diff --git a/plugins/folder_library/CLAUDE.md b/plugins/folder_library/CLAUDE.md
index f687dbb..dc69ab5 100644
--- a/plugins/folder_library/CLAUDE.md
+++ b/plugins/folder_library/CLAUDE.md
@@ -153,15 +153,14 @@ Each song object (built by `_meta()`):
   "added": 1748132400.0,
   "arrangements": ["Lead", "Rhythm", "Bass"],
   "stems": ["Drums", "Bass", "Vocals"],
-  "lyrics": true,
-  "has_preview": true
+  "lyrics": true
 }
 ```
 
 - `filename` is the full relative path from the DLC root — pass it directly to `window.playSong()`.
 - `added` is a Unix timestamp (float, seconds) from `stat().st_mtime` — convert with `new Date(added * 1000)`. Always recomputed fresh (it changes when a file moves), even on a metadata-cache hit.
 - `arrangements` / `stems` are flat lists of **strings**, even though `extract_meta()` returns them as objects.
-- `has_preview` is set from the pack's manifest `preview:` key (via core `load_manifest`) — `true` iff `song_preview` will actually serve a preview. The frontend previews **only** `has_preview` songs, so hover never requests (and 404-logs) a preview-less pack. Hover-preview audio itself is **not** resolved here — it comes from the `song_preview` endpoint (see Preview on Hover).
+- Hover-preview isn't resolved here at all — the cards just carry `data-fn`/`data-v3-play` markup and the `song_preview` plugin handles it (see Preview on Hover).
 
 ### extract_meta returns arrangements/stems as objects, not strings
 
@@ -333,17 +332,16 @@ Drag-and-drop uses **pointer events** (mousedown/mousemove/mouseup), not the HTM
 
 ## Preview on Hover
 
-Hovering a song for `_HOVER_PREVIEW_DELAY_MS` (800 ms) plays a short audio preview in place — no navigation to the player. Toggled by a play-icon toolbar button (`_injectToolbar`); **on by default** (`_previewHover`, persisted per surface under `<cfg.storePrefix>previewHover` — only an explicit stored `'false'` disables it).
+The Folder Library does **not** implement hover-preview itself — the `song_preview` plugin does, for the whole app. Its hover loop finds song elements by the selector `#v3-songs [data-fn]` (with a `[data-v3-play]` playable surface) and its `MutationObserver` watches the `#v3-songs` subtree — and the folder view (`#lib-folder-tree`) renders **inside** `#v3-songs`. So all folder_library has to do is give each card/row the standard markup:
 
-- **Audio** — a dedicated `_previewAudio` `<audio>` element (never the main player's), played **from 0**. `_startPreview` runs **only when `song.has_preview`** (a backend flag from the manifest `preview:` key), so a preview-less pack is never requested — that's what keeps hover from logging 404s (a HEAD/`<audio>` request to `song_preview`'s 404 shows in the console even via `fetch`, because a plugin wraps `window.fetch`). The URL (`_previewUrl`) points at the **`song_preview` plugin** — `/api/plugins/song_preview/audio?file=<filename>` — the same endpoint the grid/list previews use. `song_preview` serves the clip **strictly** from the manifest `preview:` key (short baked clip, Range); there is **no stem fallback by design** (a full-length stem isn't a preview), so packs without a baked preview — hand-authored, most tutorials — don't preview until `song_preview`'s backfill generates one. Don't seek by `song.duration` — the clip is short, so a full-song offset lands past its end and nothing plays.
-- **Sequence guard** — `_previewSeq` is bumped on every start/stop, so a slow load that resolves after the pointer has moved on is ignored.
-- **Drag / click safety** — `mouseenter` skips arming while a drag is in progress (`_dragState` non-null); `mousedown` clears the pending dwell timer **and** `_stopPreview()`s any already-playing one, so grabbing a song to drag (or clicking to play) never leaves a preview running.
-- **Indicator** — a waveform overlay (`.fl-wf`, 9 bars) over the art, drawn via a one-time injected `<style>` (`_ensurePreviewStyle`). It **fades in** (`fl-in`) to avoid an abrupt pop, and the bars animate only once audio actually fires (the `.playing` class, set on the `playing` event). Perf: bars animate with `transform: scaleY` + `will-change: transform` (GPU-composited, no per-frame JS), only while playing. `_showIndicator` / `_markIndicatorPlaying` / `_clearIndicator` manage it against `_previewIndHost`.
