Root cause of the intermittent white screen: the shell HTML is no-cache
(cf-cache-status: DYNAMIC), so every page-open does a synchronous round-trip
to the residential origin before any pixel renders — and the SW's network-first
navigation only fell back to the cached shell on REJECTION, never on slowness.
A stalled fetch meant staring at white with a perfectly good shell in cache.
The boot seatbelt couldn't see it either: it lives inside the HTML that hadn't
arrived yet, so slow boots left no telemetry.
- service-worker: race navigation fetch vs 2.5s grace timer. Network wins →
fresh HTML as before; timer/5xx/failure → cached shell instantly, network
response still refreshes the cache in the background. Safe due to the 14-day
immutable-chunk grace window. Caps the white screen at ~2.5s for repeat
visitors on any network.
- app.html: beacon `boot-slow: Nms (html Nms) on 4g` when mount takes >4s —
the "white screen, then it loaded" glitches finally leave a trace, with
HTML-arrival timing to separate slow-origin from slow-JS.
- admin: bot UAs (HeadlessChrome/bot/spider/crawl/…) excluded from the
headline "Load errors today" count — throttled crawlers trip the 10s boot
check routinely (the one recorded error was HeadlessChrome on X11, not a
phone). Bots stay visible in the list, tagged + dimmed.
Tests: telemetry test extended for bot flag + filtered counts. 223 pytest +
11 vitest green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
thejayman77
628cc5722c