ersatztv/docs/spa-conventions.md

# SPA conventions — "Add a screen" playbook

Purpose: a precise playbook for adding a new screen (or sub-path editor) to the ChicoryTV React SPA
(`web/`), for an agent with no prior context in this repo. **Update this doc in the same PR that
changes any convention below.**

Companion to `api-conventions.md` (the API surface the SPA talks to) and `docs/contributing.md`
(general repo conventions).

## 1. Stack & layout

Vite + React + TypeScript, builds to `ErsatzTV/wwwroot/app` (see `web/vite.config.ts`:
`base: '/app/'`, `build.outDir: '../ErsatzTV/wwwroot/app'`), served by the ASP.NET host at `/app`.

- **Composition root**: `web/src/App.tsx` — owns the active route, approved Libraries sub-path,
  navigation guard integration, and theme; it composes the shell with the matched screen.
- **Routes + nav**: `web/src/app/routes.tsx` — the ONE stable module-level array of `ScreenRoute`
  objects (`path`, `label`, `title`, `kicker`, `icon`, etc.) plus `allowSubPaths?: boolean`, the
  matcher, and sidebar group definitions. Route objects must never be rebuilt per render.
- **Shell + screen dispatch**: `web/src/app/AppShell.tsx` owns Sidebar/TopBar/Connect/version/theme
  chrome; `web/src/app/ScreenContent.tsx` exhaustively maps a matched route to its screen and owns
  the Media/Libraries route wrappers.
- **Screens**: `web/src/screens/*.tsx`, one file per top-level screen, generally with a colocated
  `*.test.tsx`.
- **API clients**: `web/src/api/<domain>.ts` (see §4).
- **Styling**: `web/src/shell.css` (+ `web/src/components/components.css`) — utility classes with a
  `ctv-` prefix (~690 occurrences across those two files). Reuse an existing `ctv-*` class before
  inventing a new one. `shell.css` carries the only base reset — `html, body { margin: 0 }` plus
  `body { background: var(--surface-app) }` (the 8px default body margin otherwise frames every
  full-viewport layout with a light border, #373). **There is no global `box-sizing` reset**
  (the SPA is authored under the default `content-box`), so any element that combines `width: 100%`
  with padding/border must set `box-sizing: border-box` locally or it overflows its container — e.g.
  `.ctv-nav-item` (#377). Prefer `width: auto` (shrink-to-fit) over `width: 100%` + padding where you can.

## 2. CRITICAL: sub-path screens must own their own pathname state

If a route sets `allowSubPaths: true` (e.g. so `/app/blocks/{id}` works under the `/app/blocks` nav
entry), **the screen component itself must track `window.location.pathname` and listen for
`popstate`** — do not rely on the composition root re-rendering `ScreenContent` when the sub-path
changes.

**Why**: `app/routes.tsx`'s `routeFromLocation()` matches an `allowSubPaths` route by prefix
(`pathname.startsWith(\`${route.path}/\`)`) and returns the **same `ScreenRoute` object reference**
for the base path and every sub-path under it. `App`'s state update is
`setActiveRoute(routeFromLocation())`; React's `useState` setter bails via `Object.is` when the new
value is reference-equal to the old one — so navigating from `/app/blocks` to `/app/blocks/42` (or
between `/app/blocks/42` and `/app/blocks/17`) **never re-invokes `ScreenContent`** at the `App`
level. See the comment block directly above `PlayoutsRouteScreen` in
`screens/PlayoutsScreen.tsx` for the canonical explanation, and its implementation
(`useState(() => window.location.pathname)` +
`useEffect` with a `popstate` listener local to the wrapper component) for the fix.

Exemplars of screens that already do this correctly: `BlocksScreen.tsx`, `TemplatesScreen.tsx`,
`DecosScreen.tsx`, `DecoTemplatesScreen.tsx`, and the `PlayoutsRouteScreen` wrapper colocated in
`PlayoutsScreen.tsx` (which owns two sibling sub-paths, `/playouts/{id}/alternate-schedules` and
`/playouts/{id}/templates`, dispatching internally via `parsePlayoutSubRoute`).

**Exception — a guarded sub-path route defers pathname ownership to App.** When a sub-path route's
screens ALSO register a dirty guard (§8), the wrapper must **not** self-listen for `popstate`; App
owns the pathname and passes the approved sub-path down as a prop. The `LibrariesRouteScreen` wrapper
in `app/ScreenContent.tsx` is the exemplar (sub-path parsed by `parseLibrariesSubRoute`, dispatched
via a flat `switch`, sub-path supplied by App's `librariesSubPath` state). See §8 for why child-before-parent
effect order makes the self-listening pattern unsafe here.

## 3. Data loading pattern

Reference implementation: `web/src/screens/LogsScreen.tsx`. Structure to copy for any screen that
fetches from the API:

- A **discriminated-union state type** covering loading/success/error, e.g.
  `type LogsState = { status: 'loading'; ... } | { status: 'success'; ... } | { status: 'error'; ... }`.
- A `seqRef` (monotonically incremented request counter) + `activeRef` (mount-tracking boolean,
  flipped in a mount/unmount `useEffect`) pair — guards against a stale, slower request overwriting
  a newer one's result, and against setting state after unmount.
- The actual fetch lives in a `useCallback` (`load`), called from a **separate** `useEffect(() => {
  load(); }, [load])`.
- **Lint rule — `react-hooks` "no set-state-in-effect"**: never call `setState` **synchronously in
  the body** of a `useEffect`. State transitions happen only inside event handlers or promise
  `.then()`/`.catch()` callbacks (as in `LogsScreen`'s `load`). This is enforced by
  `eslint-plugin-react-hooks` in `web/eslint.config.js` — a synchronous `setState` in an effect body
  will fail `npm run lint`.

## 3a. "Keep results visible during refetch" ⇒ gate mutations + show a refreshing cue

Some grid screens deliberately keep the **previous** successful result set rendered while a refetch
is in flight (no full-screen loading state on a query/kind/page change), so the grid doesn't flash
empty. `SearchScreen.tsx` and `MediaBrowseScreen.tsx` do this. If such a screen also carries
**mutation surfaces** (per-card Add-to menu, Select/select-mode, a selection action bar, "Add all",
"Save as smart collection"), those surfaces would otherwise stay live over a **stale** result set —
an add/select action then targets the about-to-be-replaced items, or (worse) a query-wide "Add all"
bulk request resolves against the previous query. This was issue #221 (adversarial-reviewer#18).

Convention — when a screen keeps stale results visible during a refetch:

- **Key the success state to the request params that produced it.** Store the identifying params on
  the `status: 'success'` variant (`SearchScreen`: the `query`; `MediaBrowseScreen`: a
  `kind|query|page` `key`), set in the seq-guarded `.then`. Derive
  `const refreshing = state.status === 'success' && state.<key> !== <current params>;` in render.
  Prefer this over a synchronously-set `refreshing` flag: setting state synchronously from the load
  path trips the `react-hooks` "no set-state-in-effect" rule (§3).
  **Invariant, not a guarantee**: the derivation is only self-correcting when *every* value the
  current params can take will actually trigger a fetch. If `load()` early-returns for some param
  value (e.g. `SearchScreen`'s blank-query guard), `state` never updates for that value and a stale
  `status: 'success'` variant lingers — so the comparison must exclude params that suppress
  fetching, or gate the whole flag on the same condition that gates the fetch (`SearchScreen`:
  `const refreshing = hasQuery && state.status === 'success' && state.query !== query.trim();` —
  fixed post-review in #222 after the naive derivation got stuck `true` once the query was cleared
  to empty, see PR discussion for #221).
- **While `refreshing`:** show a visible cue (a `role="status"` "Refreshing…" row with `<Spinner>`
  plus the `.ctv-media-grid-dim` opacity class on the grid) and **disable every mutation surface** —
  per-card Add-to menu (withhold the `actions` node), select toggle + in-grid selection
  (`const canSelect = selectMode && !refreshing;` gates `onToggleSelect`), the selection action bar,
  "Add all", "Save as smart collection". Card navigation (`onOpen`) **may** stay live — but only
  outside select mode: while `selectMode && refreshing`, `MediaPosterCard` falls back to `onOpen`
  whenever `onToggleSelect` is undefined, so both props must be withheld together or a mid-select
  click navigates away instead of no-op'ing. The select-mode toggle itself should only be disabled
  while refreshing when *entering* select mode (`refreshing && !selectMode`) — exiting only clears
  selection, not a mutation, so it must stay available.
- **Bind async bulk completions to their request params, not just mount.** A whole-query/whole-set
  request (e.g. `getSearchAllItems`) must, on resolve, check that its snapshotted params are still
  current (compare against a ref that always holds the committed value — `SearchScreen` reuses
  `lastQueryRef`) and **discard** otherwise. Checking only `activeRef` (mounted) is insufficient.

