feat(audio): loopback feeder mode + static no-cache — all app audio under exclusive/ASIO (#877)

* feat(audio): route feedpak full-mix natively under exclusive output

Song playback runs through the renderer, which WASAPI-exclusive (and
ASIO) output silences. Route single-mix feedpaks (stem-less
original_audio packs AND single-stem packs) onto the engine's backing
transport when the output device type is exclusive-style, and migrate
back to HTML5 when it isn't. Extends /api/audio-local-path to resolve
/api/sloppak/.../file/... URLs via the same containment guards as
serve_sloppak_file. Multi-stem packs stay on the WebAudio path
(Phase 2). Includes [feedpak-route] transition-gated diagnostics
logging.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(audio): renderer-bus feeder — mix renderer song audio into engine output (Phase 2)

Under exclusive-style output the native backing transport (Phase 1, #824)
carries loose /audio/ songs and feedpak full-mixes, but not the stems
plugin's multi-stem WebAudio graph or tracks JUCE rejected. The feeder taps
the renderer-side master with an AudioWorklet, re-points the owning
AudioContext at a null sink so it keeps rendering without a device, and
pushes ~10 ms chunks over IPC into the desktop engine's renderer bus
(feedBack-desktop#90 follow-up). Inert in the Docker sphere and in shared
mode. Validated by the fix12 tester spike: null-sink rendering works,
clocks hold, no overflow.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(diag): --debug ASIO routing diagnostics in static bundle

Gated on window.feedBackDesktop.audio.debugEnabled() (desktop --debug);
inert in the Docker sphere and normal desktop runs.

- [asio-diag] getCurrentDevice= full device object on outputType change
  (catches ASIO drivers reporting a non-'ASIO' type name)
- [asio-diag] renderer-bus: full feeder decision vector, change-gated
  (running/exclusive/stems/juceMode/elementSong/want/mode)
- [asio-diag] setSink: every sink flip with ctx state + rate

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(audio): loopback feeder mode — all app audio under exclusive/ASIO

Tester-confirmed (2026-07-11 log): song previews and other
plugin-private audio bypass the per-surface feeder taps and leak to the
default WASAPI device under ASIO output. Also confirmed: the element
capture path poisons itself when highway_3d already owns #audio's
one-shot MediaElementSource (InvalidStateError with _elCtx assigned
pre-throw → TypeError every later tick).

- New preferred mode 'loopback': one getDisplayMedia frame-audio capture
  (desktop main answers with the app's own frame) covers song, previews,
  and UI sounds for the whole exclusive session — engages even with no
  song loaded. Local playback silenced via suppressLocalAudioPlayback,
  page-mute IPC fallback otherwise.
- Sticky fallback to the existing stems/element surface modes when
  capture is unavailable (old desktop main, denied, Docker sphere).
- Element capture: assign module state only after the whole chain
  succeeds; close the context on failure — collision now retries clean.
- Failed engage now disables the bus and tears down loopback (no more
  bus-enabled-with-no-producer stranding).
- Tests: 12 (5 new — loopback engage/preference/mute-fallback/sticky
  fallback, collision retry).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(audio): loopback feeder mode — all app audio under exclusive/ASIO

Tester-confirmed (2026-07-11 log): song previews and other
plugin-private audio bypass the per-surface feeder taps and leak to the
default WASAPI device under ASIO output. Also confirmed: the element
capture path poisons itself when highway_3d already owns #audio's
one-shot MediaElementSource (InvalidStateError with _elCtx assigned
pre-throw → TypeError every later tick).

