fix: preview crossfade no longer strands the main volume (silent playback)

Clicking a track could play silently: the store showed status=playing with
the position advancing and no error, but the sink was at volume 0. The
open/close crossfade ramped the main player's USER VOLUME as its fade
channel and left it at 0 on an ack timeout (or when a track change
suppressed the close resume). User volume persists across tracks and a new
Play doesn't reset it, so every later click played at 0 while the slider
still read 0.7.

Give the main player a separate fade_gain channel (Cmd::SetFadeGain, sink =
volume * norm * fade_gain), reset to 1 by every Play/Stop. The open/close
fades ramp fade_gain instead of set_volume (their volume arg is gone, JS
callers too), so the user volume is never touched -- store and backend can't
desync -- and a clicked track always resets fade_gain to 1, so it can never
be stranded silent. fade_resume lands exactly at 1. Player::fade_gain()
mirrors it into a report atomic; debug overlay shows main fade + this proved
the fix live (open 1->0, close 0->1, new track 1.0, volume steady at 0.7).

39 app + 12 player + 85 JS tests pass.

crates/player/src/lib.rs

@@ -113,6 +113,10 @@ enum Cmd {
     /// exactly where loudness may step. Cleared by Play/Stop; an unknown
     /// next track falls back to unity at the boundary.
     SetNextNormGainDb(f32),
+    /// Transient fade multiplier (0..1) for the preview open/close crossfade,
+    /// SEPARATE from the user volume so a fade can never strand the volume
+    /// setting. Reset to 1 by every Play/Stop.
+    SetFadeGain(f32),
     SetOutputDevice {
         device: Option<String>,
         reply: mpsc::Sender<Result<(), String>>,
@@ -227,6 +231,9 @@ where
 #[derive(Clone)]
 pub struct Player {
     cmd_tx: mpsc::Sender<Cmd>,
+    /// f32 bits; the audio thread mirrors its live fade-gain here for
+    /// observability (the debug overlay / crossfade diagnostics). 1.0 = full.
+    fade_report: Arc<std::sync::atomic::AtomicU32>,
 }
 
 impl Player {
@@ -242,13 +249,20 @@ impl Player {
     ) -> (Player, mpsc::Receiver<PlayerEvent>) {
         let (cmd_tx, cmd_rx) = mpsc::channel::<Cmd>();
         let (event_tx, event_rx) = mpsc::channel::<PlayerEvent>();
+        let fade_report = Arc::new(std::sync::atomic::AtomicU32::new(1.0f32.to_bits()));
 
+        let thread_report = fade_report.clone();
         std::thread::Builder::new()
             .name("audio".into())
-            .spawn(move || audio_thread(cmd_rx, event_tx, output_device))
+            .spawn(move || audio_thread(cmd_rx, event_tx, output_device, thread_report))
             .expect("spawn audio thread");
 
-        (Player { cmd_tx }, event_rx)
+        (Player { cmd_tx, fade_report }, event_rx)
+    }
+
+    /// The audio thread's current fade-gain (0..1). Observability only.
+    pub fn fade_gain(&self) -> f32 {
+        f32::from_bits(self.fade_report.load(Ordering::Relaxed))
     }
 
     pub fn play_local(&self, path: impl Into<PathBuf>, duration_hint: Option<f64>) {
@@ -352,6 +366,13 @@ impl Player {
             .send(Cmd::SetNextNormGainDb(db.clamp(-18.0, 9.0)));
     }
 