## 3b. Paged list endpoints clamp server-side — page to completeness ONLY for bounded lists; a media-library picker searches instead

Every paged `/api/v1` list endpoint (rerun-collections, multi-collections, library/browse, search,
trakt-lists, …) clamps `pageSize` to its own controller's `MaxPageSize` (100, as of #644) regardless
of what the client requests. A screen that asks for `pageSize: 1000` to "get everything in one call"
gets only the first `MaxPageSize` rows back, silently — no error, no truncation indicator, no paging
UI to notice the gap. This was issue #644 (following on from #634, which fixed the first instance —
`SchedulesScreen`'s rerun-collections picker load).

**Two classes of call site, treated differently** (decision record:
`docs/decisions/records/spa/library-pickers-resolve-by-search.md`,
`spa.library-pickers-resolve-by-search`, superseding `spa.list-completeness-vs-bounded-pickers` —
the #644 follow-up got Class A right and Class B only half right):

- **Class A — bounded-by-construction lists** (collections, multi-collections, smart collections,
  playlists — admin-created, hundreds of rows at most): genuinely need the complete list, and
  completeness is cheap. Use the shared `loadAllPages` helper (`web/src/api/paging.ts`, re-exported
  via `web/src/api/index.ts`) instead of an inflated `pageSize`:

  ```ts
  const { items, complete } = await loadAllPages(getMultiCollections); // pages against totalCount, cap defaults to 100
  ```

  It pages `pageNum` from 0 (per §"paging-zero-based" in `api-conventions.md`) against the response's
  `totalCount`, stopping — and reporting `complete: false` — on an empty page (defensive guard
  against a `totalCount` that never converges) or on an aborted `signal`. **Always check `complete`**:
  a caller that needs the full list must not treat a resolved promise as proof the list is whole (a
  partial result is otherwise silently indistinguishable from a complete one — the same defect class
  as #644 itself, since `GetLibraryBrowseItemsHandler.HydrateMediaItems` can legitimately drop stale
  Lucene hits and produce a short/empty page in normal operation). Render `complete: false` as its own
  copy ("List may be incomplete — retry to reload"), never through search-narrowing text — a
  `loadPickerOptions` result that can come from either class carries a `hint: 'incomplete' | 'none'`
  discriminator, not a boolean shared with an unrelated condition (#644 follow-up round-3 review F1).
  Pass an `AbortSignal` (4th arg) from the caller's effect cleanup so a superseded load stops issuing
  further page requests, and gate any `console.warn` on `!signal?.aborted` — a superseded or
  user-aborted load returns `complete: false` too, and that's expected, not a defect. **Do not raise
  the server-side cap to work around any of this** — the `api.search-allitems-paging` precedent is
  that the client bounds itself and the server stays bounded.

- **Class B — media-library pickers** (Episode / Song / Image / Movie / MusicVideo / TelevisionShow /
  TelevisionSeason / Artist / OtherVideo / RemoteStream — the largest tables in an install, tens of
  thousands of rows possible): **resolve by search, do not window the type at all** (#651). Neither
  `loadAllPages` (~200 serial requests for a 20k-row library, each more expensive than the last since
  `LuceneSearchIndex.Search` computes `hitsLimit = skip + limit`) nor a single bounded page (an
  arbitrary alphabetical prefix, unusable as a picker even once the truncation is made visible) is
  acceptable. Render the shared `SearchPicker` (`web/src/schedules/pickers.tsx`) over
  `searchLibraryPickerOptions` (`web/src/api/libraryBrowse.ts`):

  ```ts
  const searchLibrary = useCallback(              // memoize: SearchPicker lists `search` in its effect deps
    (q: string) => searchLibraryPickerOptions('Episode', q),
    []
  );
  ```

  The helper owns both bounds: at most ONE `getLibraryBrowseItems` request per settled query, at most
  `LIBRARY_PICKER_RESULTS` (25) rows, and no request at all below `LIBRARY_PICKER_MIN_QUERY` (2)
  characters. Selecting a media-library type must issue **zero** requests. The per-kind
  `LIBRARY_PICKER_RESULTS` cap is the only truncation this class has — there is no whole-type window
  left to hint at, so the old `Showing the first 100 of 5000 — use search to narrow.` copy is gone
  from these pickers along with the window it described. Surfacing the per-kind cap is *permitted*
  wherever it is reachable, and *required* only where bulk selection makes the count actionable — see
  the `AddItemsDialog` sub-bullet below, which sums the cap across kinds and renders a `Showing N of
  M matches` hint for exactly that reason. Prove the bound with a **request-count assertion against a
  large (20k-row) fixture**, not by inspection.

  - **`SearchPicker` is the single-select SHAPE, not the rule itself.** A MULTI-select picker
    (`CollectionsScreen`'s `AddItemsDialog` — checkbox rows, many items added at once, fanned out
    over several kinds) cannot render `SearchPicker` and must not be forced to. It satisfies this
    section by taking the same *constraints* the helper enforces for single-select — the gate on
    `LIBRARY_PICKER_MIN_QUERY`, `titleContainsQuery` the typed text, `LIBRARY_PICKER_RESULTS` per
    kind — via `searchLibraryBrowseItems` (`web/src/api/libraryBrowse.ts`), a sibling of
    `searchLibraryPickerOptions` that returns full `LibraryBrowseItem` rows plus `totalCount`
    instead of `{id, name}`, so the bound lives in the helper rather than the caller (#685 review
    finding 2). There is no post-fetch `slice`, but the per-kind cap can still truncate the real
    match count — this is a bulk multi-select add, where "add the 40 matching episodes" is a
    first-class use, so `AddItemsDialog` sums each kind's `totalCount` and renders a `Showing N of
    M matches` hint once it exceeds the rendered rows (finding 4 — an earlier revision of this
    bullet called the truncation nothing left to hint at). **The gate's home is the shared HELPER,
    not the screen — however single-sink the screen's own function looks.** #685 got this wrong
    twice in a row, and the second time is the instructive one: the check sat inside `runSearch`,
    which genuinely IS the one sink both entry paths route through, so it read as correct. It was
    still a duplicate of the helper's gate, and the two masked each other: as of `4be3f247d` —
    which had no unit tests on the helper — deleting EITHER copy left the whole suite green, so the
    min-query boundary test pinned nothing. Removing the screen's copy is what made the helper's
    gate load-bearing. **The invariant, not the count: every gate must have at least one test that
    reddens when that gate ALONE is removed.** A guard you cannot redden is not a guard, and "it's
    the single sink" is not evidence that it is the only one. **Outstanding on this screen**: `AddItemsDialog` still lacks the monotonic `seqRef`
    stale-response guard and `useIsMountedRef()` — the same class of guard "Debounced typeaheads"
    below mandates there, applied to a debounced-while-typing fetch; `AddItemsDialog` is an explicit
    Search-button submission, not a typeahead, so that mandate doesn't reach it directly, but the
    same race (a superseded search settling after a newer one) can still occur here — tracked in
    **ersatztv#740**, not yet fixed here.