- New preferred mode 'loopback': one getDisplayMedia frame-audio capture
  (desktop main answers with the app's own frame) covers song, previews,
  and UI sounds for the whole exclusive session — engages even with no
  song loaded. Local playback silenced via suppressLocalAudioPlayback,
  page-mute IPC fallback otherwise.
- Sticky fallback to the existing stems/element surface modes when
  capture is unavailable (old desktop main, denied, Docker sphere).
- Element capture: assign module state only after the whole chain
  succeeds; close the context on failure — collision now retries clean.
- Failed engage now disables the bus and tears down loopback (no more
  bus-enabled-with-no-producer stranding).
- Tests: 12 (5 new — loopback engage/preference/mute-fallback/sticky
  fallback, collision retry).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(audio): close loopback capture context on teardown (release tap worklet)

The loopback context was reused across engages (_lbCtx || new), but teardown
only stopped the stream + deactivated the tap — never closing the context or
detaching the worklet node. Each exclusive<->shared switch orphaned a live
tap worklet on the long-lived context. Use a fresh context per session and
close it on disengage. Adds a test asserting the context is closed on teardown.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* feat(diag): install-time + uncaught-error diagnostics for the reroute chain

2026-07-11 tester log showed the routing watcher and renderer-bus feeder
never installed (zero [feedpak-route]/[renderer-bus] lines) plus an
uncaught SyntaxError with no source location — nothing in the log said
why. New:

- global error/unhandledrejection tap logging message + filename:line:col
  (error events carry the location even for parse errors in other scripts)
- explicit install / NOT-installed lines for watcher and feeder (incl.
  loopback capability probe)
- DOMException detail (name/message/stack head) in the feeder retry warn
  — the console-message forward stringified it to [object DOMException]

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(static): force conditional revalidation on /static (Cache-Control: no-cache)

Without Cache-Control Chromium's heuristic freshness (10% of file age)
serves /static/app.js from disk cache for hours-to-days without
revalidating. Desktop consequence: a new build's window ran the previous
build's app.js — the 2026-07-11 ASIO investigation traced 'routing
watcher never installed' + a stems module-plugin SyntaxError to exactly
this (stale loader predating scriptType support). no-cache keeps caching
but revalidates via ETag — unchanged files still cost only a 304.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(diag): gate install-time + uncaught-error [asio-diag] lines on --debug

The error tap and install lines from the previous diag commit were
unconditional. Now: error/rejection taps check _asioDiagEnabled() at
event time; install lines log deferred once the async debugEnabled()
resolves true. The NOT-installed anomaly lines stay bridge-gated
(window.feedBackDesktop present) instead — a broken bridge can't deliver
the debug flag, they fire at most once, and only in the broken state
they exist to witness. Docker sphere: fully silent.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Byron Gamatos <xasiklas@gmail.com>

server.py

@@ -2386,7 +2386,25 @@ app.include_router(ws_highway.router)
 
 
 
-app.mount("/static", StaticFiles(directory=str(STATIC_DIR)), name="static")
+class _RevalidatedStaticFiles(StaticFiles):
+    """StaticFiles that forces conditional revalidation on every request.
+
+    Without Cache-Control, Chromium applies heuristic freshness (10% of the
+    file's age since Last-Modified) and serves /static/app.js from its disk
+    cache for hours-to-days without asking the server. In the desktop app that
+    meant a new build's renderer ran the PREVIOUS build's app.js — the
+    2026-07-11 ASIO investigation lost a day to a stale loader that couldn't
+    even load module plugins. `no-cache` does NOT disable caching: the browser
+    keeps the cached copy and revalidates with If-None-Match; unchanged files
+    still cost only a 304."""
+
+    async def get_response(self, path, scope):
+        response = await super().get_response(path, scope)
+        response.headers.setdefault("Cache-Control", "no-cache")
+        return response
+
+
+app.mount("/static", _RevalidatedStaticFiles(directory=str(STATIC_DIR)), name="static")
 
