# Story: Document Back Button Returns to Where You Came From

**Phase:** `13_document_back_navigation.md` · **E2E:** `tests/e2e/test_document_back_navigation.py`

## Narrative

As **a user**, when I open a document **from the chat tab** (a source
chip), the viewer's back button must take me **back to the chat** — not
to Sources. When I open it from Sources, it takes me to Sources.

Today every chip/link opens the viewer in a **new tab**, where there is
no browser history, so the fallback always lands on `/sources.html` —
which is wrong for chat-originated visits.

- **Given** I open a document from chat (chip) or from Sources (table)
- **When** I click the viewer's back button
- **Then** I land on the page I opened it from — Chat or Sources.

## Acceptance criteria
1. Chat source chips carry `&back=%2F` (chat) in the viewer URL; Sources
   table links keep the default (no `back` param → `/sources.html`).
2. The viewer resolves the back target: `back` param wins when it is a
   same-origin relative URL (starts with `/`, not `//`); anything else
   (missing, absolute, protocol-relative, garbage) falls back to
   `/sources.html`. The back link's **label** matches (Chat / Sources).
3. Clicking back navigates to the resolved target (deterministic — no
   `history.length` heuristics).
4. Phase-10 viewer behavior and a11y frame unchanged.
5. Unit + integration green, `app/` coverage >90%, story E2E green in
   isolation, one `--no-gpg-sign` commit.

## Playwright Mapping Rule
**Test Scenario → `tests/e2e/test_document_back_navigation.py`** (seeded KB):
1. `test_back_from_chat_returns_to_chat` — question → click source chip →
   new tab at `/document.html?…&back=%2F` → back link href is `/` and
   labeled "Chat" → click → URL is the chat page.
2. `test_back_from_sources_returns_to_sources` — Sources → click path
   link → new tab (no `back` param) → back link href is `/sources.html`,
   labeled "Sources" → click → URL is Sources.
3. `test_malicious_back_param_is_rejected` — `back=https%3A%2F%2Fevil.com`
   and `back=%2F%2Fevil.com` → back link falls back to `/sources.html`.