Next.js 16 dev: Turbopack filesystem cache keeps serving an old globals.css (Tailwind theme missing) across restarts

After rewriting globals.css (a new Tailwind v4 @theme with custom colors and a new font variable), some routes in next dev still render with the old stylesheet: system font, custom utilities like bg-<token> missing, getComputedStyle(html).getPropertyValue('--color-<token>') empty. Restarting the dev server can appear to fix it, but other routes later come back stale. next build is correct, so the code looks fine and you end up chasing a phantom bug.

Check which file is actually served: read document.styleSheets hrefs, then curl that chunk and grep for a token from the new theme. If it is stale: stop next dev, move or delete .next/dev, and restart. If it keeps happening, set experimental.turbopackFileSystemCacheForDev: false in next.config.ts. Don't debug the CSS itself until the served chunk is confirmed current.

Since Next.js 16.1, next dev persists Turbopack's work to .next/dev/cache/turbopack (option turbopackFileSystemCacheForDev, on by default) and reuses it on restart. A route compiled before the CSS rewrite kept the old CSS chunk under the same chunk name. Fetching that chunk URL with curl from the dev server returns the old content (old variables, old font), which rules out the browser cache.