-- **CSS gotcha** — set the bar animation with **longhand** `animation-name`/`-duration`/… , not the `animation` shorthand: the shorthand resets `animation-delay` to 0 and (being higher-specificity) overrode the per-bar `nth-child` delays, making every bar move in sync.
+- **`_songCard`** — `card.dataset.fn = song.filename` (raw filename) + `data-v3-play` on the art wrap (the surface `song_preview` overlays its indicator on).
+- **`_songRow`** — `row.dataset.fn = song.filename` + `data-v3-play` on the thumb.
+
+`song_preview` then previews folder view exactly like the grid/list — same audio (its `/audio` endpoint), same availability/404 handling, same indicator — with **zero preview code here**. If you change the card/row structure, keep `data-fn` (raw, not URL-encoded) and a `[data-v3-play]` descendant, or `song_preview` will stop recognising the cards.
 
 ## Roadmap
 
-Implemented since the original release: **nested subfolders** (recursive tree + create-inside-folder), drag-and-drop, sort, advanced filtering, server-side tree filtering synced to the host library, the warm metadata cache, and **preview-on-hover** (see above).
+Implemented since the original release: **nested subfolders** (recursive tree + create-inside-folder), drag-and-drop, sort, advanced filtering, server-side tree filtering synced to the host library, and the warm metadata cache. Hover-preview is provided by the `song_preview` plugin via the `data-fn` markup above (not implemented here).
 
 Not yet implemented, in rough priority order:
 
diff --git a/plugins/folder_library/README.md b/plugins/folder_library/README.md
index f8f5788..b9c4605 100644
--- a/plugins/folder_library/README.md
+++ b/plugins/folder_library/README.md
@@ -30,7 +30,7 @@ A FeedBack (fee[dB]ack) plugin that organizes your `.sloppak` / `.feedpak` DLC s
 - **List & Grid views** — toggle between a compact list with thumbnails or a full album art card grid
 - **Album art** — pulls art automatically for every song in both views
 - **One-click playback** — click any song to start playing immediately
-- **Preview song on hover** — hover a song briefly to preview its audio in place, with a waveform indicator over the art; toggle from the toolbar (on by default)
+- **Preview song on hover** — hover a song to hear a quick preview of its audio (powered by the Song Preview plugin, the same way the grid and list views work)
 - **Sort options** — sort songs by title, artist, duration, year, tuning, or recently added with an asc/desc toggle
 - **Advanced filters** — filter by arrangements, stems, lyrics, and tuning with include and exclude support
 - **Folder management** — create, rename, and delete folders without leaving the plugin
@@ -55,7 +55,7 @@ Folder Library ships bundled with FeedBack as a core plugin (`"bundled": true`),
 | Switch to grid view | Click the grid icon in the toolbar |
 | Switch to list view | Click the list icon in the toolbar |
 | Play a song | Click any song row or card |
-| Preview song on hover | On by default; click the toolbar button to toggle. When on, hovering a song ~0.8s previews its audio |
+| Preview song on hover | Hover a song — the Song Preview plugin plays a quick clip (same as the grid/list views) |
 | Sort songs | Use the sort dropdown in the toolbar |
 | Toggle sort direction | Click the arrow button next to the sort dropdown |
 | Open filters | Click the filter icon in the toolbar |
diff --git a/plugins/folder_library/routes.py b/plugins/folder_library/routes.py
index d01c37f..6163f9c 100644
--- a/plugins/folder_library/routes.py
+++ b/plugins/folder_library/routes.py
@@ -12,14 +12,6 @@ from fastapi.responses import JSONResponse
 import shutil
 import re
 
-# Core manifest reader — used only to tell the frontend which packs actually
-# carry a baked preview (manifest `preview:` key), so hover never requests a
-# preview-less pack and logs a 404. Guarded so the plugin still loads without it.