+    /// Transient fade multiplier (0..1) for the preview open/close crossfade.
+    /// A separate channel from the user volume — reset to 1 by every Play/Stop
+    /// — so a fade can never leave a later track silent. See [`Cmd::SetFadeGain`].
+    pub fn set_fade_gain(&self, gain: f32) {
+        let _ = self.cmd_tx.send(Cmd::SetFadeGain(gain.clamp(0.0, 1.0)));
+    }
+
     /// Change devices immediately. The audio thread acknowledges only after
     /// the endpoint has opened successfully, so callers can safely persist it.
     pub fn set_output_device(&self, device: Option<String>) -> Result<(), String> {
@@ -476,6 +497,7 @@ fn audio_thread(
     cmd_rx: mpsc::Receiver<Cmd>,
     event_tx: mpsc::Sender<PlayerEvent>,
     initial_device: Option<String>,
+    fade_report: Arc<std::sync::atomic::AtomicU32>,
 ) {
     // Open the output device lazily and keep it across tracks. If the device
     // is momentarily busy (e.g. another instance still releasing it), we
@@ -505,7 +527,12 @@ fn audio_thread(
     let mut norm_target_db: f32 = 0.0;
     // Gain waiting for the next gapless boundary (None → unity there).
     let mut pending_norm_db: Option<f32> = None;
-    let eff = |user: f32, ndb: f32| user * 10f32.powf(ndb / 20.0);
+    // Transient fade multiplier (0..1) for the preview session's open/close
+    // crossfade — a SEPARATE channel from the user volume, so a fade can
+    // never corrupt the volume setting (leaving a track silent after the
+    // fade or across the next Play). Reset to 1 by every Play/Stop.
+    let mut fade_gain: f32 = 1.0;
+    let eff = |user: f32, ndb: f32, fg: f32| user * 10f32.powf(ndb / 20.0) * fg;
     let mut playing = false;
     // The active stream source, so we can abort its blocking reads.
     let mut active_stream: Option<Arc<StreamProgress>> = None;
@@ -568,6 +595,11 @@ fn audio_thread(
                 norm_db = 0.0;
                 norm_target_db = 0.0;
                 pending_norm_db = None;
+                // A clicked/queued track always plays at full: never inherit a
+                // preview crossfade's residual fade level (that was the
+                // "audio broken after a track click" bug).
+                fade_gain = 1.0;
+                fade_report.store(fade_gain.to_bits(), Ordering::Relaxed);
                 let track = TrackSource {
                     path,
                     stream,
@@ -672,7 +704,7 @@ fn audio_thread(
                 match decode_scratch_window(path, secs, rate) {
                     Ok((samples, speed)) => {
                         let s = Sink::connect_new(out.mixer());
-                        s.set_volume(eff(volume, norm_db));
+                        s.set_volume(eff(volume, norm_db, fade_gain));
                         s.set_speed(speed);
                         s.append(samples);
                         s.play();
@@ -700,7 +732,7 @@ fn audio_thread(
                 match local_decoder(path) {
                     Ok(decoder) => {
                         let s = Sink::connect_new(out.mixer());
-                        s.set_volume(eff(volume, norm_db));
+                        s.set_volume(eff(volume, norm_db, fade_gain));
                         queued_sources.store(1, Ordering::SeqCst);
                         s.append(EndNotifier {
                             inner: decoder,
@@ -731,7 +763,7 @@ fn audio_thread(
             Ok(Cmd::Volume(v)) => {
                 volume = v;
                 if let Some(s) = &sink {
-                    s.set_volume(eff(volume, norm_db));
+                    s.set_volume(eff(volume, norm_db, fade_gain));
                 }
             }
             Ok(Cmd::SetNormGainDb { db, immediate }) => {
@@ -739,7 +771,7 @@ fn audio_thread(
                 if immediate {
                     norm_db = db;
                     if let Some(s) = &sink {
-                        s.set_volume(eff(volume, norm_db));
+                        s.set_volume(eff(volume, norm_db, fade_gain));
                     }
                 }
                 // Non-immediate: the tick arm slews norm_db there.
@@ -747,6 +779,13 @@ fn audio_thread(
             Ok(Cmd::SetNextNormGainDb(db)) => {
                 pending_norm_db = Some(db);
             }
+            Ok(Cmd::SetFadeGain(fg)) => {
+                fade_gain = fg.clamp(0.0, 1.0);
+                fade_report.store(fade_gain.to_bits(), Ordering::Relaxed);
+                if let Some(s) = &sink {
+                    s.set_volume(eff(volume, norm_db, fade_gain));
+                }
+            }
             Ok(Cmd::SetOutputDevice { device, reply }) => {
                 output_change = Some((device, Some(reply)));
             }
@@ -762,6 +801,8 @@ fn audio_thread(
                 waiting_preload = None;
                 pending_tracks.clear();
                 pending_norm_db = None;
+                fade_gain = 1.0;
+                fade_report.store(fade_gain.to_bits(), Ordering::Relaxed);
                 playing = false;
                 scratching = false;
                 local_path = None;
@@ -776,7 +817,7 @@ fn audio_thread(
                 if diff.abs() > 0.001 {
                     norm_db += diff.clamp(-0.15, 0.15);
                     if let Some(s) = &sink {
-                        s.set_volume(eff(volume, norm_db));
+                        s.set_volume(eff(volume, norm_db, fade_gain));
                     }
                 }
             }
@@ -882,7 +923,7 @@ fn audio_thread(
                     let track_duration = ready.track_duration();
                     if !is_preload {
                         let s = Sink::connect_new(out.mixer());
-                        s.set_volume(eff(volume, norm_db));
+                        s.set_volume(eff(volume, norm_db, fade_gain));
                         sink = Some(s);
                         active_track = Some(ready.track.clone());
                         active_stream = ready.track.stream.clone();
@@ -969,7 +1010,7 @@ fn audio_thread(
             norm_db = pending_norm_db.take().unwrap_or(0.0);
             norm_target_db = norm_db;
             if let Some(s) = &sink {
-                s.set_volume(eff(volume, norm_db));
+                s.set_volume(eff(volume, norm_db, fade_gain));
             }
         }
         last_queued = q;

