This repository should help you reproducing sveltejs/kit#12580.

How to reproduce

1. clone, npm install
2. npm run dev
3. Request the development server (e. g. in a browser)
4. See the error message:

[vite] Error when evaluating SSR module /src/routes/+layout.ts:
|- ReferenceError: localStorage is not defined
    at /var/www/my-app/src/routes/+layout.ts:4:1
    at instantiateModule (file:///var/www/my-app/node_modules/vite/dist/node/chunks/dep-Cy9twKMn.js:52858:11)

The problem

Svelte runs the +layout.ts file on server-side, even though SSR is explicitly disabled.

This can't be avoided, as SSR is disbaled as late as in the +layout.ts file itself.

We either need a way to disable SSR from the begin with, or another way to add layout code that won't be run on server side.