[mbreiser]

Summary

LAB-303 follow-up from the Bergamo ScanImage day (2026-10-16, ros2-poc/scanimage-day-2026-10-16.md item 5): host-timed protocol steps ran 4.00 s instead of 3.00 s whenever the Studio window was hidden or covered (the operator working in ScanImage). ArenaRunner._sleep was a plain setTimeout, which Chrome aligns to 1 s ticks once a tab is hidden or the window is occluded (and to 1 min after 5 min hidden).

• js/arena-runner-g6.js: _sleep now runs on createSleepTimer() — a dedicated Worker built from an inline blob URL (no extra file; the runner stays a classic <script src> module per CLAUDE.md) arms the timeout and posts back. A page setTimeout is armed beside it as a backstop; whichever fires first wins, so the Worker path is never worse than the old behavior. Without Worker/Blob/URL (Node tests, old engines) or after a Worker error, the setTimeout alone runs, exactly as before. _resolveSleep() clears through the same timer, so stop() / _clear() / the runner's finally still wake the wait at once. One Worker per page (sleepTimer()), created on first use.
• Tripwire: a wait that still overshoots by more than 250 ms emits a warn event (host-timed wait ran 4.00 s instead of 3.00 s (browser timer throttling? keep the Studio window visible)) into the run log, so a throttled or starved page is visible in the record, not only in the data afterwards. This is independent of the document.hidden banner planned for the 2026-10-20 candidate; both can live together.
• arena_studio.html: bumps the runner's ?v= cache-buster (20261018-worker-timer). This one line will conflict trivially with Studio: stop the run when the FicTrac bridge is lost (BRIDGE_LOST) + ?p= links never wait on GitHub (LAB-303 S1, S2) #257's 20261015-bridge-lost bump once that lands; keep the newer token.
• Exports: createSleepTimer, sleepTimer, SLEEP_LATE_WARN_MS. ArenaRunnerG6.sleepTimer().kind reads 'worker' in a browser and 'timeout' in Node — a headless check can assert it.

Tier

next candidate (after 2026-10-20), together with #219. No footer/version bump (the release commit does it).

Release notes

Studio

• Host-timed steps keep their length when the Studio window is hidden or covered. The waits between commands (and a trial's host-side end) used a browser timer that Chrome slows to whole seconds once the tab is hidden or the window is behind another one; on the Bergamo rig every 3 s step ran 4 s while the operator worked in ScanImage. The wait now runs in a background Worker, which the browser does not throttle, with the old timer kept as a fallback.
• A wait that still runs long is flagged. If a host-timed wait overshoots by more than a quarter second, the run log gets a warning naming timer throttling and asking you to keep the Studio window visible.

Test plan

• pixi run test (61/61 files; tests/test-arena-runner-g6.js 553 checks: fallback sleep completes, stop()/_clear() wake a 5 s wait, fake-Worker path set/clear/backstop/onerror → fallback, the inline Worker source's message protocol, late-wait warn, runSequence with the real sleep completes and aborts mid-wait)
• pixi run studio-smoke 0 failures, 0 warnings
• Prettier clean on the touched JS
• Browser: open the Studio, run a protocol with 3 s host-timed steps with the window covered/minimized; ArenaRunnerG6.sleepTimer().kind === 'worker' in DevTools; step intervals 3.0 s (step_timing.py)

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