-try:
-    from sloppak import load_manifest as _load_manifest
-except Exception:
-    _load_manifest = None
-
 
 # ── Pure, testable helpers ─────────────────────────────────────────────────
 
@@ -144,13 +136,7 @@ def setup(app, context):
 
         # Cache miss — run the expensive extract.
         m = {"title": None, "artist": None, "album": None, "duration": None,
-             "year": None, "tuning": None, "arrangements": [], "stems": [], "lyrics": False,
-             "has_preview": False}
-        if _load_manifest is not None:
-            try:
-                m["has_preview"] = bool((_load_manifest(p) or {}).get("preview"))
-            except Exception:
-                pass
+             "year": None, "tuning": None, "arrangements": [], "stems": [], "lyrics": False}
         try:
             raw = context["extract_meta"](p)
             if raw:
diff --git a/plugins/folder_library/screen.js b/plugins/folder_library/screen.js
index bbbfb34..24b8ddf 100644
--- a/plugins/folder_library/screen.js
+++ b/plugins/folder_library/screen.js
@@ -50,10 +50,6 @@ function createFolderSurface(cfg) {
     let _sortDir          = _store('sortDir') || 'asc';
     let _toolbarDone      = false;
     let _hoveredFolder    = null;             // { wrap, hdr, btnGroup } — only innermost folder is active
-    let _previewHover     = _store('previewHover') !== 'false';  // default on; preview a song's audio on hover
-    let _previewTimer     = null;
-    let _previewAudio     = null;   // dedicated element — never touches the main player's <audio>
-    let _previewSeq       = 0;      // invalidates in-flight loads when the hover moves or stops
 
     // ── Core arrangement order (pinned to top of filter panel) ──────────
     const _CORE_ARRANGEMENTS = ['Lead', 'Rhythm', 'Bass', 'Combo'];
@@ -735,119 +731,15 @@ function createFolderSurface(cfg) {
     function _prompt(msg, def) { return _showModal(msg, true,  def || ''); }
 
     // ── Song card (grid view) ───────────────────────────────────────────
-    // ── Preview on hover (opt-in; toggled from the toolbar) ─────────────
-    // Streams the song's own audio into a dedicated <audio> element after the
-    // pointer dwells briefly (so skimming doesn't blast audio), stopping on
-    // leave. Shows an equalizer "now playing" indicator over the art. Never
-    // calls playSong or touches the main player's <audio> element.
-    var _HOVER_PREVIEW_DELAY_MS = 800;   // dwell before preview — long enough that a click/drag doesn't trigger it
-    var _previewIndHost = null;             // art element currently showing the indicator
-
-    // Delegate to the song_preview plugin (the canonical preview subsystem): it
-    // resolves the clip from the pack's manifest `preview:` key (falling back to
-    // the default stem), serves it with Range support, and backfills a missing
-    // preview.ogg. If song_preview isn't installed the endpoint 404s and the
-    // audio's onerror clears the indicator — preview no-ops, nothing breaks.
-    function _previewUrl(song) {
-        if (!song || !song.filename) return null;
-        return '/api/plugins/song_preview/audio?file=' + encodeURIComponent(song.filename);
-    }
-
-    function _ensurePreviewStyle() {
-        if (document.getElementById('fl-preview-style')) return;
-        var st = document.createElement('style');
-        st.id = 'fl-preview-style';
-        st.textContent =
-            '.fl-preview-ind{position:absolute;inset:0;display:flex;align-items:center;justify-content:center;background:rgba(0,0,0,0.5);z-index:4;pointer-events:none;animation:fl-in .28s ease both;}' +
-            '.fl-wf{display:flex;align-items:center;gap:2px;height:55%;max-height:22px;opacity:.55;transition:opacity .2s ease;}' +
-            '.fl-preview-ind.playing .fl-wf{opacity:1;}' +
-            '.fl-wf i{display:block;width:2px;height:100%;background:#60a5fa;border-radius:2px;transform:scaleY(.4);transform-origin:center;}' +
-            '.fl-preview-ind.playing .fl-wf i{animation-name:fl-wf;animation-duration:1s;animation-timing-function:ease-in-out;animation-iteration-count:infinite;will-change:transform;}' +
-            '.fl-wf i:nth-child(odd){animation-delay:-.2s}.fl-wf i:nth-child(3n){animation-delay:-.4s}.fl-wf i:nth-child(4n){animation-delay:-.1s}' +
-            '@keyframes fl-wf{0%,100%{transform:scaleY(.4)}50%{transform:scaleY(1)}}' +
-            '@keyframes fl-in{from{opacity:0}to{opacity:1}}';
