While building an internal dashboard on Next.js, our engineering team hit a styling failure that every tool in the chain declared healthy. This case study covers how we root-caused it at the source, isolated a bundler bug rather than a bug in our code, and contributed the reproduction upstream to Next.js.
@source directiveThe layout silently collapsed, twice, with no errors anywhere. Lint, type-checking, unit tests, and production builds were all green. Nothing in the codebase pointed to a cause, which meant every code-level check was reporting success while the running application said otherwise.
Instead of trusting the toolchain's all-clear, we verified what the dev server was actually serving: we requested the compiled CSS directly and searched it for the utility classes the UI depended on. They were missing from the output, even though the source was correct.
The root cause: Tailwind v4 classes pulled in via the @source directive were being dropped during Turbopack's incremental rebuilds. A development-only bundler bug, not a bug in our code, which is why every code-level check passed.
We documented a reliable remedy, wrote up a minimal reproduction, and contributed it upstream to Next.js (issue #93052).
This is what disciplined, AI-era engineering looks like: when the tooling reports success but the result says otherwise, you verify reality at the source, isolate the true cause, and give the fix back to the ecosystem.
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