Pre-existing, and it fired for real during the full-suite run on a cold stack.
The lightbox binds `poster={upload.thumbnail_url ?? undefined}`, so the attribute is
absent until compression produces the thumbnail. This test asserted on it immediately
after seeding, never waiting for the worker -- unlike the Range test further down the
same file, which does poll. Against a warm stack the worker usually wins; against a
freshly rebuilt one (`stack:down -v`, cold ffmpeg) it doesn't.
That is the worst possible time for a false failure: the first run after a rebuild is
exactly when you are trying to establish whether a change broke something. Poll for
`compression_status = 'done'` before the poster assertion. The `src` assertion needs
no wait and keeps none.
Verified with --repeat-each=3.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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