 
 @app.get("/")

static/app.js

@@ -4858,9 +4858,37 @@ window.jucePlayer = jucePlayer;
 // on a dead JUCE backing transport. This watcher migrates the loaded song
 // between the two paths whenever the engine's running state changes, preserving
 // playback position and play/pause state.
+// [asio-diag] global error tap: the 2026-07-11 tester log showed an uncaught
+// SyntaxError with no source location and the routing watcher/feeder never
+// installing — an error event carries filename:line even for parse errors in
+// other scripts, which console output does not. Gated on the desktop --debug
+// flag via window._asioDiagEnabled (installed just below; resolves async, so
+// errors thrown in the first ~second of a debug run may be missed — the
+// stale-cache class of failure reproduces on every later tick anyway).
+window.addEventListener('error', (e) => {
+    if (!window._asioDiagEnabled?.()) return;
+    console.warn('[asio-diag] uncaught-error:', e.message,
+        'at', (e.filename || '<unknown>') + ':' + (e.lineno || 0) + ':' + (e.colno || 0));
+});
+window.addEventListener('unhandledrejection', (e) => {
+    if (!window._asioDiagEnabled?.()) return;
+    const r = e.reason;
+    console.warn('[asio-diag] unhandled-rejection:',
+        (r && (r.name + ': ' + r.message)) || String(r));
+});
+
 (function _installJuceEngineRoutingWatcher() {
     const juceApi = window.feedBackDesktop?.audio;
-    if (!juceApi || typeof juceApi.isAudioRunning !== 'function') return;
+    if (!juceApi || typeof juceApi.isAudioRunning !== 'function') {
+        // Desktop bridge present but audio API incomplete — the whole
+        // exclusive reroute chain is dead and this line is the only witness.
+        // (Docker sphere has no bridge at all: stay silent, nothing to
+        // diagnose there and no debug flag to gate on.)
+        if (window.feedBackDesktop) {
+            console.log('[asio-diag] routing watcher NOT installed (audio api incomplete)');
+        }
+        return;
+    }
 
     let _rerouteInFlight = false;
     // URL that JUCE's loadBackingTrack *explicitly rejected* (ok === false —
@@ -4889,7 +4917,13 @@ window.jucePlayer = jucePlayer;
     // renderer-bus feeder below via window._asioDiagEnabled.
     let _asioDiag = false;
     if (typeof juceApi.debugEnabled === 'function') {
-        juceApi.debugEnabled().then((v) => { _asioDiag = !!v; }).catch(() => {});
+        juceApi.debugEnabled().then((v) => {
+            _asioDiag = !!v;
+            // Deferred install line: the flag resolves async, so logging at
+            // IIFE entry would race it. Change-detection isn't needed — this
+            // runs once per page load.
+            if (_asioDiag) console.log('[asio-diag] routing watcher installed');
+        }).catch(() => {});
     }
     window._asioDiagEnabled = () => _asioDiag;
     async function _outputIsExclusive() {
@@ -5311,7 +5345,23 @@ window.jucePlayer = jucePlayer;
 (function _installRendererBusFeeder() {
     const api = window.feedBackDesktop?.audio;
     if (!api || typeof api.setRendererBus !== 'function'
-             || typeof api.pushRendererAudio !== 'function') return;
+             || typeof api.pushRendererAudio !== 'function') {
+        // Silent in the Docker sphere (no bridge, no debug flag); a desktop
+        // bridge missing the bus API is the diagnostic case.
+        if (window.feedBackDesktop) {
+            console.log('[asio-diag] renderer-bus feeder NOT installed (api=' + !!api
+                + ' setRendererBus=' + typeof api?.setRendererBus
+                + ' pushRendererAudio=' + typeof api?.pushRendererAudio + ')');
+        }
+        return;
+    }
+    // Deferred like the watcher's install line: gate on the async debug flag.
+    if (typeof api.debugEnabled === 'function') {
+        api.debugEnabled().then((v) => {
+            if (v) console.log('[asio-diag] renderer-bus feeder installed (loopback-capable='
+                + (typeof window.navigator?.mediaDevices?.getDisplayMedia === 'function') + ')');
+        }).catch(() => {});
+    }
 