src-tauri/src/diagnostics.rs

@@ -79,6 +79,9 @@ pub struct DebugStats {
     pub normalization_enabled: bool,
     pub target_lufs: f64,
     pub loudness: Option<crate::loudness::CurrentTrack>,
+    /// Main player's live fade-gain (1.0 = full; <1 only during a preview
+    /// open/close crossfade). Proves a fade never strands the volume.
+    pub main_fade_gain: Option<f32>,
     /// The preview voices' resting volume (mirrors the app volume; 0 = muted).
     pub preview_volume: Option<f32>,
     /// The active preview clip's loudness-normalization gain in dB.
@@ -173,6 +176,9 @@ pub fn debug_stats(app: tauri::AppHandle, state: State<'_, DiagState>) -> DebugS
         preview_norm_db: app
             .try_state::<crate::preview::PreviewState>()
             .map(|s| crate::preview::active_norm_db(&s)),
+        main_fade_gain: app
+            .try_state::<crate::playback::PlayerState>()
+            .map(|p| p.player.fade_gain()),
     }
 }
 

src-tauri/src/preview.rs

@@ -688,27 +688,26 @@ pub fn preview_dispose(state: State<'_, PreviewState>) -> Result<(), String> {
 /// the preview voices' `fade_gen`.
 static MAIN_FADE_GEN: AtomicU64 = AtomicU64::new(0);
 