  - **Compile typed text; never forward raw Lucene.** Send `titleContainsQuery(text)` →
    `title:*<escaped>*`. The index's default field does not match bare title words (`Alpha` finds
    nothing for "Show Alpha" — see `e2e-local.md`), so a raw forward looks broken in a *name* picker.
    Reuse the helper; do not re-implement the escaping (same rule as the #440 Auto-Tune typeahead,
    same shape `builder/rules/compile.ts` emits for `contains`). The escaped set includes `&` and
    `|`, because Lucene's boolean operators are `&&`/`||` and a title like `Rock & Roll` otherwise
    compiles to something Lucene parses as syntax. **Drive the escaping test from the exported
    character set** (`LIBRARY_PICKER_LUCENE_SPECIALS`), one character per case — a test carrying its
    own hand-copied "every special" sample cannot see what is missing from that sample.
  - **The bound belongs to the helper, not the caller.** `searchLibraryPickerOptions` *clamps*
    `pageSize` to `LIBRARY_PICKER_RESULTS`; a documented bound a caller can exceed by passing a
    bigger number is not a bound.
  - **Render the current selection from the owning record, not from the result set.** An item already
    selected but outside the current results must still display — losing it on edit is data loss, not
    a cosmetic defect. Rerun collections and playlist items carry `selectedName` on their own DTOs;
    `FillerPresetFullResponseModel` stores only an id, so its edit path resolves the name with a
    single by-id detail read (`getShow`/`getSeason`/`getArtist`) and degrades to `#id` on failure —
    never to a cleared field.
  - **A read model that derives an id and its name from the same eager-loaded navigation reports
    *no selection at all* when that navigation isn't loaded** — a successful 200 indistinguishable
    from "the user cleared it". (`RerunCollectionsController.ProjectToResponseModel` does exactly
    this, and `MediaCollections/Mapper` maps RemoteStream through `_ => null`; tracked as **#671**.)
    An earlier revision of this section required a client-side guard that preserved the id across
    such a response. **That guard is gone and must not be rebuilt** — it only ever preserved a value
    seeded from the list row, which is itself null in production for every row, and the reconciliation
    it required is what the initialize-once rule below replaced. The correct handling is to show the
    server's answer honestly: no selection, Save disabled, and the validation badge saying why.
  - **An id NEVER travels without its namespace — in results, in options, in cached result sets.**
    A media/collection id only means anything inside the type that produced it, so any structure
    holding ids must hold the type too, and identity is compared as `(type, id)`. Three places this
    bites, all the same bug:
    1. A typeahead's cached results must be keyed on `(source, query)`, not the query text. Same
       query, different source ⇒ the results are not *stale*, they are *wrong*: hide them and
       re-query. Keying on text alone lets a re-query guard **suppress** the new source's request
       and leave the old namespace's hit clickable under the new label.
    2. List-backed `<select>` options must carry the type they were loaded for and be dropped the
       moment the active type differs — otherwise the previous type's rows stay selectable during
       the replacement load on a slow connection.
    3. A local edit whose type contradicts the server's is a **conflict**, not something to
       reconcile (below).
  - **Initialize an edit draft ONCE, from the detail read — never reconcile a late response against
    an open form.** This supersedes an earlier prescription here for merging a refresh into a draft
    field-by-field/atomically with touched-field tracking. That reconciliation layer produced a HIGH
    finding in three consecutive review rounds of #651, including three cross-user lost updates, and
    the last of them (a merge with no immutable baseline, so it could not tell a local edit from a
    server change) is unfixable without adding a third-way baseline — more machinery on the surface
    that was generating the bugs. Instead:
    - `draft` starts as `null` for an existing record and the form does not render until the detail
      GET lands. There is then no draft for a late response to reconcile against, and no window in
      which the user can edit something about to be replaced.
    - **Do not seed from the list row.** It is not authoritative: for rerun collections the list
      handler applies zero `.Include()`s while the detail handler applies fourteen, and both project
      through the same mapper, so the list response is a strict SUBSET of the detail one (#671). A
      seed can only add a race, never information. Verify that claim for your endpoint before
      relying on it.
    - **Fail CLOSED on a missing concurrency token.** Writing the ETag in the same callback that
      sets the draft is *not* the same as "a draft implies an ETag" — the response can simply omit
      the header, and then the PUT carries no `If-Match` and silently force-writes. No token ⇒ no
      editable draft (error + Retry/Back). Note this makes your test mocks load-bearing: a detail
      mock that omits `ETag` was previously exercising the force-write path without saying so, so
      give every single-record GET mock a real ETag and test the absent case explicitly.
    - **Bound the load and always offer a way out.** A caller-supplied fetch with no abort signal can
      hang forever; race it against a deadline, and give the loading view a Back control so a hung
      request is never a dead end.
    - **Detect conflicts at save time** via the existing `If-Match` → 412 → Reload path. Reload sets
      the draft back to `null` and re-runs the same initialize-once load, so "replace" needs no
      separate policy and the form is unmounted while the replacement is in flight.

    `FillerPresetsScreen` and `PlaylistsScreen` already worked this way; `RerunCollectionsScreen` was
    the outlier that seeded from its list row, which is where every one of these defects lived.
  - **A name resolved asynchronously must be keyed to the id it was resolved FOR**, and must refuse
    to overwrite a label that already names a different id. A slow by-id read landing after the user
    has picked something else would otherwise label the new selection with the old item's title
    while the id — and therefore what gets saved — says otherwise. Keep the guard at the *writer*,
    where it is reachable and testable; a second render-time id comparison is unreachable once
    every writer sets the label and the id together, and an unreachable guard is an untested one.
  - **Only Lucene-backed types.** `GetLibraryBrowseItemsHandler` applies `query` as a Lucene clause
    for media items but as a plain SQL `LIKE` on `Name` for the collection-family types (Collection /
    SmartCollection / MultiCollection / RerunCollection / Playlist). A compiled `title:*x*` sent at
    those matches nothing literally. Keep the collection-family pickers on their Class A / bounded
    single-page loads — `FillerPresetsScreen`'s `COLLECTION_TYPES` marks the search-driven entries
    with `searchable: true` for exactly this reason.

**If a screen shows a bounded preview or has real paging UI** (a "load more" button, a page-size
selector, a fixed-size typeahead result list), a `pageSize` at or below the cap is correct as-is —
`loadAllPages` is only for "I need literally everything, and the list is small by construction"
call sites.

**Debounced typeaheads: arm on focus, and guard on mounted as well as on sequence.** A typeahead that
fetches on *mount* multiplies by the number of rows on screen (an N-rule tree fired N unrequested
facet lookups before #578); arm the effect on the input's `onFocus` instead. And pair the monotonic
`seqRef` stale-response guard with the shared `useIsMountedRef()` (`web/src/hooks.ts`) in every async
callback — `seqRef` drops an *older* response, but says nothing about whether the component still
exists.

**A custom picker replacing a native control owes you its keyboard behaviour.** A `<select>` is
fully keyboard-operable; swapping in a listbox-and-input is an accessibility *regression* unless it
implements the ARIA combobox pattern — `role="combobox"` + `aria-expanded`/`aria-controls`/
`aria-autocomplete` on the input, ArrowUp/ArrowDown to move a virtual cursor exposed via
`aria-activedescendant`, Enter to commit, Escape to dismiss, options as non-tab-stops
(`tabIndex={-1}`) marked with `aria-selected`. Note this changes what `getAllByRole('combobox')`
matches in tests: count `<select>` elements when that is what you mean. Two failure modes that only
appear once the widget is asynchronous:

- **Freshness is `(source, query)`, and cached failures are not answers — but they are not licences
  to retry either.** A `SearchPicker`-style cache must record which source produced the results and
  whether the attempt *succeeded*. Caching a failure as an authoritative empty result turns a
  transient 500 into a permanent "No matches" that reopening can never clear. But simply declining
  the cached failure re-runs the effect and schedules another request every debounce — a **request
  storm** on a persistent outage. Keep the two apart: a `resultsFor.ok` flag says whether the held
  answer is authoritative, and a separate *attempted* key (a ref, so writing it doesn't re-render)
  suppresses automatic retries until an explicit user action — reopen, focus, or edit — re-arms it.
- **Put a validity predicate at the BOUNDARY the class crosses, not at the site the bug was found.**
  An entity-reference id (`selectedId`, `collectionId`, `mediaItemId`, …) is bound by the API as a
  32-bit integer, so `1.5` or `2147483648` renders and commits fine and then fails on write. Such
  ids enter editor state through *several* doors — search results, list-backed `<select>` options,
  and the selection restored from a detail read — so a check added to whichever one surfaced the
  defect leaves the others open (this is how #651 produced the same finding in two consecutive
  rounds). Share one predicate (`isSelectionId` / `selectionIdOrNull` in `web/src/api/selectionId.ts`)
  and apply it on every path — including the ones that don't look like pickers, such as a
  `playlistGroupId` seeded from the wire into a create dialog. **Treat an unbindable id as ABSENT,
  never coerce it** — rounding `1.5` to `1` would submit a *different* record — and **clear its
  label with it**: a row still reading "Blade Runner" over a null id makes two contradictory
  statements about the same item. Drop rather than render an option that cannot be selected safely.
  "Surfaces as no selection" is only true if that screen's Save gate actually checks for one — on
  `PlaylistsScreen` it did not, so this claim was false there for a full round after being written
  here. **Verify an invariant on every screen it names before writing it down.** Prove it per
  ingress by asserting zero writes are reachable *after attempting the write*: a write-count
  assertion on a path that never attempts one is trivially true. And unit-test the predicate's
  INCLUSIVE endpoints directly — once it is the single point of failure for every ingress, a `>`
  for `>=` slip passes an entire screen suite.
- **A caller-supplied promise needs a deadline, and a 2xx body is not a contract.** `client.ts`
  turns malformed JSON into `undefined` rather than rejecting, so `setResults(undefined)` throws on
  the next render. Validate the **elements, not just the container**: `Array.isArray` accepts
  `[null]`, which then throws on `option.id` during render, and an element with a wrong-typed `id`
  commits an invalid value through `onSelect`. Treat any malformed payload as a failed attempt (so
  it stays retryable), not as an empty answer. And a `search` prop carries no abort signal, so race
  it against a timeout — otherwise a never-settling request leaves the picker spinning with no way
  back.
- **A stale result set must not be committable — by ANY modality.** Between a keystroke and its
  response, `results` still describe the *previous* query, so highlighting an option, retyping, and
  pressing Enter commits the old option while the box reads the new text. Drop the highlight on
  **input change** (not when the next response arrives) and put the guard in the single `choose()`
  sink rather than on each call site — gating Enter and leaving `onClick` open is the same defect in
  another modality, and the next path added would be ungated too. Keep the stale list *visible*
  (hiding it flickers on every keystroke) but genuinely inert: `aria-disabled` plus a dimmed style,
  not merely a handler that silently no-ops on a normal-looking button.
- **Escape must not strand the user.** Closing the popup while focus stays in the input means
  `onFocus` can never re-arm it, so typing does nothing and the user has to blur and refocus to
  recover. Typing and ArrowDown must both reopen it — and reopening onto results that are already
  current must **not** re-query: the duplicate response lands later and resets the cursor the user
  has since moved, so Enter silently does nothing. Reopening also places the cursor (ARIA APG)
  rather than swallowing the keypress.

