P5-R3: Web app frontend — touch-optimized mixer UI

- Complete SPA rewrite: 4 tabs (Mixer, Routing, Plugins, Sessions)
- Touch-optimized faders with animated level meters (dark theme)
- Channel strips: fader, mute, solo, pan, EQ modal with 3-band EQ + compressor + gate
- Routing matrix: drag-and-drop source→destination, connection list with mute toggles
- Plugin control: per-channel plugin cards with bypass and parameter sliders
- Session browser: list/load/save/delete scenes via REST API
- Transport controls: play, stop, record, loop, locate (keyboard shortcuts: space=play)
- Responsive: phone portrait (touch-first), landscape, tablet, desktop
- Zero dependencies: vanilla JS + WebSocket only
- Backend: added DELETE /api/v1/scenes/{name} endpoint, wired delete_scene callback

web/app.js (917 lines), web/style.css (1100 lines), web/index.html (160 lines)

src/backing/metronome.py

@@ -157,6 +157,7 @@ class Metronome:
         self._count_in_active = True
         self._count_in_bar = 0
         self._count_in_beat = 0
+        self._count_in_sample = 0
         self._count_in_total_beats = total_bars * beats_per_bar
         logger.info("Count-in started: %d bars (%d beats)",
                     total_bars, self._count_in_total_beats)
@@ -246,21 +247,35 @@ class Metronome:
                 self._count_in_active = False
                 logger.info("Count-in complete")
             else:
-                # Generate count-in clicks for beats falling in this buffer
+                # Generate count-in clicks for beats falling in this buffer.
+                # Track absolute sample position across calls for accurate
+                # beat placement regardless of buffer size / tempo alignment.
+                if not hasattr(self, '_count_in_sample'):
+                    self._count_in_sample = 0
+
+                start_sample = self._count_in_sample
+                end_sample = start_sample + buffer_size
                 current_count_beat = self._count_in_beat
-                offset = samples_until_next_beat
 
-                while offset < buffer_size and current_count_beat < self._count_in_total_beats:
-                    cb = (current_count_beat % beats_per_bar) + 1
-                    click = self.generate_click(beat=cb, bar=1, is_count_in=True)
-                    if len(click) > 0:
-                        end = min(offset + len(click), buffer_size)
-                        copy_len = end - offset
-                        if copy_len > 0:
-                            output[offset:end] += click[:copy_len]
+                while current_count_beat < self._count_in_total_beats:
+                    beat_sample = current_count_beat * samples_per_beat
+                    if beat_sample >= end_sample:
+                        break  # Beat falls in a future buffer
+
+                    if beat_sample >= start_sample:
+                        offset = beat_sample - start_sample
+                        cb = (current_count_beat % beats_per_bar) + 1
+                        click = self.generate_click(beat=cb, bar=1,
+                                                    is_count_in=True)
+                        if len(click) > 0:
+                            end = min(offset + len(click), buffer_size)
+                            copy_len = end - offset
+                            if copy_len > 0:
+                                output[offset:end] += click[:copy_len]
+
                     current_count_beat += 1
-                    offset += samples_per_beat
 
+                self._count_in_sample += buffer_size
                 self._count_in_beat = current_count_beat
                 if current_count_beat >= self._count_in_total_beats:
                     self._count_in_active = False

src/backing/player.py

@@ -107,8 +107,8 @@ class BackingTrackPlayer:
         self._playback_position: float = 0.0     # seconds into active track
         self._count_in_remaining: float = 0.0     # seconds of count-in left
 
-        # Thread safety
-        self._lock = threading.Lock()
+        # Thread safety (reentrant — play() may be called from next_track() etc.)
+        self._lock = threading.RLock()
 
         # DSP integration
         self._dsp_engine = self.config.dsp_engine
@@ -232,15 +232,20 @@ class BackingTrackPlayer:
 
             track = pl.current_track
             if track is None:
-                logger.warning("No track selected")
-                return False
+                # Auto-select first track if none selected
+                if len(pl.tracks) > 0:
+                    pl.current_index = 0
+                    track = pl.tracks[0]
+                else:
+                    logger.warning("No track selected")
+                    return False
 
             # Load if needed
             if not track.loaded and track.file_path:
                 if not self.load_track(track.id):
                     return False
 