-/// Session-open crossfade, main side: ramp the main player to silence over
-/// `fade_ms` (equal-power cos, so the preview's sin ramp-in sums level),
-/// then pause it WITH the desired-paused latch, then restore `volume` on the
-/// now-silent sink so the later resume has its level back. Returns as soon
-/// as the fade thread is RUNNING — the caller starts the preview's ramp-in
-/// over it; the overlap is the point (deliberate relaxation of the Phase
-/// 0.4 "acknowledged silence before any preview audio" rule, user decision
-/// 2026-07-23). If the ack never lands, the volume stays at 0 — silent is
-/// the safe failure — and the close-side fade-resume self-heals it.
+/// Session-open crossfade, main side: ramp the main player's FADE-GAIN (a
+/// channel separate from the user volume) 1→0 over `fade_ms` (equal-power
+/// cos, so the preview's sin ramp-in sums level), then pause with the
+/// desired-paused latch. Returns as soon as the fade thread is RUNNING — the
+/// caller starts the preview's ramp-in over it; the overlap is the point
+/// (deliberate relaxation of the Phase 0.4 "acknowledged silence before any
+/// preview audio" rule, user decision 2026-07-23).
+///
+/// Fade-gain is used INSTEAD of the user volume precisely so a fade can never
+/// strand the volume: fade-gain resets to 1 on the next Play, and the
+/// close-side fade-resume ramps it back — so even if the ack times out or a
+/// track change suppresses the resume, a later clicked track still plays at
+/// full (the "audio broken after a track click" bug this replaces).
 #[tauri::command]
-pub fn player_fade_pause_acked(
-    app: tauri::AppHandle,
-    fade_ms: u64,
-    volume: f64,
-) -> Result<(), String> {
+pub fn player_fade_pause_acked(app: tauri::AppHandle, fade_ms: u64) -> Result<(), String> {
     let gen = MAIN_FADE_GEN.fetch_add(1, Ordering::Relaxed) + 1;
     std::thread::Builder::new()
         .name("main-fade-out".into())
         .spawn(move || {
             let players = app.state::<PlayerState>();
-            let v = volume as f32;
             let dur = fade_ms.max(1);
             let t0 = Instant::now();
             loop {
@@ -718,7 +717,7 @@ pub fn player_fade_pause_acked(
                 let t = (t0.elapsed().as_millis() as f32 / dur as f32).min(1.0);
                 players
                     .player
-                    .set_volume(v * (t * std::f32::consts::FRAC_PI_2).cos());
+                    .set_fade_gain((t * std::f32::consts::FRAC_PI_2).cos());
                 if t >= 1.0 {
                     break;
                 }
@@ -727,28 +726,27 @@ pub fn player_fade_pause_acked(
             if MAIN_FADE_GEN.load(Ordering::Relaxed) != gen {
                 return;
             }
+            // Pause with the latch. Fade-gain stays 0 while paused (silent),
+            // but the user volume is untouched, so nothing is stranded.
             let rx = players.player.pause_acked();
-            match rx.recv_timeout(Duration::from_millis(1_500)) {
-                Ok(_) => players.player.set_volume(v),
-                Err(_) => eprintln!("preview: main fade-pause ack timed out; volume left silent"),
-            }
+            let _ = rx.recv_timeout(Duration::from_millis(1_500));
         })
         .map_err(|e| e.to_string())?;
     Ok(())
 }
 
-/// Session-close crossfade, main side: come back from the paused snapshot at
-/// volume 0 and ramp to `volume` while the preview's fade-out rings down.
-/// Resume also clears the desired-paused latch (player-crate contract).
+/// Session-close crossfade, main side: resume the paused main and ramp its
+/// FADE-GAIN 0→1 while the preview's fade-out rings down. Resume clears the
+/// desired-paused latch (player-crate contract); the user volume is never
+/// touched, so the track returns at exactly the level it left.
 #[tauri::command]
-pub fn player_fade_resume(app: tauri::AppHandle, fade_ms: u64, volume: f64) -> Result<(), String> {
+pub fn player_fade_resume(app: tauri::AppHandle, fade_ms: u64) -> Result<(), String> {
     let gen = MAIN_FADE_GEN.fetch_add(1, Ordering::Relaxed) + 1;
     std::thread::Builder::new()
         .name("main-fade-in".into())
         .spawn(move || {
             let players = app.state::<PlayerState>();
-            let v = volume as f32;
-            players.player.set_volume(0.0);
+            players.player.set_fade_gain(0.0);
             players.player.resume();
             let dur = fade_ms.max(1);
             let t0 = Instant::now();
@@ -759,12 +757,16 @@ pub fn player_fade_resume(app: tauri::AppHandle, fade_ms: u64, volume: f64) -> R
                 let t = (t0.elapsed().as_millis() as f32 / dur as f32).min(1.0);
                 players
                     .player
-                    .set_volume(v * (t * std::f32::consts::FRAC_PI_2).sin());
+                    .set_fade_gain((t * std::f32::consts::FRAC_PI_2).sin());
                 if t >= 1.0 {
                     break;
                 }
                 std::thread::sleep(FADE_TICK);
             }
+            // Land exactly at unity so a stale tick can't leave it short.
+            if MAIN_FADE_GEN.load(Ordering::Relaxed) == gen {
+                players.player.set_fade_gain(1.0);
+            }
         })
         .map_err(|e| e.to_string())?;
     Ok(())