-        document.head.appendChild(st);
-    }
-    function _showIndicator(host) {
-        _clearIndicator();
-        if (!host) return;
-        _ensurePreviewStyle();
-        var ind = document.createElement('div');
-        ind.className = 'fl-preview-ind';
-        ind.innerHTML = '<span class="fl-wf"><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i></span>';
-        host.appendChild(ind);
-        _previewIndHost = host;
-    }
-    function _markIndicatorPlaying() {
-        var ind = _previewIndHost && _previewIndHost.querySelector('.fl-preview-ind');
-        if (ind) ind.classList.add('playing');
-    }
-    function _clearIndicator() {
-        if (!_previewIndHost) return;
-        var ind = _previewIndHost.querySelector('.fl-preview-ind');
-        if (ind) ind.remove();
-        _previewIndHost = null;
-    }
-
-    function _previewEl() {
-        if (_previewAudio) return _previewAudio;
-        var a = document.createElement('audio');
-        a.preload = 'none';
-        a.volume  = 0.7;
-        _previewAudio = a;
-        return a;
-    }
-    function _stopPreview() {
-        _previewSeq++;                       // any pending load/error/playing handler is now stale
-        _clearIndicator();
-        if (_previewAudio) {
-            _previewAudio.onerror = _previewAudio.onloadedmetadata = _previewAudio.onplaying = null;
-            try { _previewAudio.pause(); } catch (_) {}
-            _previewAudio.removeAttribute('src');
-            try { _previewAudio.load(); } catch (_) {}
-        }
-    }
-    function _startPreview(song, host) {
-        // Only packs the backend flagged with a baked preview are requested at
-        // all — so hover never hits song_preview's 404 for a preview-less pack
-        // (which would log a console error). Play from 0; the clip is short.
-        if (!song || !song.has_preview) return;
-        var url = _previewUrl(song);
-        if (!url) return;
-        var a   = _previewEl();
-        var seq = ++_previewSeq;
-        _showIndicator(host);
-        a.onerror          = function () { if (seq === _previewSeq) _clearIndicator(); };
-        a.onloadedmetadata = function () { if (seq === _previewSeq) a.play().catch(function () {}); };
-        a.onplaying        = function () { if (seq === _previewSeq) _markIndicatorPlaying(); };
-        a.src = url;
-        a.load();
-    }
-    function _armHoverPreview(el, song, host) {
-        el.addEventListener('mouseenter', function () {
-            if (!_previewHover || _dragState) return;   // don't preview while dragging a song
-            clearTimeout(_previewTimer);
-            _previewTimer = setTimeout(function () {
-                if (_previewHover && !_dragState) _startPreview(song, host);
-            }, _HOVER_PREVIEW_DELAY_MS);
-        });
-        el.addEventListener('mouseleave', function () {
-            clearTimeout(_previewTimer);
-            _stopPreview();
-        });
-        // A click or drag-start cancels a pending preview AND stops one that's
-        // already playing, so it never runs mid-interaction.
-        el.addEventListener('mousedown', function () { clearTimeout(_previewTimer); _stopPreview(); });
-    }
-
     function _songCard(song, folderName) {
         var card = document.createElement('div');
         card.className = 'flex flex-col rounded-lg overflow-hidden cursor-pointer group transition-transform duration-100 hover:scale-105';
         card.style.background = '#1a1d2e';
-        card.dataset.filename  = song.filename;
+        card.dataset.fn = song.filename;   // data-fn (raw): song_preview's hover loop finds cards by this
 
         var artWrap = document.createElement('div');
         artWrap.style.cssText = 'position:relative; width:100%; padding-bottom:100%; background:#111827; overflow:hidden;';
+        artWrap.setAttribute('data-v3-play', '');   // the playable surface song_preview overlays its indicator on
         var img = document.createElement('img');
         img.style.cssText = 'position:absolute; inset:0; width:100%; height:100%; object-fit:cover;';
         img.alt = ''; img.loading = 'lazy';
@@ -906,7 +798,6 @@ function createFolderSurface(cfg) {
             if (typeof window.playSong === 'function') window.playSong(song.filename);
         });
         _makeDraggable(card, song, folderName);
-        _armHoverPreview(card, song, artWrap);
         return card;
     }
 