**The web typecheck gate is `npm run typecheck`, never `npx tsc --noEmit`.** `web/tsconfig.json` is
solution-style (`"files": []` + `references`), so a bare `tsc --noEmit` resolves to zero input files
and exits 0 **without checking anything** — a green that means "I looked at nothing". CI runs
`npm run typecheck` (`tsc -b --pretty false`), which builds the referenced projects and includes the
test files. Verified by planting a deliberate type error: `--noEmit` stayed green, `-b` caught it.

**Testing an is-mounted guard: React 19 does not warn on a setState-after-unmount, and an unmounted
tree renders nothing either way** — so no DOM assertion can distinguish "the guard stopped it" from
"React discarded it". Prove the *mechanism* (a `useIsMountedRef` unit test, with a StrictMode
double-invoke for the re-arm) **and** the *integration* (mock the hook module and assert the
component actually read `current` — and saw `false` — when the late response landed). Verify each by
removing the mechanism and confirming the test fails.

## 4. API client modules

One file per domain in `web/src/api/`, e.g. `logs.ts`, `blocks.ts`, `playouts.ts`. Pattern (see
`web/src/api/logs.ts`):

- Re-export the generated response/DTO types from `./generated/v1`:
  `export type LogEntry = components['schemas']['LogEntryResponseModel'];`
- A typed params interface for the endpoint's query string (e.g. `GetLogsParams`).
- The fetch function builds a `URLSearchParams` from only the params that are set, then calls the
  shared `request<T>(url)` helper from `./client`.
- An error-message helper (e.g. `messageFromLogsError`) that narrows `unknown` → `ApiError` (from
  `./client`) → a human string, with a fallback message — screens use this instead of stringifying
  errors themselves.
- `web/src/api/index.ts` re-exports everything so screens import from `'../api'`, not from the
  individual domain file directly.

**Trust the generated key casing — it mirrors the runtime.** Since #198 the OpenAPI spec is generated
to match the runtime Newtonsoft serializer exactly (a schema transformer runs the same contract
resolver; see `api-conventions.md` §5a), so the generated types carry the real wire keys — including
Newtonsoft's acronym quirks like `ffmpegProfileId` (channel FFmpeg-profile id) and `ffmpegProfile`
(channel FFmpeg-profile display name). **Do not** hand-cast responses to "fix" a key or dual-read a
spec-cased vs runtime-cased key (the old `PlaybackTroubleshootingScreen` `#198` escape hatch that read
`data.channel.fFmpegProfileId` has been removed — read `ffmpegProfileId` straight off the typed
response). When you mock an API response in a test, use the generated (runtime) casing.

## 4a. Optimistic-concurrency editors (ETag / If-Match / 412)

Replace-all editors (Blocks is the reference; see `api-conventions.md` §7a for the server contract,
issue #253) must round-trip the aggregate's concurrency ETag so a stale tab can't silently overwrite a
fresher edit:

- **Transport seam** (`web/src/api/client.ts`): `requestWithMeta<T>(path, options)` returns
  `{ data, etag }` (reads the `ETag` response header). `request<T>` delegates to it and drops the meta —
  keep using `request` for endpoints without a concurrency token.
- **Domain module** (`web/src/api/blocks.ts`): expose a `…WithMeta` load
  (`getBlockItemsWithMeta` → `{ data, etag }`) and make the replace accept the last-seen ETag and return
  the new one: `replaceBlock(id, body, ifMatch?)` → `requestWithMeta(..., { headers: ifMatch ? {'If-Match': ifMatch} : undefined })`.
- **Editor**: hold the ETag in a `useRef`; set it from the load GET, and **replace it from the PUT
  response's ETag on every successful save** (a same-tab second save otherwise 412s against its own
  write). Keyed reload: a `reloadKey` state in the load `useEffect` dep array lets the conflict flow
  re-fetch.
- **412 UX**: catch `error instanceof ApiError && error.status === 412` on save and open a blocking
  "changed elsewhere — reload (unsaved changes discarded)" `ConfirmDialog` (Reload bumps `reloadKey`),
  distinct from a 409 ("build in progress — retry shortly"). All other errors stay the generic save-error
  path. Reference: `web/src/screens/BlocksScreen.tsx` `BlockEditor`.
- **List-derived editors** (Multi/Rerun collections open the editor from a paged-list row, not a per-record
  GET): fetch the single record via the `…WithMeta` by-id helper (`getMultiCollectionWithMeta` /
  `getRerunCollectionWithMeta`) when the editor mounts, and build the draft from **that** response — so the
  ETag and the draft data come from one read and stay consistent (the fail-safe ordering; don't pair a
  list-row draft with a separately-fetched ETag). Editors that navigate back to the list on save (Playout,
  Multi/Rerun, Collection reorder) need only the 412 branch — no post-save ETag rotation.
- **Replace-all PUT → round-trip every persisted field, even ones the editor doesn't visibly edit.** A
  replace-all update (Multi/Rerun collection, schedule items, block items) submits the *whole* item list,
  so any field the draft mapping drops is silently reset to its default server-side. The multi-collection
  editor's per-source `weight` (#404) is the canonical trap: it must be read in `itemsFromMultiCollection`
  and written in `toItemRequest`, or every source resets to weight 1 on the next save of an unrelated edit
  (rename, add a source). When you add a field to a replace-all DTO, add it to **both** the response→draft
  and draft→request mappings and cover the round-trip in a test.
- **Numeric inputs with API bounds**: hold the value as a string in the draft so the field edits smoothly
  (transient empty/partial), pass `type="number"` + `min`/`max`/`inputMode="numeric"` to `Input` for the
  native stepper, clamp to the API's validator range on `onBlur` (immediate feedback) **and** again at save
  (authoritative) — never let an out-of-range value reach the server and surface a raw 400. Mirror the
  server's bound as a shared const (e.g. multi-collection weight `WEIGHT_MIN`/`WEIGHT_MAX` = 1..1000, #404).

## 5. Artwork rendering

Render `item.artwork` / `item.poster` (or whatever the DTO field is named) **directly as an `<img
src>`** — since PR #181, API responses already return rooted, directly-usable URLs (see
`api-conventions.md` §4). **Do not** client-side-prefix artwork paths (no `/artwork/posters/` string
building in SPA code) — if you see that pattern, it's stale/wrong.

