docs: add how-to-test.md (browser E2E + out-of-band) and link from AGENTS.md

Adds a testing guide covering how to verify features against a running stand:
drive the behaviour under test through the browser (not the API), verify
out-of-band in the DB/git, and the non-obvious traps. Notably the page has two
ProseMirror editors — [aria-label='Page title'] (non-collab) and
[aria-label='Page content'] (the collab body); querySelector('.ProseMirror')
returns the title, so tests must target the body editor and wait ~10s for the
hocuspocus store debounce. Links the new doc from AGENTS.md next to dev-stand.md
and adds a matching gotcha #8 to dev-stand.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

AGENTS.md

@@ -214,6 +214,12 @@ Run from the repo root unless noted. The dev workflow needs **Postgres (with the
 > server, `APP_SECRET` mismatch between processes, a stale `editor-ext` white-
 > screening the client, LAN exposure. See **[docs/dev-stand.md](docs/dev-stand.md)**
 > for the step-by-step and the traps.
+>
+> **Testing the app against a stand** (browser E2E + out-of-band verification) has
+> its own non-obvious traps — the page has two ProseMirror editors (only the body is
+> collab-bound), a ~10s store debounce, and API-seeding the thing under test is a
+> silent no-test. See **[docs/how-to-test.md](docs/how-to-test.md)** before writing
+> UI tests.
 
 ```bash
 pnpm install                 # install all workspaces (uses pnpm patches; see package.json `pnpm.patchedDependencies`)

docs/dev-stand.md

@@ -131,5 +131,14 @@ const { Client } = require("pg");
 7. **Migrations don't auto-run in dev** — run `migration:latest` after every pull
    or branch switch.
 
+8. **Automation (Playwright): type into the BODY editor, not the title.** A page has
+   two `.ProseMirror` editors — `[aria-label='Page title']` (non-collab) and
+   `[aria-label='Page content']` (the collab body). `document.querySelector('.ProseMirror')`
+   returns the TITLE editor, so typing there never changes body content and `mod+S`
+   versions nothing. Target `[aria-label='Page content']`, confirm it's collab-bound
+   (`el.editor.extensionManager.extensions.some(e=>e.name==='collaboration')`), and
+   wait ~10-12s for the store debounce before asserting `pages.content` changed. Full
+   testing methodology + traps: **[how-to-test.md](how-to-test.md)**.
+
 See also the **Commands** and **Architecture → Two server processes** sections in
 [`AGENTS.md`](../AGENTS.md).

docs/how-to-test.md

@@ -0,0 +1,108 @@
+# How to test the application (browser E2E + out-of-band)
+
+How to actually verify a feature end-to-end against a running stand — driving the
+**real app in a browser** and confirming results **out-of-band** in the DB/git, not
+through the same API you're supposed to be testing. Written from real false-positives
+that wasted hours (see **Traps** — read them before you write a test).
+
+Prereq: a running stand — see **[dev-stand.md](dev-stand.md)**. Automation uses
+Playwright (`pip install playwright && python -m playwright install chromium`).
+
+## Principles
+
+1. **Drive the behaviour under test through the browser.** The stand exists so you
+   exercise the real UI + realtime-collab + server path. Using `POST /api/pages/*` to
+   perform the action you're validating tests the API, not the app — an e2e suite can
+   do that. API calls are fine ONLY for one-time setup/fixtures, never for the
+   interaction you're asserting on.
+2. **Evidence before claim.** Nothing "passes" without an artifact: a DB row, a git
+   diff, a screenshot looked at as an image. If you can't show it, you didn't verify it.
+3. **Verify out-of-band.** Judge results from a source independent of the UI: `psql`
+   against the DB, a fresh `git clone` of a synced repo, a hard reload. Optimistic UI
+   lies about persistence.
+4. **Disconfirm by default.** For each feature, actively try to prove it's broken
+   before concluding it works. Reload after every create/edit/save.
+5. **Recon actuatability FIRST.** Before building editor tests, confirm the