src/components/DebugOverlay.svelte

@@ -92,7 +92,10 @@
             </div>
             <div class="sec">
                 <div><b>FPS</b> {fps}</div>
-                <div><b>PLAYER</b> {player.status}{player.track ? ` · ${player.track.title}` : ""}</div>
+                <div>
+                    <b>PLAYER</b> {player.status}{player.track ? ` · ${player.track.title}` : ""}
+                    {#if stats.mainFadeGain != null && stats.mainFadeGain < 0.999}· fade {stats.mainFadeGain.toFixed(2)}{/if}
+                </div>
                 {#if dlTotal > 0}
                     <div>
                         <b>DL</b>

src/lib/api.js

@@ -138,10 +138,10 @@ export const streamWaveform = (username, path, size, buckets) =>
 export const previewResolve = (trackId) => invoke("preview_resolve", { trackId });
 export const previewLoad = (leaseId, startPaused, crossfadeMs = 0, startAtSecs = null) =>
     invoke("preview_load", { leaseId, startPaused, crossfadeMs, startAtSecs });
-export const playerFadePauseAcked = (fadeMs, volume) =>
-    invoke("player_fade_pause_acked", { fadeMs, volume });
-export const playerFadeResume = (fadeMs, volume) =>
-    invoke("player_fade_resume", { fadeMs, volume });
+export const playerFadePauseAcked = (fadeMs) =>
+    invoke("player_fade_pause_acked", { fadeMs });
+export const playerFadeResume = (fadeMs) =>
+    invoke("player_fade_resume", { fadeMs });
 export const debugStats = () => invoke("debug_stats");
 export const previewControl = (action, value = null) =>
     invoke("preview_control", { action, value });

src/lib/preview/focusBridge.js

@@ -105,14 +105,15 @@ export function createProductionFocus({ readPlayer, readPlaybackGeneration, resu
     // Deferred open crossfade: fade the main track out NOW, synchronised with
     // the preview's fade-in. Idempotent, and only fades if the main is
     // actually sounding (guards a commitment that leaked past a handoff or an
-    // idle-main open). Fresh volume read at fire time.
+    // idle-main open). The fade rides a separate fade-gain channel — the user
+    // volume is never touched.
     coordinator.beginMainSilence = () => {
         if (!mainSilenceArmed) return;
         mainSilenceArmed = false;
         const p = readPlayer();
         const audible = p.status === "playing" || p.status === "buffering";
         if (!audible) return;
-        void api.playerFadePauseAcked(CROSSFADE_MS, p.muted ? 0 : (p.volume ?? 0.7));
+        void api.playerFadePauseAcked(CROSSFADE_MS);
     };
 
     return { coordinator, gateway };

src/lib/state.svelte.js

@@ -3570,7 +3570,7 @@ export function initEvents() {
         resumeMain: () => {
             // Close crossfade: come back from the preview at volume 0 and
             // ramp to the user's level while the preview rings down.
-            api.playerFadeResume(PREVIEW_CROSSFADE_MS, player.muted ? 0 : player.volume);
+            api.playerFadeResume(PREVIEW_CROSSFADE_MS);
         },
     });