fix(xtiles): the loaded-view gate now matches the app, and passes

The xTiles UI check has run to a pass for the first time, both phases, after a
session was captured with `just xtiles-auth`. Getting there found two things that
only a real run could find.

**The gate's purpose was right and its mechanism was wrong.** It waited for `main`,
`nav` or `[role=main]` before believing an absence. The real signed-in app has none
of those — zero landmark elements on any page — so the check could only ever fail.
It now states the property directly instead of through a proxy: something rendered,
and the body does not carry xTiles' not-found text.

**It earned its place on the way, against exactly the case it was written for.**
`https://xtiles.app/my/tasks` redirects to `/en/tasks`, which is a 404 page. The
gate stopped there rather than reporting the probe as absent — which is the failure
mode a validation council flagged: an absence on an error page proves nothing about
a deletion. Without it, the removal test would have passed on a not-found page and
reported the round trip closed.

A third finding belongs to the test rather than the code: the first removal run
failed with "2 element(s) carrying 'StudyLoop live-check probe' are still visible",
correctly. A standalone task from an earlier attempt carried the same prefix and had
never been deleted. A scoped-prefix check found leftover probe content that the
person driving it had lost track of.

The owner's planner page is byte-identical to how it was found, including their own
three items. That is also why the screenshot stays cropped to the matched element:
this run is the concrete case behind the earlier finding that a full-page capture of
a signed-in workspace puts someone's real content into an artefact.

Evidence: reviews/2026-09-03-second-brain/evidence/m8/xtiles-live/01-first-passing-run.md

Author: NetDevAutomate

packages/studyloop/tests/test_xtiles_live.py

@@ -124,6 +124,11 @@
 #: timeout before anyone looked.
 DEFAULT_LOGIN_PATH = "/user/login"
 
+#: Text that means the page is xTiles' not-found page rather than a view.
+#:
+#: Needed because an absence on a 404 looks exactly like a successful deletion.
+_NOT_FOUND_MARKERS = ("Page not found", "Oops!", "Return to Home")
+
 #: A prefix shorter than this is not a scope, it is a substring that could match
 #: anything in the owner's workspace.
 MIN_PROBE_LENGTH = 12
@@ -429,19 +434,41 @@ def _require_loaded_xtiles_view(page, probe: str) -> None:
             f"ended up on {_origin(page.url)}, not the host under test ({expected}). "
             "An absence there proves nothing."
         )
-    # Something of the app's own chrome must be present. Deliberately weak and
-    # structural rather than a brittle selector: what matters is "this is an
-    # application view that finished rendering", not which build of it.
+    # "The app rendered, and this is not an error page."
+    #
+    # The first version waited for `main`/`nav`/`[role=main]`. Measured against the
+    # real signed-in app: it has NONE of those — zero landmark elements on any page.
+    # So that check could only ever fail, which is its own kind of useless.
+    #
+    # It did earn its place immediately though: `https://xtiles.app/my/tasks`
+    # redirects to `/en/tasks`, which is a **404 page**, and the check refused to
+    # read an absence from it. That is exactly the failure mode a council flagged —
+    # an absent probe on an error page proving nothing about deletion.
+    #
+    # So the property is stated directly instead of through a proxy: something
+    # rendered, and it is not the not-found page.
+    body = page.locator("body")
     try:
-        page.locator("main, [role='main'], nav, [role='navigation']").first.wait_for(
-            state="visible", timeout=20000
+        body.wait_for(state="visible", timeout=20000)
+        page.wait_for_function(
+            "() => document.body && document.body.innerText.trim().length > 40",
+            timeout=20000,
         )
     except Exception as exc:
         pytest.fail(
-            f"no application chrome rendered at {_redact(page.url)} within 20s, so an "
-            f"absent {probe!r} would prove nothing about deletion. ({exc})"
+            f"nothing rendered at {_redact(page.url)} within 20s, so an absent "
+            f"{probe!r} would prove nothing about deletion. ({exc})"
         )
 
+    text = body.inner_text()
+    for marker in _NOT_FOUND_MARKERS:
+        if marker.lower() in text.lower():
+            pytest.fail(
+                f"{_redact(page.url)} is an error page ({marker!r}), so nothing can "
+                f"be concluded from {probe!r} being absent. Measured: /my/tasks "
+                "redirects to a 404 — check the URL your assistant returned."
+            )
+
 
 def test_the_assistants_write_is_visible_to_the_learner(
     signed_in_page, probe: str, evidence_dir: Path