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fix(station-boards): push last snapshot on subscribe when one exists #482

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Merged
ciotlosm merged 1 commit into from
Aug 7, 2026
Merged

fix(station-boards): push last snapshot on subscribe when one exists #482

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22 changes: 16 additions & 6 deletions src/lib/workers/gtfs/stationSubscribers.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -43,12 +43,22 @@ export async function subscribeStationBoards(
const key = Symbol();
const sub: StationSub = { stopIds: new Set(initialStopIds), cb };
subscribers.set(key, sub);
// NOTE: no immediate catch-up push here. Every `tickLive` call
// broadcasts to all station subscribers via `pushAllStationSubscribers`,
// so new subscribers automatically receive the next poll's merged
// data. Pushing immediately with `getReconciledSnapshot()` before the
// first tick would send scheduled-only vehicles (no GPS) and cause
// a flicker in StationCard groups before the merged push arrives.
// Late-subscribe catch-up. If the worker already has a merged
// snapshot (the user is navigating between views while data is
// flowing — e.g. station view -> home, then home -> station
// again), hand the new subscriber the same merged-with-GPS data
// the stream is currently showing. Without this, the new view
// renders scheduled-only vehicles for up to one poll cycle
// (15 s today) before the next tickLive broadcast arrives.
//
// The first-ever subscription, before the first tick has run,
// has `getReconciledSnapshot() === null`; the catch-up is
// skipped for that case so the new subscriber doesn't paint
// with an empty snapshot and then re-paint with the merged one
// (the regression that PR #454 closed — see that PR for the
// incoming-first flicker this guards against).
const snap = getReconciledSnapshot();
if (snap) void pushOne(sub, snap);
// Proxy the whole handle so Comlink keeps it as a remote reference.
// Per-method Comlink.proxy() doesn't help here — Comlink only checks
// the proxy marker at the TOP of the returned value, so a plain
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