     const TAP_WORKLET = `
         class FeedbackBusTap extends AudioWorkletProcessor {
@@ -5381,15 +5431,94 @@ window.jucePlayer = jucePlayer;
         if (_elCtx) return;
         const el = document.getElementById('audio');
         if (!el) throw new Error('no core audio element');
-        _elCtx = new AudioContext();
+        // Assign the module state ONLY after the whole chain succeeded.
-        _elSource = _elCtx.createMediaElementSource(el);
+        // createMediaElementSource throws InvalidStateError when another
-        _elSource.connect(_elCtx.destination);
+        // consumer (highway_3d's analyser tap) already owns the element's
-        _elTap = _makeTap(_elCtx);
+        // one-shot source — assigning _elCtx before that throw poisoned every
-        await _elTap.attach(_elSource);
+        // later tick into `_elTap.active` TypeErrors (tester log 2026-07-11)
+        // while the song kept playing on the default device.
+        const ctx = new AudioContext();
+        let source, tap;
+        try {
+            source = ctx.createMediaElementSource(el);
+            source.connect(ctx.destination);
+            tap = _makeTap(ctx);
+            await tap.attach(source);
+        } catch (e) {
+            try { await ctx.close(); } catch (_) { /* already closed */ }
+            throw e;
+        }
+        _elCtx = ctx; _elSource = source; _elTap = tap;
+    }
+
+    // ── Whole-app loopback capture ───────────────────────────────────────────
+    // Preferred mode: one getDisplayMedia frame-audio capture covers EVERY
+    // sound the app makes (song, previews, UI) — no per-surface taps, so
+    // plugin-private AudioContexts (song-preview, future plugins) survive
+    // exclusive/ASIO output too. The desktop main process answers the request
+    // with this window's own frame (frame-scoped — no other apps' audio).
+    // Local playback is silenced via the suppressLocalAudioPlayback track
+    // constraint, with a page-mute IPC fallback (capture taps frame audio
+    // before the output mute, so a muted page still feeds the stream).
+    let _lbStream = null, _lbCtx = null, _lbTap = null, _lbPageMuted = false;
+    let _loopbackUnavailable = false;   // sticky: probe once, then fall back
+    async function _engageLoopback() {
+        const stream = await navigator.mediaDevices.getDisplayMedia({
+            video: true,
+            audio: { suppressLocalAudioPlayback: true },
+        });
+        for (const t of stream.getVideoTracks()) t.stop();   // required, unused
+        const track = stream.getAudioTracks()[0];
+        if (!track) {
+            for (const t of stream.getTracks()) t.stop();
+            throw new Error('no loopback audio track');
+        }
+        try {
+            // Fresh context per session (not reused) so teardown's close()
+            // fully releases the tap worklet node — see _teardownLoopback.
+            _lbCtx = new AudioContext();
+            if (_lbCtx.state !== 'running') await _lbCtx.resume().catch(() => {});
+            const source = _lbCtx.createMediaStreamSource(stream);
+            const tap = _makeTap(_lbCtx);
+            await tap.attach(source);
+            const suppressed = track.getSettings?.().suppressLocalAudioPlayback === true;
+            if (!suppressed && typeof api.setPageMuted === 'function') {
+                _lbPageMuted = (await api.setPageMuted(true)) === true;
+            }
+            if (window._asioDiagEnabled?.()) {
+                console.log('[asio-diag] loopback: suppressed=', suppressed,
+                    'pageMuted=', _lbPageMuted, 'rate=', _lbCtx.sampleRate);
+            }
+            await api.setRendererBus(true, 1.0);
+            tap.active = true;
+            _lbStream = stream; _lbTap = tap;
+            _mode = 'loopback';
+            console.log('[renderer-bus] engaged: app loopback → engine bus');
+        } catch (e) {
+            for (const t of stream.getTracks()) t.stop();
+            throw e;
+        }
+    }
+    async function _teardownLoopback() {
+        if (_lbTap) _lbTap.active = false;
+        if (_lbStream) for (const t of _lbStream.getTracks()) t.stop();
+        _lbStream = null; _lbTap = null;
+        // Close the capture context so its tap worklet node is released. The
+        // context is per-session (not reused): without this, each exclusive⇄
+        // shared switch orphaned a live worklet on a long-lived context.
+        if (_lbCtx) {
+            try { await _lbCtx.close(); } catch (_) { /* already closed */ }
+            _lbCtx = null;
+        }
+        if (_lbPageMuted && typeof api.setPageMuted === 'function') {
+            try { await api.setPageMuted(false); } catch (_) { /* engine gone */ }
+        }
+        _lbPageMuted = false;
     }
 