## 5b. HLS video preview

Screens that preview an ErsatzTV HLS stream use the reusable `HlsPlayer` component
(`web/src/media/HlsPlayer.tsx`, introduced with #145). Pass it a `src` (the `.m3u8` URL, or `null`
for idle) and — when the manifest GET itself starts a server-side session (e.g. troubleshooting
`playback.m3u8`) — a `playToken` you increment per play, so a repeat play with an identical URL
still tears down and re-attaches (an unchanged `src` alone is a state no-op that never issues a new
request); it attaches `hls.js` when Media Source Extensions are available and falls back to native
HLS (`video.canPlayType('application/vnd.apple.mpegurl')`, i.e. Safari) otherwise, and tears down the
`hls.js` instance on `src` change and unmount. Its config mirrors the legacy `_Host.cshtml`
`previewChannel` (`liveDurationInfinity: true` + an unbounded manifest `maxTimeToFirstByteMs`) because
the troubleshooting `playback.m3u8` endpoint blocks until segments exist before it 302s to the live
manifest. **In tests, mock `hls.js` wholesale** (`vi.mock('hls.js', …)` with a class exposing
`static isSupported()`, `static Events`, and `loadSource`/`attachMedia`/`on`/`destroy`) so jsdom never
touches a real `MediaSource`; assert the manifest URL via the mocked `loadSource` spy (see
`PlaybackTroubleshootingScreen.test.tsx`).

It also takes an optional `onError(message)` wired to `Hls.Events.ERROR` (fatal errors only) and,
on the Safari native path, the `<video>` element's own `error` event. Fatal errors are reported,
never auto-recovered — a diagnostic surface must show the fault rather than retry past it. Pass a
**stable** `onError` (`useCallback`); it is in the attach effect's dependency array, so an unstable
identity restarts the stream every render. When mocking `hls.js` in a test that exercises errors,
include `ERROR` in the mock's `static Events`.

It also takes an optional `onPlaying()`, called once playback has actually started — wired to the
`<video>` element's own `playing` event on **both** the hls.js/MSE path and the Safari native-HLS
path. It is deliberately **not** wired to `Hls.Events.MANIFEST_PARSED`: that fires before any media
has decoded (the manifest for an `HttpLiveStreamingDirect` channel always parses, even over a black
video), so it only issues the `video.play()` kick. `onPlaying` is purely additive (omitting it is
safe) and mirrors `onError`'s requirements exactly: pass a **stable** `onPlaying` (`useCallback`),
since it too sits in the attach effect's dependency array and an unstable identity would restart the
stream every render.

**Resolving a `/iptv/*` src under JWT auth (#552).** Before feeding an `/iptv/*` manifest URL to
`HlsPlayer`, pass it through `withIptvToken(url)` (`web/src/media/iptvToken.ts`): under a JWT-enabled
deployment it appends the short-lived `?access_token=` the `/iptv/*` scheme requires (the `ctv-session`
cookie does not satisfy it), and it is a **no-op** when JWT is off (the endpoint answers 204, cached).
It is async, so resolve it into state and gate the player on the resolved src — the channel-preview
panel does this in an effect keyed on `playToken`; the troubleshooting screen awaits it inline in its
`onPlay` handler. Only the top-level manifest needs it (segments and the multi-variant→variant hop
carry or bypass the token server-side). **In tests, mock `../media/iptvToken` to identity**
(`withIptvToken: (url) => Promise.resolve(url)`) so opening a preview fires no real `iptv-token` fetch;
the append logic itself is unit-tested in `media/iptvToken.test.ts`.

## 5c. Media "Add to…" affordances

Screens that let the user add media items to a collection/playlist/schedule use the shared layer in
`web/src/media/addTo/` — `AddToMenu` (popover for a `MediaPosterCard` `actions` slot or a detail
page's action row) and the `AddToCollectionDialog` / `AddToPlaylistDialog` / `AddToScheduleDialog` /
`SaveAsSmartCollectionDialog` it drives. Do **not** build screen-local target pickers. Multi-select
on grid screens is an explicit "Select" toggle (see `docs/decisions.md` 2026-07-10 for the rationale
and the accepted deviations from Blazor).

## 5d. Client-local preferences: `localStorage`, namespaced `ctv-*` keys

Per-browser UI preferences (theme, an auth token, a screen's remembered page size) live in
`window.localStorage` under a namespaced `ctv-` key, **not** a round-trip through the API — the
established pattern is `designSystem.ts`'s `getStoredDesignSystemTheme`/`applyDesignSystemTheme`
(`ctv-theme`): a small `getStorage()` helper that returns `window.localStorage` wrapped in a
try/catch (so a disabled/unavailable storage API degrades to the default instead of throwing), a
getter that validates the stored value against the known option set before trusting it, and a
setter that writes straight through. `LogsScreen.tsx`'s page-size persistence (`ctv-logs-page-size`,
#213) follows the same shape. Reserve this for state that's genuinely local to the browser/user
session — if a preference needs to be shared across devices or is really server/business state
(e.g. Blazor's `ConfigElement`-backed settings), it belongs behind an API endpoint instead; see
`docs/decisions.md` 2026-07-11 for the specific reasoning on logs page-size.

## 5e. Session auth: the boot gate, the machine-key screen + the global 401 signal (#295)

Since #295 the browser authenticates with a **session cookie**, not an API key. The whole `/api`
surface still answers a missing/expired session with **401**, and mutating verbs additionally require a
CSRF header. The former keyless-bootstrap `ApiKeyScreen` (the `X-Api-Key`/`localStorage` model of #197)
is gone. The SPA seams now are:

- **The request client sends no API key; it relies on the cookie and adds CSRF automatically**
  (`web/src/api/client.ts`): the session cookie rides along with every same-origin request, and every
  mutating verb (POST/PUT/PATCH/DELETE) gets an `X-Csrf` header set automatically — screens making a
  POST get CSRF for free. There is **no** `getStoredApiKey`/`X-Api-Key` path anymore; the only residual
  `localStorage` touch is `clearLegacyStoredApiKey()` (`web/src/api/auth.ts`), a one-shot cleanup the
  boot gate runs once to purge any stale `ctv-api-key`.
- **The boot gate owns login/setup** (`web/src/AuthGate.tsx`), wrapping `<App/>` outside the
  shell/router. It asks the PUBLIC `/api/v1/auth/config`, then `/api/v1/auth/session`, and renders Setup /
  Login / the app. It also publishes `AuthContext` (`{ username, method, signOut, requireLogin }`) —
  read `method` via `useContext(AuthContext)` to branch on the auth kind (e.g. `'local'` vs OIDC).
- **The machine-key screen is a normal authenticated screen** (`web/src/screens/ApiKeyScreen.tsx`,
  route `/app/api-key`, System nav group). It loads the server machine key from `/api/v1/auth/machine-key`
  (via `getMachineKey()`) and displays it **masked** (a masked `<code>` that never puts the key in the
  DOM until Reveal) with Reveal + Copy affordances — the key is for MCP / external REST clients; the
  browser no longer uses it. For **local** accounts only (`AuthContext.method === 'local'`) it also
  renders a "Local admin password" card that calls `changePassword(current, new)`, mapping a wrong-
  current-password **401** to the server ProblemDetails detail inline.
- **File-download endpoints go through a fetch-blob helper, never a browser tab.** Endpoints that
  return a binary body (the troubleshooting archive / media-sample POSTs) must **not** go through
  `request()` (it JSON-parses) and must **not** be opened with `window.open` (a session-cookie GET in a
  new tab can't carry the CSRF header and bypasses error handling). Instead `fetch(path, { method:
  'POST', headers: { 'X-Csrf': '1' } })`, then `response.blob()` → object URL → anchor-click →
  revoke, reading the filename from `Content-Disposition`; a non-ok response throws an `ApiError`. See
  `downloadTroubleshootingArchive` / `downloadTroubleshootingMediaSample` in `web/src/api/troubleshoot.ts`.
- **401 UX is one shell-level banner, not per-screen handling.** Rather than special-casing 401 in
  every screen's error path, the request client emits a single app-wide signal on 401
  (`auth.ts` `notifyUnauthorized` / `subscribeUnauthorized`), and `web/src/UnauthorizedBanner.tsx`
  (rendered once in the App shell) subscribes and prompts re-login. This is the DRY seam for "the server
  rejected us" — reuse it instead of adding bespoke 401 branches. Auth flows that expect a 401 as an
  inline answer (login, change-password) pass `suppressUnauthorizedSignal` so they don't trip it.

