The "Back restores the feed scroll position" spec failed against the production
build while the app was in fact correct. Root cause, found by instrumenting
PageScroller and comparing the baked :8080 build to live :5173:
`getByTestId("video-card-channel").first()` grabs the first card in the DOM, which
after scrolling 3000px is in the virtualizer's overscan ABOVE the viewport.
Playwright scrolls it into view to click it, moving the feed from 2879 to ~1017–1253
BEFORE navigating; PageScroller's save-on-scroll records that moved position, so Back
faithfully restores the moved position — under the test's own tolerance. A real user
clicks a card they can see, which does not move the feed.
New helper clickVisibleChannelLink() clicks a card whose bounding box is inside the
viewport. Restore now lands at ~2804 (was ~1017). 17/17 pass; stable over 3 reruns.
No app change — the scroll-restore logic was already correct.
Locks the five flows the refactor must not break (feed, channel, player,
settings, nav) so later sprints can prove they changed nothing. Adds a thin
data-testid layer and a dedicated seeded non-demo account; the suite runs on
the baked :8080 stack (authoritative) or :5173. No product-behaviour change.
peter