     // ── Engagement state machine ─────────────────────────────────────────────
-    // 'off' | 'element' | 'stems'
+    // 'off' | 'loopback' | 'element' | 'stems' (element/stems = fallback when
+    // loopback capture is unavailable: old desktop main, denied capture)
     let _mode = 'off';
     let _stemsGraph = null;   // { context, masterNode } snapshot while engaged
     let _stemsTap = null;
@@ -5414,7 +5543,9 @@ window.jucePlayer = jucePlayer;
         const prev = _mode;
         _mode = 'off';
         try { await api.setRendererBus(false, 0); } catch (_) { /* engine gone */ }
-        if (prev === 'element' && _elCtx) {
+        if (prev === 'loopback') {
+            await _teardownLoopback();
+        } else if (prev === 'element' && _elCtx) {
             _elTap.active = false;
             await _setSink(_elCtx, false).catch(() => {});
         } else if (prev === 'stems' && _stemsGraph) {
@@ -5473,9 +5604,20 @@ window.jucePlayer = jucePlayer;
 
             let want = 'off';
             if (running && exclusive) {
-                if (stems) want = 'stems';
+                // Loopback covers ALL app audio (song, previews, UI), so it
+                // engages for the whole exclusive session — not just while a
+                // song is loaded. Per-surface modes remain as fallback when
+                // loopback capture is unavailable (old desktop main without
+                // the display-media handler, capture denied).
+                if (!_loopbackUnavailable) want = 'loopback';
+                else if (stems) want = 'stems';
                 else if (elementSong) want = 'element';
             }
+            // Song audio riding the native transport must not ALSO ride the
+            // loopback (double-carry into the same engine output). The native
+            // transport plays from the engine, not the page, so page loopback
+            // never hears it — no conflict; loopback stays engaged for
+            // previews/UI while the transport owns the song.
 
             // [asio-diag] full decision vector, change-gated (500ms poll —
             // steady state must not flood the buffer). This is the feeder-side
@@ -5487,6 +5629,7 @@ window.jucePlayer = jucePlayer;
                     + ' stems=' + !!stems + ' songAudio=' + !!songAudio
                     + ' juceMode=' + !!window._juceMode
                     + ' elementSong=' + elementSong
+                    + ' loopbackUnavailable=' + _loopbackUnavailable
                     + ' want=' + want + ' mode=' + _mode;
                 if (d !== window._lastRendererBusDecision) {
                     window._lastRendererBusDecision = d;
@@ -5498,12 +5641,33 @@ window.jucePlayer = jucePlayer;
             const stemsGraphChanged = _mode === 'stems' && stems !== _stemsGraph;
             if (want !== _mode || stemsGraphChanged) {
                 await _disengage();
-                if (want === 'stems') await _engageStems(stems);
+                try {
-                else if (want === 'element') await _engageElement();
+                    if (want === 'loopback') await _engageLoopback();
+                    else if (want === 'stems') await _engageStems(stems);
+                    else if (want === 'element') await _engageElement();
+                } catch (e) {
+                    if (want === 'loopback') {
+                        // Capture unavailable (no handler in an old desktop
+                        // main, permission denied) — remember and fall back to
+                        // the per-surface modes on the next tick.
+                        _loopbackUnavailable = true;
+                        console.warn('[renderer-bus] loopback capture unavailable — falling back to surface taps:', e);
+                    }
+                    throw e;
+                }
             }
         } catch (e) {
-            console.warn('[renderer-bus] reevaluate failed (will retry):', e);
+            // Explicit name/message/stack head — the console-message forward
+            // stringifies a DOMException to the useless "[object DOMException]".
+            console.warn('[renderer-bus] reevaluate failed (will retry):',
+                (e && e.name ? e.name + ': ' + e.message : String(e)),
+                (e && e.stack ? '| ' + String(e.stack).split('\n')[1] : ''));
             _mode = 'off';
+            // A partial engage may have left the bus enabled with no producer
+            // and the page muted — undo both so a failed tick can't strand
+            // audio in silence until the next successful engage.
+            try { await api.setRendererBus(false, 0); } catch (_) { /* engine gone */ }
+            await _teardownLoopback().catch(() => {});
         } finally {
             _busy = false;
         }