+   interaction even works in your harness (does a typed edit reach the DB?). Skipping
+   this is how you ship a pile of tests that all silently exercised the wrong thing.
+
+## The editor: two ProseMirror instances (READ THIS)
+
+A page has **two** `.ProseMirror` editors:
+
+| index | selector | role | collab? |
+|---|---|---|---|
+| 0 | `[aria-label='Page title']` | title field | **NO** (16 exts, no `collaboration`) |
+| 1 | `[aria-label='Page content']` | body | **YES** (95 exts, has `collaboration`) |
+
+`document.querySelector('.ProseMirror')` returns the **title** editor (first match).
+Type there and you edit the title only — body page content never changes, so `mod+S`
+"versions" unchanged content and every content test silently no-ops.
+
+**Always target the body editor** and confirm it's collab-bound before typing:
+
+```js
+const el = document.querySelector("[aria-label='Page content']");
+el.editor.extensionManager.extensions.some(e => e.name === 'collaboration'); // must be true
+```
+
+Body edits emit ~20 `/collab` websocket frames while typing and land in
+`pages.content` after the **hocuspocus store debounce (~10s)** — so **wait ~12s**
+before asserting persistence (checking at 6–8s is a false negative). `mod+S` (the
+`save-version` stateless message) flushes immediately, so a version created right
+after a settled body edit holds the typed text.
+
+## A known-good browser flow
+
+```
+1. goto /s/<space-slug>            # the "Create page" button lives in the space sidebar, not /home
+2. click button[aria-label='Create page']   # fully UI-driven page creation
+3. type into [aria-label='Page title']       # optional title
+4. click [aria-label='Page content'] → type body text
+5. wait ~12s (store debounce)
+6. assert pages.content changed (psql)       # out-of-band
+7. mod+S / menu Save → assert page_history row (psql)
+8. reload / fresh context → re-assert (persistence round-trip)
+```
+
+Auth: log in ONCE, save `storage_state.json`, reuse it across pages/agents (re-login
+per run trips shared rate-limits). Cookie-based session authorizes both REST and the
+collab websocket.
+
+## Judging out-of-band
+
+```bash
+# page content / history
+docker exec <db> psql -U docmost -d docmost -tAc \
+  "select coalesce(kind,'null'), content::text from page_history where page_id='<id>' order by created_at;"
+# git-sync round-trip: clone the space repo and diff against what you pushed
+git clone http://<user>:<pass>@127.0.0.1:3000/git/<spaceId>.git /tmp/x
+```
+
+`page_history.content` is full JSON — parse it, don't truncate the snippet, or a
+marker check misses. For sync/async features (autosave, git-sync, idle-flush) use an
+active probe: write a unique marker, wait past the debounce/poll window, re-read
+out-of-band, ≥2 iterations — never conclude "broken" from a single snapshot.
+
+## Traps (each of these produced a false result in a real run)
+
+- **Wrong editor.** Typed into `.ProseMirror` (= title). Edits never touched body
+  content. → target `[aria-label='Page content']`.
+- **Checked persistence too early.** Store debounce ~10s; a 6–8s check reads stale.
+- **Truncated the DB snapshot** below where the test marker sits → false "content
+  missing".
+- **API-seeded the content under test**, then "verified" the feature — that validated
+  the API, not the app.
+- **Reused a fixed marker on a non-rebooted stand** → title/row collisions inflate
+  counts (`count==2`). Use a unique per-run marker (timestamp).
+- **Idle/async read once** and called it "permanently broken" — it was mid-debounce.
+- **Concluded env-limitation without a cross-build control.** If unsure whether a
+  failure is your harness or the product, run the SAME harness against a known-good
+  build; a divergence localizes it.
+
+## Scope note
+
+Some paths genuinely need a human in a real browser (rich drag-drop, native file
+pickers, clipboard, and anything the harness can't actuate). Label those UNTESTED in
+the report — "handled gracefully" is not "works". Keep four states distinct:
+verified-working, defect, untested, env-limitation.