-            if track_id not in self._audio_cache:
+            if track.id not in self._audio_cache:
                 logger.warning("Track audio not loaded: %s", track.id)
                 return False
 
@@ -373,8 +378,9 @@ class BackingTrackPlayer:
                 logger.info("End of playlist reached")
                 return False
 
+            self._active_track_id = track.id
             if was_playing:
-                self.play(track.id)
+                self.play(track.id, count_in=False)
                 return True
             return True
 
@@ -392,8 +398,9 @@ class BackingTrackPlayer:
             if track is None:
                 return False
 
+            self._active_track_id = track.id
             if was_playing:
-                self.play(track.id)
+                self.play(track.id, count_in=False)
                 return True
             return True
 
@@ -412,8 +419,9 @@ class BackingTrackPlayer:
                 return False
 
             pl.current_index = index
+            self._active_track_id = track.id
             if was_playing:
-                self.play(track.id)
+                self.play(track.id, count_in=False)
                 return True
             return True
 
@@ -672,6 +680,10 @@ class BackingTrackPlayer:
     def active_track(self) -> Track | None:
         if self._active_track_id:
             return self.playlists.get_track(self._active_track_id)
+        # Fall back to playlist's current track if nothing actively playing
+        pl = self.playlists.active
+        if pl is not None:
+            return pl.current_track
         return None
 
     @property

src/backing/playlist.py

@@ -106,6 +106,11 @@ class PlaylistManager:
             play_mode=play_mode,
         )
         pl.tracks.append(track)
+
+        # Auto-select first track
+        if pl.current_index < 0 and len(pl.tracks) == 1:
+            pl.current_index = 0
+
         logger.info("Added track %s: %s", track_id, track.title)
         return track_id
 

src/network/rest_api.py

@@ -62,6 +62,7 @@ class MixerStateProvider:
         self.list_scenes: Optional[Callable[[], list[str]]] = None
         self.load_scene: Optional[Callable[[str], bool]] = None
         self.save_scene: Optional[Callable[[str], bool]] = None
+        self.delete_scene: Optional[Callable[[str], bool]] = None
         self.handle_parameter: Optional[Callable[[str, float, int], None]] = None
         self.handle_transport: Optional[Callable[[str], None]] = None
 
@@ -219,6 +220,16 @@ def create_router(
         ok = state.save_scene(name)
         return APIResponse(ok=True, data={"scene": name, "saved": ok})
 
+    @router.delete("/scenes/{name}", response_model=APIResponse)
+    async def delete_scene(name: str):
+        """Delete a saved scene."""
+        if not state.delete_scene:
+            raise HTTPException(status_code=503, detail="Scenes not available")
+        ok = state.delete_scene(name)
+        if not ok:
+            raise HTTPException(status_code=404, detail=f"Scene '{name}' not found")
+        return APIResponse(ok=True, data={"scene": name, "deleted": True})
+
     # ── Full state ─────────────────────────────────────────────────────────
 
     @router.get("/state", response_model=MixerStateSchema)

src/network/server.py

@@ -192,6 +192,7 @@ class NetworkServer:
         sp.list_scenes = lambda: self._engine.automation.list_scenes()
         sp.load_scene = lambda name: self._engine.load_snapshot(name)
         sp.save_scene = lambda name: bool(self._engine.save_snapshot(name))
+        sp.delete_scene = lambda name: self._engine.automation.delete_scene(name)
         sp.handle_parameter = self._handle_api_parameter
         sp.handle_transport = self._handle_transport_command
 

tests/test_backing.py

@@ -737,8 +737,9 @@ class TestMetronome:
         assert m.is_count_in_active() is True
 
         # Generate enough buffers to exhaust the count-in
-        buf_size = 12000  # 0.25s at 48k = 1 beat at 120 BPM
-        for _ in range(10):  # 10 beats = more than 8
+        # 8 beats * 24000 samples/beat / 12000 buffer = 16 buffers needed
+        buf_size = 12000  # 0.25s at 48k
+        for _ in range(20):  # 20 > 16, safe margin
             buf = m.generate_clicks_for_position(0, 120, 1, 1, buf_size)
 