tests/js/renderer_bus_feeder.test.js

@@ -50,17 +50,42 @@ function makeFakeContext(sampleRate = 48000) {
             this.mediaSourceEl = el;
             return { connect() {}, disconnect() {} };
         },
+        createMediaStreamSource(stream) {
+            this.mediaStreamSource = stream;
+            return { connect() {}, disconnect() {} };
+        },
+        close() { this.closed = true; return Promise.resolve(); },
     };
     return ctx;
 }
 
-function makeSandbox({ isAudioRunning = () => true, exclusive = () => true } = {}) {
+// Fake getDisplayMedia stream for the loopback-capture path.
-    const calls = { setRendererBus: [], pushRendererAudio: [] };
+function makeLoopbackStream({ suppressed = true } = {}) {
+    const stopped = [];
+    const audioTrack = {
+        kind: 'audio',
+        stop() { stopped.push('audio'); },
+        getSettings: () => (suppressed ? { suppressLocalAudioPlayback: true } : {}),
+    };
+    const videoTrack = { kind: 'video', stop() { stopped.push('video'); } };
+    return {
+        __stopped: stopped,
+        getAudioTracks: () => [audioTrack],
+        getVideoTracks: () => [videoTrack],
+        getTracks: () => [videoTrack, audioTrack],
+    };
+}
+
+// `displayMedia`: undefined → loopback capture unavailable (Docker sphere /
+// old desktop main); a function → used as navigator.mediaDevices.getDisplayMedia.
+function makeSandbox({ isAudioRunning = () => true, exclusive = () => true, displayMedia } = {}) {
+    const calls = { setRendererBus: [], pushRendererAudio: [], setPageMuted: [] };
 
     const api = {
         isAudioRunning: () => Promise.resolve(isAudioRunning()),
         setRendererBus: (en, g) => { calls.setRendererBus.push([en, g]); return Promise.resolve(); },
         pushRendererAudio: (buf, rate) => { calls.pushRendererAudio.push([buf.length, rate]); },
+        setPageMuted: (m) => { calls.setPageMuted.push(m); return Promise.resolve(m); },
     };
 
     class FakeWorkletNode {
@@ -85,6 +110,7 @@ function makeSandbox({ isAudioRunning = () => true, exclusive = () => true } = {
         __createdContexts: [],
         __audioEl: { id: 'audio' },
         __calls: calls,
+        navigator: { mediaDevices: displayMedia ? { getDisplayMedia: displayMedia } : {} },
         window: null,
     };
     sandbox.window = {
@@ -111,12 +137,21 @@ function makeStemsGraph() {
     };
 }
 
-test('stems graph + exclusive output → bus enabled, stems ctx null-sinked', async () => {
+// Surface-mode (stems/element) tests run WITHOUT getDisplayMedia: the first
+// tick probes loopback, fails, and latches _loopbackUnavailable; the second
+// tick exercises the fallback surface mode. This mirrors an old desktop main
+// without the display-media handler.
+async function reevaluateWithFallback(sb) {
+    await sb.window._reevaluateRendererBus();   // loopback probe → unavailable
+    await sb.window._reevaluateRendererBus();   // surface fallback
+}
+
+test('stems graph + exclusive output → bus enabled, stems ctx null-sinked (loopback unavailable)', async () => {
     const sb = makeSandbox({ exclusive: () => true });
     const graph = makeStemsGraph();
     sb.window.feedBack.stems.audioGraph = graph;
 