@@ -914,10 +805,11 @@ function createFolderSurface(cfg) {
     function _songRow(song, folderName) {
         var row = document.createElement('div');
         row.className = 'flex items-center gap-3 px-3 py-2 rounded cursor-pointer hover:bg-dark-500 group transition-colors duration-100';
-        row.dataset.filename = song.filename;
+        row.dataset.fn = song.filename;   // data-fn (raw): song_preview's hover loop finds rows by this
 
         var thumb = document.createElement('div');
         thumb.style.cssText = 'width:36px; height:36px; border-radius:4px; overflow:hidden; background:#111827; flex-shrink:0; position:relative;';
+        thumb.setAttribute('data-v3-play', '');   // marks the row previewable for song_preview
         var tImg = document.createElement('img');
         tImg.loading = 'lazy';
         tImg.src = '/api/song/' + song.filename.split('/').map(encodeURIComponent).join('/') + '/art';
@@ -977,7 +869,6 @@ function createFolderSurface(cfg) {
             if (typeof window.playSong === 'function') window.playSong(song.filename);
         });
         _makeDraggable(row, song, folderName);
-        _armHoverPreview(row, song, thumb);
         return row;
     }
 
@@ -1695,30 +1586,10 @@ function createFolderSurface(cfg) {
         );
         colBtn.addEventListener('click', _collapseAll);
 
-        // Preview-on-hover toggle (persisted per surface via cfg.storePrefix)
-        var previewBtn = document.createElement('button');
-        previewBtn.style.cssText = 'display:flex; align-items:center; gap:6px; padding:7px 12px; border:1px solid #374151; border-radius:10px; cursor:pointer; font-size:13px; white-space:nowrap; transition:color 0.1s, background 0.1s, border-color 0.1s;';
-        previewBtn.innerHTML = '<svg viewBox="0 0 20 20" fill="currentColor" style="width:14px;height:14px"><path d="M6 4l10 6-10 6V4z"/></svg>';
-        function _applyPreviewBtn() {
-            previewBtn.style.background  = _previewHover ? '#1d4ed8' : '#171a22';
-            previewBtn.style.color       = _previewHover ? '#ffffff' : '#565f6d';
-            previewBtn.style.borderColor = _previewHover ? '#3b82f6' : '#2a303b';
-            previewBtn.style.opacity     = _previewHover ? '1' : '0.5';
-            previewBtn.title = 'Preview song on hover: ' + (_previewHover ? 'on' : 'off');
-        }
-        _applyPreviewBtn();
-        previewBtn.addEventListener('click', function () {
-            _previewHover = !_previewHover;
-            _store('previewHover', _previewHover ? 'true' : 'false');
-            if (!_previewHover) { clearTimeout(_previewTimer); _stopPreview(); }
-            _applyPreviewBtn();
-        });
-
         ctrl.appendChild(viewGroup);
         ctrl.appendChild(newBtn);
         ctrl.appendChild(expBtn);
         ctrl.appendChild(colBtn);
-        ctrl.appendChild(previewBtn);
 
         _toolbarDone = true;
     }
@@ -1735,7 +1606,6 @@ function createFolderSurface(cfg) {
     // ── Unload (lib surface) ────────────────────────────────────────────
     function _unload() {
         _clearVirtualLists();   // don't leave scroll listeners behind on teardown
-        _stopPreview();         // silence any in-progress hover preview
         if (!cfg.searchInputId) return;
         var el = _el(cfg.searchInputId);
         if (el) el.style.maxWidth = '';
@@ -1839,8 +1709,16 @@ function createFolderSurface(cfg) {
         init: _init,
         onScreenChanged: _onScreenChanged,
         render: _render,
-        // Pure helpers, exposed for tests (no DOM needed).
-        __test: { visibleWindow: _visibleWindow, VIRTUAL_MIN: VIRTUAL_MIN, VIRTUAL_BUFFER: VIRTUAL_BUFFER, previewUrl: _previewUrl },
+        // Helpers exposed for tests. visibleWindow is pure; songCard/songRow
+        // need a DOM (the tests supply a minimal element mock) and pin the
+        // song_preview integration markup (data-fn + a data-v3-play surface).
+        __test: {
+            visibleWindow: _visibleWindow,
+            VIRTUAL_MIN: VIRTUAL_MIN,
+            VIRTUAL_BUFFER: VIRTUAL_BUFFER,
+            songCard: _songCard,
+            songRow: _songRow,
+        },
     };
 }
 
diff --git a/plugins/folder_library/tests/preview_url.test.js b/plugins/folder_library/tests/preview_url.test.js
deleted file mode 100644
index 4ec7aec..0000000
--- a/plugins/folder_library/tests/preview_url.test.js
+++ /dev/null
@@ -1,71 +0,0 @@
-// Hover-preview audio URL (folder_library preview-on-hover).
