# Phase 73 — Hidden tab never stops a generating answer

**Source:** `TODO.md` L3 — "Clicking on another tab while an answer is generating stops that answer from being generated. Reponses should continue to generate unless you outright close the tab."
**Story:** n/a (TODO-derived)
**Context:** `frontend/assets/app.js` (the SSE turn machine: `runTurn` ~L1859, the `pagehide` partial-persist handler ~L2162, the 120s pre-token guard `TURN_TIMEOUT_MS` ~L287 / `armTurnTimeout` ~L982, `readSSE` ~L1044, the settle paths `done` ~L2011 / stop ~L2079), `app/api/chat.py` (the `finally` "turn cancelled" log line — the server-side signal that the SSE consumer really went away), phase-48 teardown contract (a REAL consumer departure — tab closed, navigation, Stop — still cancels the fetch and stops the model: that behavior is correct and must survive this phase).

## Objective
An in-flight answer keeps generating while the browser tab is merely hidden (switched away from) and completes when the user returns; only closing the tab, navigating away, or clicking Stop aborts the turn. Also fixes the latent record-corruption on that path (a `pagehide` partial persist can leave a duplicated brain turn in the saved conversation, which makes the answer *look* truncated on restore).

## Dependencies
— (none)

## Tasks
1. `01_repro_root_cause.md` — bounded repro with an instrumentation decision tree; pin down WHICH mechanism stops the answer on tab switch (no permanent code changes).
2. `02_fix_hidden_tab.md` — the fix: correlate the pagehide partial with the turn's settle so `done`/stop *replaces* it (never appends a second brain turn); if the repro implicates the 120s guard, make hidden time not count toward it; keep phase-48 teardown for real departures.
3. `03_e2e_hidden_tab_stream.md` — Playwright regression: synthetic `pagehide` mid-stream → the answer completes exactly once, the record has one brain turn, reload restores it; regressions + commit.

## Testing & Quality
- Unit/integration: frontend-only phase — no `app/` changes expected (coverage floor unaffected, must stay **>90%** on `app/`).
- E2E: new story suite `tests/e2e/test_hidden_tab_stream.py`, run in isolation (`uv run pytest tests/e2e/test_hidden_tab_stream.py -v --no-cov`) against the deterministic mock LLM (long/slow deterministic streams give a guaranteed mid-stream window).
- Regression runs in isolation: `test_chat_rag.py`, `test_chat_persistence.py`, `test_chat_history.py` (phase 50), `test_stop_generation.py` (phase 48 contract: real Stop/cancel still tears down), `test_retry_answer.py`.

## Completion Criteria
- [ ] The repro's root cause is named in the phase-73 commit message body (one line: which candidate from task 01 fired, or "none of C1–C3 — <finding>").
- [ ] `uv run pytest tests/e2e/test_hidden_tab_stream.py -v --no-cov` green in isolation: a tab switch (synthetic `pagehide`) mid-turn never stops the answer, and the persisted conversation holds exactly one brain turn for that question.
- [ ] Real departures unchanged: Stop button, tab close, and navigation still cancel the fetch (phase-48 `test_stop_generation.py` + `test_chat_persistence.py` green).
- [ ] `uv run pytest` green; `uv run pytest --cov=app --cov-report=term-missing` >90%; `uv run ruff check . && uv run pyright` clean.
- [ ] One atomic `--no-gpg-sign` Conventional-Commits commit (e.g. `fix(chat): keep generating while the tab is hidden`); phase dir moved to `.agents/phases/complete/`.