## 6. Tests

- **vitest**, colocated `*.test.ts` / `*.test.tsx` next to the source file.
- Every screen with meaningful logic gets a screen test; every API client module gets a
  param-mapping / URL-building test (e.g. `logs.test.ts` next to `logs.ts`).
- `web/src/App.test.tsx` covers composition, navigation, shell behavior, and integration regressions
  such as sub-path-to-sub-path rendering and guarded popstate restoration. Detailed screen behavior
  belongs in the screen's colocated test. `web/src/app/routes.test.tsx` independently pins stable
  route-object identity and query/path matching.
- **Nav-label test-selector care**: `getByRole('link'/'button', { name: /Regex/ })` matches by
  substring by default — a loose regex can match more than one nav item. Verified example: the
  System nav button is matched with an **anchored** regex (`name: /^System/`) rather than a bare
  `/System/`, specifically to avoid ambiguous matches against other labels that start with or
  contain "System". Anchor (`^`/`$`) or use exact strings in `getByRole` name matchers whenever a
  new label could be a substring of (or share a substring with) an existing one — check
  `app/routes.tsx`'s nav `label:` list for collisions before picking a new label.
- **Extracted-screen tests own their own fetch mock** (Schedules #207, Channels #244, Playouts
  #245): when a
  screen is pulled out of `App.tsx` into `web/src/screens/<Name>Screen.tsx`, its colocated
  `<Name>Screen.test.tsx` builds a **self-contained** `vi.spyOn(window, 'fetch')` mock scoped to
  that screen's own endpoints (plus local `jsonResponse`/fixture-factory helpers) and renders the
  screen component **directly** — it must not import from `App.test.tsx` or reuse the monolithic
  `mockDashboardApi()`. `App.test.tsx` keeps only a thin **nav-smoke** test for the extracted
  screen (route to it, assert it mounted and hit its own endpoint) plus genuinely cross-cutting
  shell concerns (route table, sub-path ownership §2, the unsaved-changes/popstate guard §8, the
  TopBar action wiring). Detailed screen behavior lives in the screen's own test file. See
  `ChannelsScreen.test.tsx` / `SchedulesScreen.test.tsx` / `PlayoutsScreen.test.tsx` for the shape.

## 7. Verification gate — run before every commit touching `web/`

From `web/`:
```bash
npm test          # vitest
npm run lint      # eslint .
npm run build     # tsc -b && vite build
```
Also run `npm run check:api` if you touched anything OpenAPI-relevant (see `api-conventions.md` §5)
— it regenerates `src/api/generated/v1.d.ts` and fails the build if it's out of sync with what's
committed.

## 8. Unsaved-changes navigation guard

Screens with a **draft / explicit-Save** model (edits accumulate locally, one Save flushes them —
e.g. the schedules editor, `web/src/screens/SchedulesScreen.tsx`) must guard against losing the
draft to navigation. The shared module `web/src/navigationGuard.ts` is the seam:

- The screen **registers a guard** on mount: `registerNavigationGuard(() => !dirtyRef.current ||
  window.confirm(...))`, returning the unregister fn from the mount `useEffect` for cleanup. Only one
  guard is active at a time (the mounted screen); a stale unregister only clears its own guard.
- `App.tsx`'s `navigate` handler calls `canLeaveCurrentScreen()` before `pushState` — a `false`
  return aborts the in-app sidebar/nav click.
- **Browser Back/Forward (`popstate`) is also covered.** `App.tsx`'s `popstate` handler consults
  `canLeaveCurrentScreen()` too. A `popstate` cannot be cancelled — by the time it fires the URL has
  already changed — so on a veto the handler **re-pushes the pre-pop path** (tracked in a
  `currentPathRef` updated on every approved navigation) via `history.pushState` and leaves
  `activeRoute` untouched, effectively undoing the browser's URL change. This same handler also runs
  for the **synthetic** pop `navigateToPath()` dispatches, so programmatic in-app navigation is
  guarded as well. The re-push does **not** re-fire `popstate`; that's safe because only one screen
  is mounted at a time.

- **Guarded sub-path routes: App owns pathname/popstate (resolves the old §2 caveat).** A sub-path
  wrapper that BOTH tracks its own pathname (§2) AND whose sub-screens register a dirty guard cannot
  self-listen for `popstate`: React commits **child** passive effects before the **parent**, so a
  wrapper-owned `popstate` listener fires *before* App's guard-restore handler and would switch
  sub-screen before App could veto and re-push — desyncing the two. **Resolution (design #202 §D.2,
  finding 4):** App is the single `popstate` owner. On a pop it consults `canLeaveCurrentScreen()`,
  and **only on approval** does it update a `currentSubPath` state value (`librariesSubPath`) that it
  passes **down** into the wrapper; on a veto it re-pushes the pre-pop path and updates nothing, so
  the wrapper never sees a vetoed path. The wrapper (`LibrariesRouteScreen`) derives its sub-path
  purely from that prop — never from the raw event — so effect-commit order is irrelevant. **Scope
  the state write to the guarded route only:** the `setLibrariesSubPath` call is gated on the arrived
  route being `libraries`, so unguarded sub-path routes (Playouts/Media) skip it entirely and stay
  byte-identical (they still bail App's `setActiveRoute` via `Object.is` and self-own their pathname
  per §2). The regression test is the App-owned-popstate case in `App.test.tsx` (dirty editor at
  `/app/libraries/local/3`: `confirm→false` keeps URL + mounted sub-screen; `confirm→true`
  navigates). Unguarded sub-path screens (`PlayoutsRouteScreen`, `MediaRouteScreen`) keep
  self-owning their pathname — only screens that register a dirty guard defer to App.
- The screen **also** guards the paths the module still can't see — an in-screen action that would
  replace the draft (schedule switch, opening the properties editor) uses its own
  `window.confirm(...)`, and a `beforeunload` listener (installed while `dirty`) covers full-page
  unloads (reload / close tab / external link).

Keep the guard predicate reading a **ref** (`dirtyRef`), not the `dirty` state value, so
`canLeaveCurrentScreen()` sees the current dirtiness synchronously at click time.

- **Successful save + same-callback navigation:** `useDirtyGuard` returns a `markClean` callback for
  the narrow case where a successful save queues the new draft/baseline and immediately calls
  `navigateToPath(...)` in that same promise callback. React has not committed the clean render before
  the synthetic `popstate`, so call `markClean()` after the durable save succeeds and immediately before
  navigating. Still queue the real clean draft/baseline state; the callback only transfers the synchronous
  guard truth across that one event boundary. Only do this while the successful request still owns the
  current draft: disable or otherwise gate draft mutations for the entire in-flight save, or revision-check
  the completion before marking clean. Otherwise the completion can discard edits made after the request
  started. Do not replace the callback with a timer or a forced render. Screens that remain mounted after
  saving do not need the callback.

## 9. Review checklist — temporal semantics

- **For every effect / timer / async completion, ask: *when* does it fire (mount, dependency change,
  unmount, StrictMode double-invoke) and *which* render/request does it still own?** A debounce timer
  fires on mount too (§3, the `lastQueryRef` no-change guard exists precisely for that); a `.then`
  can resolve after the params it was launched for have moved on (§3a, the `refreshing` gate and the
  Add-all query binding exist for that). A guard that only checks "still mounted" (`activeRef`) does
  not answer "still current".
- **For any "make X consistent with Y" change, re-validate the exemplar Y's temporal behavior before
  copying it.** #221 came from copying a fetch model that keeps stale results visible onto screens
  that had gained mutation surfaces — the exemplar was safe read-only, the copy was not. Copying a
  pattern copies its *assumptions*; confirm they still hold in the new context.

## 10. TopBar primary-action button (`usePrimaryAction`)

The shell TopBar (`app/AppShell.tsx`) renders at most **one** primary-action button (top-right, Plus
icon) for the active screen. `App.tsx` wraps the shell and matched screen in the minimal
`PrimaryActionProvider` from `web/src/primaryAction.ts`: the active screen explicitly registers one
handler, and the TopBar reads that matching registration directly from React context. No window event,
string-keyed dispatcher, or generic screen-action framework participates in normal screen actions.

