What happens if you passport application is not ready and your visa runs out?

I’m applying for a new passport, out of pages.
Now there is plenty of time left, but this is THailand…
So what happens if it takes longer than expected and my passport isn’t ready before my visa expires..
Do I need to pay overstay? Or can I get an extension at immigration in Jomptien?
I’m on a 30 days extension now…!