App router eagerly tries to match routes

[moy2010]

I want to gradually migrate my NextJs application to use the App router. For this, I started with a single, dummy page for testing purposes.

Soon after I found a nasty surprise in that my app started to misbehave. I think the reason being that unmatched routes were no longer being handled by the pages router, so the 404 page could not be found. This resulted in the "NextRouter was not mounted" error.

Even if my diagnostic could be wrong, I think that there should be a more declarative way of migrating apps to the App router. The official documentation is rather poor, and gives a feeling of being an immature feature.