- **A screen opts in** by calling `usePrimaryAction(routeId, handler)` at the **top of the component, before
  any early return** (it's a hook). The handler must be reachable there — a create/navigate handler or a
  `useState` setter, not something defined below a loading/error `return`. Reference: `SchedulesScreen`
  (`usePrimaryAction('schedules', () => setForm('create'))`).
- **The provider owns exactly one registration.** Each mounted hook gets an opaque owner token; registering a
  newer screen replaces the previous registration, and cleanup clears only the registration owned by that
  hook. A stale unmount therefore cannot erase the newer screen's action. The hook keeps the latest handler in
  a ref, so ordinary screen renders update behavior without reclaiming/churning ownership. Directly-rendered
  screen tests outside the provider remain harmless; there is simply no shell action consumer.
- **The route must ALSO declare a matching non-empty `primaryAction` label** in the stable
  `app/routes.tsx` table. The TopBar renders the button only when that label is non-empty **and** the active
  route owns a matching registration — metadata alone can no longer produce a dead button, while a
  registration without a label remains intentionally invisible. Keep the two in lockstep.
  The `#238` tests in `App.test.tsx` guard this: a data-driven `it.each` asserts each URL-navigating create
  screen's banner actually navigates (so a typo'd route id → a button that navigates nowhere → red), plus a
  drop test that an action screen shows no banner button. `primaryAction.test.tsx` separately pins matching,
  latest-handler, route-change, cleanup, and stale-owner semantics. The dialog/editor create screens
  (schedules, multi/rerun collections, trakt) are exercised by their own create-flow tests.
- **When to WIRE vs DROP (issue #238).** The Plus icon makes the button semantically a *"create new item"*
  affordance. **Keep + wire it only on list screens with a single, unambiguous create flow** ("Add Channel",
  "Add Schedule", "Add Multi-Collection", "Add Rerun Collection", "Add Trakt List", "Add Filler Preset", "Add
  Profile", "Add Watermark"). **Drop it** (`primaryAction: ''`) where the primary action is **not** a create
  (Save / Refresh / Play / Validate / Reset / Scan — the "+" is wrong, and those screens already carry the
  correct in-body control), or where a single banner action would be **ambiguous** (Collections has two create
  types behind tabs), **a silent no-op** (the Channel builder's Create is disabled until the form is valid),
  or **semantically misplaced** (Dashboard is a status page; Libraries "Scan" is per-row). Coexisting with an
  in-body create control is fine (Schedules has both) — the banner is a convenience, not the sole entry point.
  See `docs/decisions.md` 2026-07-12 for the full rationale.

## 11. Slide-over panels + the shared advanced-options model

- **`SlideOver` (right-edge detail/edit panel)** lives in `web/src/components/overlay.tsx` alongside
  `Dialog`. Use it for a per-item edit/detail surface layered over a screen (the Auto-Tune "Configure"
  panel is the reference, #386); use `Dialog` for a centered confirm/short form. Both share one
  private `useOverlayBehavior(open, onClose, panelRef)` hook (focus + body-scroll lock on the
  closed→open transition, Escape-to-close via a latest-`onClose` ref, scrim-click dismiss). It
  portals to `document.body` and takes `title`/`subtitle`/`children`/`footer`/`width`. Don't build a
  bespoke edge panel — extend `SlideOver`.
- **Draft-until-commit slide-overs need the §8 guard at the SCREEN, not the panel.** When per-item
  edits accumulate in the parent screen's state and are flushed by one later action (Auto-Tune's
  bulk-create), closing the panel does **not** lose data (it's still in screen state) — so the
  `window.confirm` guard belongs on *screen navigation / unload* while any uncommitted edit exists,
  not on panel close. Show an "Edited" badge on customised rows so the pending edits are visible.
- **Per-source correction rows: weight stepper + include/exclude toggle, keyed by the server's source id**
  (#440 wiring #425's backend into the Auto-Tune DetailPanel's Content-sources pane). The row model is
  reusable wherever an item list carries per-item rotation weights:
  - The row's draft (`SourceDraft` in `AutoTuneScreen.tsx`) holds `weight` as a **string** per §4a, plus a
    boolean `excluded`; the bound `WEIGHT_MIN`/`WEIGHT_MAX` = 1..1000 mirrors the server's
    `MultiCollectionItemWeight` validator — the same 1..1000 the multi-collection editor enforces (#404), but
    via its **own screen-local pair**: same values, separate consts, not a shared module. It is clamped on
    `onBlur` **and** again in the draft→request mapping.
  - **Only genuinely customised rows go on the wire.** The draft→request mapper (`sourcesRequest`) drops any
    row that is still fair-share (weight 1, not excluded, not added) and returns `undefined` when nothing is
    left, so the field is **omitted** rather than sent as an all-default array. That predicate mirrors the
    server's own `customized` check exactly — an all-default array is a documented no-op there, and omitting
    it keeps the channel on the cheaper single-SmartCollection shape. The same predicate drives the row's
    "Edited" badge and the §8 dirty guard, so a value touched and set back to the default is correctly *not*
    an edit (same rule as a name cleared back to empty).
  - **A library picker compiles typed text; it never forwards raw Lucene.** The "add a source not in the
    base set" typeahead sends `title:*<escaped>*` (`titleContainsQuery`), not the user's literal text: the
    search index's default field does **not** match bare title words (`Alpha` finds nothing for "Show Alpha"
    — see `e2e-local.md`), so a raw forward like `SearchScreen`'s (where the box is explicitly a query box)
    would look broken in a *name* picker. Escape every Lucene special + whitespace so the boundary stars are
    the only live wildcards — the same shape `builder/rules/compile.ts` emits for `contains`.
  - Rows are read-keyed by the **server's** id (`GET /api/v1/channels/auto-tune/members`), and an added id
    that turns out to already be a base member patches the existing row instead of appending a duplicate.
- **Advanced channel-options overrides are shared, not duplicated** (`web/src/builder/advancedOptions.tsx`).
  The `CreateChannelFromLineupAdvancedOptionsRequest` override model — the enum catalogs,
  `ADVANCED_KEYS`, `effectiveValue`, and the field adapters (`useAdvancedOverrides`) — is one module
  consumed by both the Channel Builder and the Auto-Tune DetailPanel, and must not be re-implemented
  per screen. Each screen writes its own field JSX over the shared hook; `playbackOrder`/`playoutMode`
  are surfaced as dedicated controls (Builder state / Auto-Tune's Shuffle + Always-playing toggles) and
  merged into `advanced` at create, not carried in the override map.
- **Three per-field states: INHERIT / set / CLEAR** (#135, `api.from-lineup-clear-to-none`). A select on
  `INHERIT` (or a cleared text input) **omits** the field so the create handler coalesces it with the
  template value. A concrete value overrides the template. The **`CLEAR` sentinel** ("None") is stored in
  the overrides map like any override but is folded into the request's `advanced.clear` list at build
  time by `applyOverridesToRequest`/`collectClears` — so the sentinel never reaches the wire as a field
  value, and `effectiveValue` reads a `CLEAR` override as none. **Build the request only through
  `applyOverridesToRequest`**, never a raw spread of the overrides map (a spread leaks the `CLEAR`
  sentinel). Today only the five id selects (watermark + fillers) expose a "None" option; the backend
  `clear` enum also covers the preferred audio/subtitle language strings for machine clients, but the SPA
  text inputs keep "empty = inherit" (a tri-state text control is deferred) — API-ahead-of-UI, matching
  the "REST API is a real audience" posture.

## 12. Reusable rule builder (`web/src/builder/rules/`)

A visual rule builder for Lucene-backed queries, introduced for the SmartCollection create/edit
dialog (#176) and built as a **standalone, controlled component module** — not a SmartCollection
screen concern — so it can be embedded by later screens without duplicating the rule model.

- **`types.ts`** — the rule tree: `Rule` (field/operator/value), `Group` (`match: all|any` over
  `Rule`s and/or **recursively nested** sub-`Group`s), `Operator`, `FieldType`, an `isGroup`
  narrowing helper, and **`MAX_GROUP_DEPTH`** — the authoring cap on nesting (root group = depth 0,
  so up to `MAX_GROUP_DEPTH` levels of sub-groups below it; currently 5). #436 replaced #176's
  one-level "Kodi" model with bounded-arbitrary depth — see the `spa.rulebuilder-nesting` decision.
  `MAX_GROUP_DEPTH` is the **single** source of truth: the UI's "Add group" gate
  (`RuleBuilder.tsx`), `parse.ts` (anything deeper is out of subset ⇒ `null` ⇒ raw-text fallback)
  and `roundtrip.test.ts`'s generator all read it — never re-hardcode a depth.
- **`compile.ts`** / **`parse.ts`** — `compile(group)` turns a rule tree into a query string covering
  a **closed subset** of the Lucene grammar; `parse(input, fieldTypes)` is its exact inverse, returning
  `null` (never a lossy best-effort tree) for any query outside that subset. Escaping is total, so a
  compile→parse round-trip is lossless for any builder-authored value, including Lucene special
  characters — pinned by a 500-tree property test (`roundtrip.test.ts`, which generates trees to the
  full `MAX_GROUP_DEPTH` and asserts the corpus actually reached that depth).
- **`fieldCatalog.ts`** — `useSearchFields()` wraps `GET /api/v1/search/fields` and exposes a clean,
  non-null `RuleField[]` (plus a `fieldTypes` lookup and a `byGroup` grouping) for field pickers; this
  is the only place the raw API response shape is unwrapped.
- **`RuleBuilder.tsx`** — the controlled component itself: `{ group, onChange }` in, add/remove
  rule and sub-group UI out. It holds no query-string state — the embedding screen owns the compiled
  `query` string (`compile(group)` on change) and, symmetrically, seeds `group` via `parse(query,
  fieldTypes)` on load (falling back to raw-text editing on a `null` parse).

