vue/compat is complaining about a missing SSR runtime module

[FrankieBue]

I'm currently looking at upgrading our app to Vue 3. It's pretty complex, so a full lift would take a long time to accomplish, so I'm using vue/compat library so I can do it piecemeal as time goes on.

The basic lift was pretty simple; add vue/compat, upgrade vue. I'd already updated the compiler to Vite, so that bit was already done. A couple changes to vite.config for compatability mode, and the project builds. Unfortunately, when trying to run it, it's complaining about a missing SSR module, which is stumping me. I can't find any SSR I'm doing in the project, and nothing I try will even generate a module name I can search for. Any ideas?