-//
-// The preview delegates to the canonical `song_preview` plugin instead of
-// resolving pack audio itself: `song_preview` reads the pack's manifest
-// (`preview:` key -> default stem) and serves the clip with Range support.
-// So `_previewUrl` just has to build that endpoint's URL from the song's
-// filename — with the filename URL-encoded as a query value (a '/', '#', '&',
-// or space in a folder/song name must not break the query). Returns null when
-// there's nothing to build from.
-
-const { test } = require('node:test');
-const assert = require('node:assert/strict');
-const fs = require('node:fs');
-const path = require('node:path');
-const vm = require('node:vm');
-
-function load() {
-    const window = {
-        console,
-        document: {
-            readyState: 'complete',
-            addEventListener() {},
-            getElementById() { return null; },
-            querySelector() { return null; },
-            querySelectorAll() { return []; },
-            createElement() { return { style: {}, classList: { add() {}, remove() {}, contains() { return false; } }, addEventListener() {}, appendChild() {} }; },
-        },
-        addEventListener() {},
-        localStorage: { getItem() { return null; }, setItem() {} },
-        performance: { now: () => 0 },
-        setInterval() { return 0; },
-        clearInterval() {},
-        requestAnimationFrame() { return 0; },
-        cancelAnimationFrame() {},
-        getComputedStyle() { return { overflowY: 'visible', paddingTop: '0px', paddingBottom: '0px' }; },
-        innerHeight: 800,
-        encodeURIComponent,   // the plugin encodes the filename query value with this
-    };
-    window.window = window;
-    window.globalThis = window;
-    const ctx = vm.createContext(window);
-    vm.runInContext(fs.readFileSync(path.join(__dirname, '..', 'screen.js'), 'utf8'), ctx, { filename: 'screen.js' });
-    assert.ok(window.folderLibrary && window.folderLibrary.__test, 'plugin must expose __test');
-    return window.folderLibrary.__test;
-}
-
-const { previewUrl } = load();
-
-test('null when there is no filename to build from', () => {
-    assert.equal(previewUrl(null), null);
-    assert.equal(previewUrl({}), null);
-    assert.equal(previewUrl({ filename: '' }), null);
-});
-
-test('delegates to the song_preview endpoint with the filename as a query value', () => {
-    const url = previewUrl({ filename: 'Test/song.sloppak' });
-    assert.equal(url, '/api/plugins/song_preview/audio?file=Test%2Fsong.sloppak');
-});
-
-test('encodes the whole filename (slashes, #, &, spaces) as one query value', () => {
-    const url = previewUrl({ filename: 'AC#DC/Back & Forth.sloppak' });
-    // As a query value, '/' is encoded too — song_preview decodes it back to the path.
-    assert.equal(url, '/api/plugins/song_preview/audio?file=AC%23DC%2FBack%20%26%20Forth.sloppak');
-});
-
-test('does not read a per-pack audio member (resolution is song_preview\'s job)', () => {
-    // Even if a caller passes a stale audio_member, it must be ignored — the
-    // folder plugin no longer resolves pack audio itself.
-    const url = previewUrl({ filename: 'CH/Song.sloppak', audio_member: 'stems/guitar.ogg' });
-    assert.equal(url, '/api/plugins/song_preview/audio?file=CH%2FSong.sloppak');
-});
diff --git a/plugins/folder_library/tests/song_preview_markup.test.js b/plugins/folder_library/tests/song_preview_markup.test.js
new file mode 100644
index 0000000..c56642c
--- /dev/null
+++ b/plugins/folder_library/tests/song_preview_markup.test.js
@@ -0,0 +1,116 @@
+// song_preview integration markup (feedBack — Folders view hover preview).
+//
+// The Folder Library does NOT implement hover-preview itself. It relies on the
+// separate `song_preview` plugin, exactly like the grid and list views. That
+// plugin's host adapter finds previewable elements with the selector
+// `#v3-songs [data-fn]` and requires each to contain a `[data-v3-play]`
+// descendant (the surface it overlays its indicator on), reading the raw
+// filename from `data-fn`.
+//
+// So the ENTIRE contract Folder Library owns is: every song card and row it
+// renders must carry `data-fn` (raw filename) and expose a `[data-v3-play]`
+// surface. If a refactor drops either, folder cards silently stop previewing
+// while grid/list keep working — a regression that's invisible without a live
+// song_preview install. These tests pin the markup so that can't happen.