**Usage pattern**: a screen owns two things — the raw `query` string (what actually gets saved) and
the parsed `Group | null` (what the builder edits). Toggling between "Builder" and raw-text modes is
just switching which of the two is the source of truth for that render, re-deriving the other via
`compile`/`parse` at the toggle boundary. See `SmartCollectionDialog` for the reference integration.

- **`SmartCollectionDialog.tsx`** (`web/src/builder/`) — the name + query authoring dialog wrapping
  `RuleBuilder` (Builder/Advanced toggle + a live preview count), extracted from CollectionsScreen so
  it is a **shared, self-contained editor** (#437). Its only coupling to a host is the interface
  `{ open, busy, error, initial, onCancel, onSubmit({name, query}) }` — the parent owns the `onSubmit`
  side effect (persist, add to a lineup, …). Consumed by **CollectionsScreen** (create/edit) and the
  **ChannelBuilder** (`/app/new-channel`, "New smart query" in the Collections source): the Builder
  authors an ad-hoc query, `createSmartCollection` persists it as a real named SmartCollection, and the
  new collection is added to the lineup by its `smartCollectionId` — no new REST/MCP surface (the MCP
  already exposes `ersatztv_create_smart_collection`).

Generalizing **Auto-Tune's** fixed axis picker into an arbitrary-field builder is a *different*
primitive (group-by, not single-query filtering) and a backend epic — designed separately in
`docs/superpowers/specs/2026-07-23-auto-tune-arbitrary-field-design.md`, tracked as its own issue,
not wired here.

- **Relative-date operators** (#435, `rulebuilder.relative-date-macros`) — `release_date`/`added_date`
  gain `inLast`/`notInLast` alongside `before`/`after`/`between`, each with a numeric value plus a
  `day|week|month|year` `unit` picker (`types.ts`'s `unit?: DateUnit`). `dateMacro.ts` is the single
  seam: a `field↔macro-prefix` table (`release_date↔released`, `added_date↔added`) plus the
  `inLast/notInLast ↔ inthelast/notinthelast` suffix maps, delegated to from `compile.ts`/`parse.ts`
  for these two fields — the compiled query is the existing `released_inthelast:"7 day"`-style
  `CustomMultiFieldQueryParser` macro, so nothing downstream changes. `validation.ts`'s `ruleError`
  requires the value to parse as a positive integer before it's compiled.
- **Facet-value typeahead** (#434/#578, `api.search-field-values-sources`) — the value input for a `text` field
  (not enum) is a combobox backed by `getSearchFieldValues` (`web/src/api/search.ts` →
  `GET /api/v1/search/fields/{name}/values?q=&limit=`), debounced on keystroke, prefix-matching the
  in-progress value against distinct terms already in the index. It always allows free-text entry as a
  fallback — a 404 (non-text field) or an empty result list (e.g. ElasticSearch backend) degrades to a
  plain text input rather than blocking the rule. Since #578 (`api.search-field-values-sources`)
  `album_artist` returns values instead of 404ing, and `artist` covers free-text music-video/song credits
  as well as entity artists; for those two the server's list is **bounded best-effort** on a very large
  library, so the free-text fallback stays load-bearing — never treat an absent suggestion as an invalid
  value.
- **Single-child-group normalization** (#438) — `normalizeGroup` (`validation.ts`) coerces a group's
  `match` to `all` whenever it has fewer than two children, recursively. A one-child `any` group is
  semantically identical to `all` but doesn't round-trip through `compile`→`parse` (the compiled Lucene
  for a lone child carries no `AND`/`OR`), so the builder normalizes on every change rather than let the
  UI and the compiled query silently diverge; `roundtrip.test.ts`'s property test asserts against
  `normalizeGroup(tree)`, not the raw generated tree.

## 13. Collapsible sidebar + nav-group accordions (#396)

The shell sidebar (`web/src/app/AppShell.tsx`) supports two independent, persisted collapse states.
Both are shell chrome — no screen participates.

- **State + persistence** live in `web/src/app/sidebarState.ts` (`useSidebarState()`), consumed by
  `AppShell` alone. It follows the §5d client-local-prefs pattern (a try/catch `getStorage()`, a
  validating getter, a write-through setter) over two namespaced keys:
  - `ctv-sidebar-collapsed` — `"1"`/`"0"`; is the sidebar collapsed to the 60px **icon rail**?
  - `ctv-sidebar-groups` — JSON `{groupKey: boolean}` where the boolean is **collapsed**. A labeled
    group with **no stored entry defaults to collapsed**, so a fresh load shows only the always-open
    **Primary** group. (Keys are the `ctv-` hyphen form, not the prototype's `ctv.sidebar.*` — see
    `decisions.md` 2026-07-18.)
- **`AppShell` stamps `ctv-app-shell-collapsed` on the shell root** when collapsed; the rail look is
  entirely CSS-driven from that one class (`shell.css` narrows the tracked `--sidebar-w` to 60px and
  transitions `grid-template-columns`; `@media (prefers-reduced-motion: reduce)` drops the transition).
- **Nav is inventory-driven** from `sidebarNavGroups` (`app/routes.tsx`). Only **labeled** groups are
  collapsible; each has an explicit stable `key` (`'media'`, `'system'`) used for the persisted map —
  don't derive the key from the label (a rename would orphan persisted state). The unlabeled Primary
  group is always rendered.
- **Accordions apply only in the expanded sidebar.** In the rail, group-collapse is ignored: every
  item renders (icon-only), groups separated by a `.ctv-nav-divider`. The nav item's **label stays in
  the a11y tree** (visually hidden via CSS, not `display:none`) so the accessible name — and every
  `getByRole('link', { name })` test — still resolves; the label is also passed as the native `title`
  tooltip (`NavItem` gained a `title` prop). Numeric badges collapse to a corner dot. The active-route
  indicator works in both states.
- **Testing note**: `App.test.tsx`'s shell/nav suite clicks Media/System nav links directly, so its
  `beforeEach` **seeds both groups open** (`ctv-sidebar-groups`); the default-collapsed / accordion /
  rail behavior is covered in its own `describe('collapsible sidebar (#396)')`, and the persistence
  helpers have a colocated `sidebarState.test.ts`.

## 14. Server-derived rollup verdict: one filter, per-item fault badges (#415)

When the server folds several fault classes into one verdict object (see `api-conventions.md` →
"server-derived rollup verdict" and `decisions.md` → `api.channel-health-object`), the SPA gets **one**
list filter/count off the rollup `status`, never a filter per individual fault:

- **One rollup filter, not N fault filters.** `ChannelsScreen.tsx`'s `problems` filter/count reads only
  `health.status === 'Problems'` (`hasProblems(channel)`); it replaced the earlier single-fault
  `willNeverPlay`/"No playout" filter now that the taxonomy is real (`#415` superseded the "don't
  broaden the label" comment that guarded against a false all-clear). Do not add a second filter per
  fault class — that re-fragments the one signal the rollup exists to unify.
- **Per-row badges still name the specific fault(s)**, spelling out `health.faults` (`No playout` /
  `Never built` / `Build failed` / `Empty` / `Broken source`) rather than a generic "Problems" chip —
  the rollup unifies the *filter*, not the *detail* a user reads on a given row. `Unknown` renders no
  badge (it is deliberately not `Healthy` and not `Problems`).
- **Defensive coercion at the read boundary, even though the generated type is non-null.** `health` is in
  the OpenAPI `required` list, so the generated `ChannelSummary['health']` types as
  `{status, faults, playoutCount, brokenSourceItemCount}` — not nullable (contrast
  `core-dtos-generate-nullable-in-spa`, which covers *other* Core DTO fields that do generate
  `| null | undefined`). Still always read it through optional chaining with an explicit fallback at the
  point of use (`channel.health?.status`, `channel.health?.faults ?? []`) rather than asserting it
  non-null — a genuinely absent `health` (e.g. an older cached response predating this field) must read
  as "no verdict", never crash or silently read as `Healthy`.
- **Status/fault string unions are hand-maintained**, matching `ChannelPreviewAvailability`: the server
  emits plain strings backed by a `const string` class (`ChannelHealthStatus`, `ChannelFault`), not a
  generated enum, so the SPA's local TS union (`'Healthy' | 'Problems' | 'Unknown'`, and the fault
  union) is kept in sync by hand when the server adds a value.