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
 
     assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'bus enabled');
     assert.equal(graph.context.sinkIdCalls.at(-1)?.type, 'none', 'stems ctx re-pointed at null sink');
@@ -128,7 +163,7 @@ test('output returns to shared → bus disabled, sink restored', async () => {
     const graph = makeStemsGraph();
     sb.window.feedBack.stems.audioGraph = graph;
 
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
     excl = false;
     await sb.window._reevaluateRendererBus();
 
@@ -145,26 +180,27 @@ test('stems graph + shared output → feeder stays off (no double audio)', async
     assert.equal(sb.__calls.setRendererBus.length, 0, 'bus never touched in shared mode');
 });
 
-test('element song + exclusive → element captured into bus', async () => {
+test('element song + exclusive → element captured into bus (loopback unavailable)', async () => {
     const sb = makeSandbox({ exclusive: () => true });
     sb.window._currentSongAudio = { url: '/api/sloppak/x.sloppak/file/stems/full.ogg' };
     sb.window._juceMode = false;
 
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
 
     assert.equal(sb.__createdContexts.length, 1, 'capture context created');
     assert.equal(sb.__createdContexts[0].mediaSourceEl, sb.__audioEl, 'element source captured');
     assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'bus enabled');
 });
 
-test('song riding the native transport (_juceMode) → feeder stays off', async () => {
+test('native-transport song, loopback unavailable → surface modes stay off', async () => {
     const sb = makeSandbox({ exclusive: () => true });
     sb.window._currentSongAudio = { url: '/audio/song.ogg' };
     sb.window._juceMode = true;
 
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
 
-    assert.equal(sb.__calls.setRendererBus.length, 0, 'native transport owns the song');
+    assert.ok(!sb.__calls.setRendererBus.some(([en]) => en === true),
+        'bus never ENABLED (failed-probe cleanup may disable it)');
     assert.equal(sb.__createdContexts.length, 0, 'no capture context created');
 });
 
@@ -172,7 +208,7 @@ test('stems graph replaced mid-engagement → re-engages on the new graph', asyn
     const sb = makeSandbox({ exclusive: () => true });
     const g1 = makeStemsGraph();
     sb.window.feedBack.stems.audioGraph = g1;
-    await sb.window._reevaluateRendererBus();
+    await reevaluateWithFallback(sb);
 
     const g2 = makeStemsGraph();
     sb.window.feedBack.stems.audioGraph = g2;
@@ -182,6 +218,105 @@ test('stems graph replaced mid-engagement → re-engages on the new graph', asyn
     assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 're-enabled for new graph');
 });
 