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const vm = require('node:vm');
+
+// A minimal DOM element mock — just enough for _songCard / _songRow to run.
+// No jsdom in this repo (see virtual_list.test.js); the element tracks the few
+// things the contract cares about: dataset, attributes, and a child tree that
+// querySelector('[data-v3-play]') can walk.
+function makeEl(tag) {
+    const attrs = {};
+    const el = {
+        tagName: String(tag || '').toUpperCase(),
+        style: {},              // supports .cssText and arbitrary props
+        dataset: {},
+        className: '',
+        children: [],
+        parentNode: null,
+        addEventListener() {},
+        removeEventListener() {},
+        setAttribute(k, v) { attrs[k] = String(v); },
+        getAttribute(k) { return k in attrs ? attrs[k] : null; },
+        hasAttribute(k) { return k in attrs; },
+        appendChild(child) { el.children.push(child); if (child) child.parentNode = el; return child; },
+        classList: { add() {}, remove() {}, contains() { return false; }, toggle() {} },
+        remove() {},
+        // Only the '[data-v3-play]'-style attribute selector is needed.
+        querySelector(sel) {
+            const attr = sel.replace(/^\[|\]$/g, '');
+            const stack = el.children.slice();
+            while (stack.length) {
+                const n = stack.shift();
+                if (n && n.hasAttribute && n.hasAttribute(attr)) return n;
+                if (n && n.children) stack.push(...n.children);
+            }
+            return null;
+        },
+    };
+    return el;
+}
+
+function load() {
+    const window = {
+        console,
+        document: {
+            readyState: 'complete',
+            addEventListener() {},
+            getElementById() { return null; },
+            querySelector() { return null; },
+            querySelectorAll() { return []; },
+            createElement(tag) { return makeEl(tag); },
+        },
+        addEventListener() {},
+        localStorage: { getItem() { return null; }, setItem() {} },
+        performance: { now: () => 0 },
+        setInterval() { return 0; },
+        clearInterval() {},
+        requestAnimationFrame() { return 0; },
+        cancelAnimationFrame() {},
+        getComputedStyle() { return { overflowY: 'visible', paddingTop: '0px', paddingBottom: '0px' }; },
+        innerHeight: 800,
+    };
+    window.window = window;
+    window.globalThis = window;
+    const ctx = vm.createContext(window);
+    vm.runInContext(fs.readFileSync(path.join(__dirname, '..', 'screen.js'), 'utf8'), ctx, { filename: 'screen.js' });
+    assert.ok(window.folderLibrary && window.folderLibrary.__test, 'plugin must expose __test');
+    return window.folderLibrary.__test;
+}
+
+const { songCard, songRow } = load();
+
+// A raw filename with a subfolder + spaces — the kind of value song_preview
+// URL-encodes downstream, so it must reach data-fn verbatim, not pre-encoded.
+const FILENAME = 'Some Artist/A Song.sloppak';
+const SONG = { filename: FILENAME, title: 'A Song', artist: 'Some Artist' };
+
+test('song_preview helpers are exposed for the markup contract', () => {
+    assert.equal(typeof songCard, 'function');
+    assert.equal(typeof songRow, 'function');
+});
+
+test('grid card carries data-fn (raw) and a data-v3-play surface', () => {
+    const card = songCard(SONG, 'Unsorted');
+    assert.equal(card.dataset.fn, FILENAME, 'data-fn must be the raw, un-encoded filename');
+    assert.ok(card.querySelector('[data-v3-play]'), 'card must contain a [data-v3-play] surface');
+});
+
+test('list row carries data-fn (raw) and a data-v3-play surface', () => {
+    const row = songRow(SONG, 'Unsorted');
+    assert.equal(row.dataset.fn, FILENAME, 'data-fn must be the raw, un-encoded filename');
+    assert.ok(row.querySelector('[data-v3-play]'), 'row must contain a [data-v3-play] surface');
+});
+
+test('card renders without depending on any optional song metadata', () => {
+    // song_preview only needs filename; the card must build from a bare song
+    // (no duration/arrangements/stems/lyrics/tuning/year) without throwing.
+    assert.doesNotThrow(() => songCard({ filename: FILENAME }, 'Unsorted'));
+    assert.doesNotThrow(() => songRow({ filename: FILENAME }, 'Unsorted'));
+});