The blurred 60fps worker-orb canvas is the main remaining Firefox lag source after the
#935 sweep (multiple Discord lag reports). So for a FIRST-TIME user with no saved
preference, default the orbs OFF on Firefox (smooth first impression where it's needed)
and ON everywhere else (full polish where the browser handles it). An explicit saved
choice ALWAYS wins — this only picks the default when the user hasn't chosen.
Done kettui-style with a SINGLE source of truth, not the dual browser-detection I first
floated (server UA + client _isFirefox would be the same fact in two places that can
drift — exactly the server/client class #943's green-flash fix just cleaned up):
- core/ui_appearance.py (new, pure + importable): is_firefox_user_agent +
resolve_worker_orbs_default(explicit, is_firefox) — explicit wins, unset → !firefox.
- web_server: the SERVER decides (UA via _request_is_firefox, request-context-safe) and
injects initial_worker_orbs_enabled; config default flipped None so "unset" is
distinguishable from an explicit False. The client just consumes the injected value
(init.js unchanged) — no client-side re-derivation of "is Firefox".
- settings.js: the orb checkbox default now reflects the server value when unset, so
saving Settings can't silently flip a first-time Firefox user's orbs back on.
No regression: Chrome users unchanged; users with an explicit setting unchanged (it
wins regardless of browser); /api/settings returns raw config so it can't clobber the
default for an unset value. Verified end-to-end through a real Flask request context
(Firefox→off, Chrome→on, explicit wins both ways, no crash outside a request). 8 pure
seam tests pin the contract; ruff clean.
