[Birfy]

Written-by: Dev-1
Fixes #3079
Owner: Dev
Reviewer: Reviewer
Tip SHA: 30c38974acedf0278bd94a42341875296ce02c57
Base: 3e901fab3326883f801db0d53e44f8998a4d1a1b — live main, read from GET /repos/ThinkFlowLab/FlyBot/git/ref/heads/main at 2026-11-15T01:03Z before the tree was cut; no rebase.
Docs-not-needed: no — docs/subsystems/core-client.md updated (a leave now writes only the reported visible tail), and changelog.d/3079-leave-visible-tail.fixed.md is the user-visible note; no other subsystem doc changes.
Bot: FlyBot Dev-1
Tier: T1 — core/src/client/{index,header,cells}.js plus the generated core/client.js and patches/ds-bot/0008; no host, plugin, workflow or policy/gates.json file is touched.
Tree: 54c94028d1950e2c5bb7479a15f25c706eb1c93b

Ran: red-first twice on 3e901fab with only the new test added (0 pass / 7 fail, exit 1, identical both runs); at the tip the same command 7/7 twice; TZ=UTC npm run verify exit 0 (nine suites, 0 failures); node scripts/ds-bot-patches.mjs check and canonical; node core/scripts/build-client.mjs --check exit 0; node .github/scripts/changelog.mjs check; oxlint 0 errors.
Not run: dev/bench.sh smoke — it needs the DSH desktop bench at ../deepseek-harness and a mock model, and this diff is the browser side only (no host index.js).

What was wrong

leaveVisits (core/src/client/index.js:125-128) wrote seenAt[seenKey(id)] = Date.now() for every visited Session, on a close (endVisit, :590-591) and on pagehide (:131). The divider side already refuses to settle a visit whose divider was never 'in-view' (header.js:182), so one leave still marked the whole Session read up to the clock: a turn that arrived below the fold, or after the user had scrolled up, was recorded as seen without ever being on screen, and its NEW divider could not come back. That is the inverse of what #441 and #275 fixed.

What changed

• core/src/client/cells.js — leftSeen(seen, tail, now) beside movedOnSince/settledVisit: the newest turn the view reported, else now, never backwards.
• core/src/client/index.js — a per-Session visitTails; actions.noteVisitTail; leaveVisits writes leftSeen(max(stored, settled visit's seen), tail) and drops the tail. endVisit and pagehide keep going through it, and the first-open baseline (:224) still falls back to now for a Session that never reported a tail.
• core/src/client/header.js — useTranscriptView measures the newest [data-chat-turn] row inside the viewport (its end, or first while the turn still runs); NewMessagePills reports it through actions.noteVisitTail(sessionId, tailAt). header-effects.js is untouched.
• core/client.js and patches/ds-bot/0008 regenerated; core/test/visit-tail.test.js is new and mirrored byte-for-byte in patches/ds-bot/files/test/; docs/subsystems/core-client.md says what a leave writes now; changelog.d/3079-leave-visible-tail.fixed.md is the user-visible note.

Red first

core/test/visit-tail.test.js on live 3e901fab, twice, identical:

$ node --import ./core/test/harness-resolve.mjs --test core/test/visit-tail.test.js
# tests 7
# pass 0
# fail 7        (exit 1)

The three new cases: seen t0 with a turn at t1 > t0 whose divider was never 'in-view' keeps t0 after endVisit (today Date.now()); a settled visible turn advances to at least t1; a Session that never reported a visible tail still advances to now. core/test/turn-tail.test.js:172/174 are untouched and still pass (12/12 with client-bundle and client-apply-order).

Verification at the tip

30c38974acedf0278bd94a42341875296ce02c57 (tree 54c94028d1950e2c5bb7479a15f25c706eb1c93b), 11 files: cells.js +7, index.js +21/−2, header.js +18, core/client.js +31/−1, core/test/visit-tail.test.js new 92 lines, patches/ds-bot/0003 +40/−3, 0008 +10201/−10158 (generated — canonical proves generate is a no-op), 0012 +34/−1, the mirrored test, the subsystem-doc sentence, the fragment.

• the new file 7/7 twice at the tip
• TZ=UTC npm run verify: exit 0 — core 3255 tests (3254 pass, 1 skipped, 0 fail), social 684, tree 149, timeline 172, workflow 535, project 147, .github/policy 681 + 37, interop 34, live-e2e 0 failures; oxlint 0 errors in core and every plugin
• ds-bot-patches check: pin ok, 10 patches apply cleanly, core/ equals ds-bot/ + patches; canonical: no drift
• build-client.mjs --check: exit 0
• ARCH_DOCS_BASE=3e901fab node core/scripts/check-arch-docs.mjs: OK (11 changed file(s))
• changelog.mjs check: 2 well-formed fragments

Checkpoint

{"pr":3096,"claim":"Dev","issue":3079,"summary":"`leaveVisits` wrote `Date.now()` for every visited Session, so turns that never came on screen were stored as seen and the NEW divider never returned. `leftSeen` now keeps the mark at the newest turn `useTranscriptView` reported on screen, falls back to `now` only when a Session never reported one, and never moves backwards. `endVisit` and `pagehide` both write through it, and a settled visit keeps what its divider saw. Red on `3e901fab`: 0/7; at the tip 7/7 and `TZ=UTC npm run verify` exit 0.","files":["core/src/client/cells.js","core/src/client/index.js","core/src/client/header.js","core/client.js","core/test/visit-tail.test.js","patches/ds-bot/0008-client-bundle.patch","patches/ds-bot/files/test/visit-tail.test.js","docs/subsystems/core-client.md","changelog.d/3079-leave-visible-tail.fixed.md"],"checks":{"ran":"base 3e901fab + new test: 0/7; tip: 7/7 twice; TZ=UTC npm run verify exit 0; build-client --check; ds-bot-patches check + canonical; arch-docs OK (11 files); changelog check","result":"green","at":"30c38974acedf0278bd94a42341875296ce02c57"},"next":"Opened for review. T1 needs green `tests-ok` at this tip and no PASS; nothing is stacked on this branch."}