+// ── Loopback mode (whole-app capture) ────────────────────────────────────────
+
+test('exclusive output + loopback available → engages without any song loaded', async () => {
+    const stream = makeLoopbackStream();
+    const sb = makeSandbox({ exclusive: () => true, displayMedia: () => Promise.resolve(stream) });
+
+    await sb.window._reevaluateRendererBus();
+
+    assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'bus enabled for whole session');
+    assert.ok(stream.__stopped.includes('video'), 'unused video track stopped');
+    assert.equal(sb.__createdContexts.at(-1)?.mediaStreamSource, stream, 'loopback stream captured');
+    assert.equal(sb.__calls.setPageMuted.length, 0, 'suppress constraint honoured — no page mute');
+});
+
+test('loopback context is closed on disengage (no orphaned tap worklet)', async () => {
+    let excl = true;
+    const stream = makeLoopbackStream();
+    const sb = makeSandbox({ exclusive: () => excl, displayMedia: () => Promise.resolve(stream) });
+
+    await sb.window._reevaluateRendererBus();          // engage loopback
+    const lbCtx = sb.__createdContexts.at(-1);
+    assert.equal(lbCtx?.mediaStreamSource, stream, 'loopback engaged');
+    assert.notEqual(lbCtx.closed, true, 'context live while engaged');
+
+    excl = false;
+    await sb.window._reevaluateRendererBus();          // disengage
+    assert.equal(lbCtx.closed, true, 'loopback context closed on disengage');
+    assert.ok(stream.__stopped.includes('audio'), 'capture stream stopped');
+});
+
+test('loopback preferred over stems when both available', async () => {
+    const stream = makeLoopbackStream();
+    const sb = makeSandbox({ exclusive: () => true, displayMedia: () => Promise.resolve(stream) });
+    const graph = makeStemsGraph();
+    sb.window.feedBack.stems.audioGraph = graph;
+
+    await sb.window._reevaluateRendererBus();
+
+    assert.equal(graph.context.sinkIdCalls.length, 0, 'stems ctx untouched — loopback owns capture');
+    assert.equal(sb.__createdContexts.at(-1)?.mediaStreamSource, stream, 'loopback engaged');
+});
+
+test('suppressLocalAudioPlayback unsupported → page-mute fallback, unmuted on disengage', async () => {
+    let excl = true;
+    const stream = makeLoopbackStream({ suppressed: false });
+    const sb = makeSandbox({ exclusive: () => excl, displayMedia: () => Promise.resolve(stream) });
+
+    await sb.window._reevaluateRendererBus();
+    assert.deepEqual(sb.__calls.setPageMuted, [true], 'page muted as fallback');
+
+    excl = false;
+    await sb.window._reevaluateRendererBus();
+    assert.deepEqual(sb.__calls.setPageMuted, [true, false], 'page unmuted on disengage');
+    assert.deepEqual(sb.__calls.setRendererBus.at(-1), [false, 0], 'bus disabled');
+});
+
+test('getDisplayMedia rejected → sticky fallback to surface modes', async () => {
+    const sb = makeSandbox({
+        exclusive: () => true,
+        displayMedia: () => Promise.reject(new DOMException('denied', 'NotAllowedError')),
+    });
+    const graph = makeStemsGraph();
+    sb.window.feedBack.stems.audioGraph = graph;
+
+    await sb.window._reevaluateRendererBus();   // probe fails, latches unavailable
+    await sb.window._reevaluateRendererBus();   // falls back to stems
+
+    assert.equal(graph.context.sinkIdCalls.at(-1)?.type, 'none', 'stems fallback engaged');
+    assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'bus enabled via fallback');
+});
+
+test('element capture collision (createMediaElementSource throws) → no poisoned state, clean retry', async () => {
+    const sb = makeSandbox({ exclusive: () => true });   // loopback unavailable
+    sb.window._currentSongAudio = { url: '/api/sloppak/x.sloppak/file/stems/full.ogg' };
+    // First capture attempt collides (highway analyser owns the element).
+    let collide = true;
+    const origFactory = sb.AudioContext;
+    sb.__createdContexts.length = 0;
+    // Patch contexts so createMediaElementSource throws while colliding.
+    sb.AudioContext = function () {
+        const c = origFactory();
+        const orig = c.createMediaElementSource.bind(c);
+        c.createMediaElementSource = (el) => {
+            if (collide) throw new DOMException('already connected', 'InvalidStateError');
+            return orig(el);
+        };
+        c.close = () => Promise.resolve();
+        return c;
+    };
+
+    await reevaluateWithFallback(sb);            // element engage fails (collision)
+    assert.ok(!sb.__calls.setRendererBus.some(([en]) => en === true), 'bus never left enabled');
+
+    collide = false;
+    await sb.window._reevaluateRendererBus();    // retry succeeds — no TypeError, fresh ctx
+
+    assert.deepEqual(sb.__calls.setRendererBus.at(-1), [true, 1.0], 'element engaged after collision cleared');
+});
+
 test('engine stops → bus disabled', async () => {
     let running = true;
     const sb = makeSandbox({ isAudioRunning: () => running, exclusive: () => true });