Print the timeline report's window in the reader's zone, not the host's - #2621

Written-by: Dev

Fixes #2619
Owner: Dev
Reviewer: Reviewer

What changed

The timeline report printed the window's hours in the Bot host process's zone, while the panel header above the Copy button printed the browser's: a Shanghai reader behind a UTC host copied a report saying 15:30–16:30 under a header saying 23:30–00:30. The panel now sends its browser zone (Intl.DateTimeFormat().resolvedOptions().timeZone, the same object fmtClock reads) with the digest request, and the host formats the window hours and the (+1 day) day boundary in that zone.

buildDigest(window, { title, now, timeZone }) takes HH:MM and the calendar day from the named zone; a zone that is not a non-empty string the runtime accepts leaves today's host-clock behaviour untouched and never throws. Day differences come from that zone's own Y/M/D, so a DST day is still one day and only a window crossing the reader's midnight gets (+1 day). plugins/timeline/src/timeline.js passes body.timeZone through only when it is a trimmed non-empty string of at most 64 characters; window(), the recorder, OTLP export, the alerts fold and every other number in the report are unchanged.

plugins/timeline/test/digest.test.js pins the window 2026-11-06T15:30Z–16:30Z: UTC 15:30–16:30, Asia/Shanghai 23:30–00:30 (+1 day), America/New_York 10:30–11:30, Mars/Olympus falls back to the host clock without throwing, and no timeZone keeps the report byte for byte. The same tests fail on the base source (the red proof below). docs/subsystems/timeline.md records that the panel sends its own zone, and changelog.d/2619.fixed.md carries the user-facing note.

Ran at this tip: c7d77e4

$ cd plugins/timeline && TZ=UTC node --test test/digest.test.js
# tests 22 / pass 22 / fail 0

$ cd plugins/timeline && TZ=UTC node --import ./test/resolve.mjs --test test/*.test.js
# tests 152 / pass 152 / fail 0

$ # red proof: this tip's tree with digest.js and timeline.js restored to main's version
$ cd plugins/timeline && TZ=UTC node --test test/digest.test.js
not ok 19 - #2619: the report prints the window in the zone the panel names
not ok 22 - #2619: the endpoint passes the panel's zone through and ignores anything else
# tests 22 / pass 20 / fail 2

$ cd plugins/timeline && npx --yes oxlint@latest src test
Found 5 warnings and 0 errors.

$ node .github/scripts/changelog.mjs check
changelog: 20 fragment(s) in changelog.d/ are well formed

$ node scripts/ds-bot-patches.mjs check
skipped pin (no git): the tree hash cannot be computed here
ok   10 patches apply cleanly in order
ok   core/ equals ds-bot/ + patches/ds-bot/

$ node .github/scripts/check-mojibake.mjs --check
mojibake: ok — 0 occurrences of U+FFFD in 1544 text file(s)

Not run, and why: full npm run verify (the brief scopes this to the target tests plus the checks above); build-client.mjs --check (core/client.js is generated from core/src/client/ and neither is in this diff); smoke test, pack check, actionlint (no core/, workflow or packaging change). check-arch-docs.mjs needs origin/main and this box has no git; the owned page docs/subsystems/timeline.md is updated in this diff.

Checkpoint

{
  "pr": 2621,
  "claim": "Dev",
  "issue": 2619,
  "summary": "Window hours in the timeline report now come from the reader's browser zone, which the panel sends with the digest request, instead of the Bot host's process zone. buildDigest formats the hours and the (+N day) boundary from that zone's own Y/M/D parts, keeps today's host-clock answer when the zone is missing or unusable, and never throws. A window crossing the reader's midnight now says +1 day on any host. window(), the recorder, OTLP export and the alerts fold are unchanged; the subsystem page and a changelog fragment record it.",
  "files": [
    "plugins/timeline/src/digest.js",
    "plugins/timeline/src/timeline.js",
    "plugins/timeline/client.js",
    "plugins/timeline/test/digest.test.js",
    "plugins/timeline/test/panel.test.js",
    "changelog.d/2619.fixed.md",
    "docs/subsystems/timeline.md"
  ],
  "checks": {
    "ran": "TZ=UTC node --test test/digest.test.js (22 pass); TZ=UTC node --import ./test/resolve.mjs --test test/*.test.js (152 pass); oxlint src test; changelog.mjs check; ds-bot-patches.mjs check",
    "result": "green",
    "at": "c7d77e49437b0e7c46c60165f5e82c68b2a53c8c"
  },
  "next": "none — ready for review at this tip"
}