

[ChristophWurst]

Post mortem analysis

Mail used asynchronously loaded components, as suggested at https://v2.vuejs.org/v2/guide/components-dynamic-async. This worked for years. For Mail v4.1.x we switched from Webpack to Vite for smaller frontend bundles. Things still worked for everyone but Greta's setup, and very few other people for a week or so.
Deployment to a production system broke the app for every user. The app content stayed blank because the main entrypoint of the app errored out.
Analysis showed that mail-main.mjs was loaded twice. Once with the cache buster ?v=xyz and once without. The Chromium network inspector revealed that once mail-main.mjs?v=xyz was included by the page and mail-main.mjs was included by home-abc.mjs. Home.vue is an async'ly loaded component. Inspecting the code showed that it has an import of the main bundle. This is a cyclic dependency: mail-main.js?v=xyz -> home-abc.mjs -> mail-main.mjs. The main bundle registers the $router property on the Vue instance, which fails the second time.