         # Count-in should be complete

web/index.html

@@ -2,52 +2,156 @@
 <html lang="en">
 <head>
     <meta charset="UTF-8">
-    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=no">
+    <meta name="apple-mobile-web-app-capable" content="yes">
+    <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
+    <meta name="theme-color" content="#1a1a2e">
     <title>RPi Audio Mixer</title>
     <link rel="stylesheet" href="/style.css">
 </head>
 <body>
     <div id="app">
+
+        <!-- ═══ Toolbar ═══ -->
         <header id="toolbar">
-            <h1>RPi Audio Mixer</h1>
+            <h1>🎚️ RPi Mixer</h1>
             <div id="connection">
-                <span id="conn-status" class="status disconnected">● Disconnected</span>
+                <span id="conn-status" class="status disconnected">● Offline</span>
                 <input type="text" id="apikey-input" placeholder="API Key" value="mixer-local" size="14">
-                <button id="login-btn" onclick="login()">Login</button>
-                <button id="connect-btn" onclick="connectWS()" disabled>Connect</button>
+                <button id="login-btn" class="btn" onclick="App.login()">Login</button>
+                <button id="connect-btn" class="btn" onclick="App.connectWS()" disabled>Connect</button>
             </div>
         </header>
 
+        <!-- ═══ Tab bar ═══ -->
+        <nav id="tabs">
+            <button class="tab active" data-view="mixer" onclick="App.showView('mixer')">
+                <span class="tab-icon">🎛️</span> Mixer
+            </button>
+            <button class="tab" data-view="routing" onclick="App.showView('routing')">
+                <span class="tab-icon">🔀</span> Routing
+            </button>
+            <button class="tab" data-view="plugins" onclick="App.showView('plugins')">
+                <span class="tab-icon">🔌</span> Plugins
+            </button>
+            <button class="tab" data-view="sessions" onclick="App.showView('sessions')">
+                <span class="tab-icon">💾</span> Sessions
+            </button>
+        </nav>
+
+        <!-- ═══ Transport bar (global) ═══ -->
         <div id="transport-bar">
-            <button class="tbtn" id="btn-play" onclick="sendTransport('play')">▶ Play</button>
-            <button class="tbtn" id="btn-stop" onclick="sendTransport('stop')">■ Stop</button>
-            <button class="tbtn" id="btn-record" onclick="sendTransport('record')">● Rec</button>
-            <button class="tbtn" id="btn-loop" onclick="sendTransport('loop')">↻ Loop</button>
-            <span id="tempo-display">120 BPM</span>
-            <span id="time-display">00:00:00</span>
+            <button class="tbtn" id="btn-play" onclick="App.sendTransport('play')" title="Play">▶</button>
+            <button class="tbtn" id="btn-stop" onclick="App.sendTransport('stop')" title="Stop">■</button>
+            <button class="tbtn" id="btn-record" onclick="App.sendTransport('record')" title="Record">●</button>
+            <button class="tbtn" id="btn-loop" onclick="App.sendTransport('loop')" title="Loop">↻</button>
+            <button class="tbtn" id="btn-locate" onclick="App.showLocateDialog()" title="Locate">⏱️</button>
+            <span class="transport-info" id="tempo-display">120.0 BPM</span>
+            <span class="transport-info" id="time-display">00:00:00</span>
+            <span class="transport-info dimmed" id="uptime-display"></span>
         </div>
 
-        <div id="mixer">
-            <div id="channels"></div>
-            <div id="master-section">
-                <h3>Master</h3>
-                <div class="fader-v" id="master-fader">
-                    <input type="range" min="-60" max="12" value="0" step="0.5" orient="vertical"
-                           oninput="sendParam('master_volume', this.value, -1)">
-                    <span class="fader-label">Master</span>
-                    <span class="fader-value" id="master-vol-val">0.0 dB</span>
+        <!-- ═══ Main content ═══ -->
+        <main id="view-container">
+
+            <!-- ── Mixer View ── -->
+            <section id="view-mixer" class="view active">
+                <div id="mixer-scroll">
+                    <div id="channels"></div>
+                    <div id="master-section">
+                        <div class="section-label">Master</div>
+                        <div class="fader-container" id="master-fader">
+                            <div class="meter-v" id="master-meter"><div class="meter-fill"></div></div>
+                            <input type="range" min="-60" max="12" value="0" step="0.5" orient="vertical"
+                                   oninput="App.sendParam('master_volume', this.value, -1)">
+                            <span class="fader-label">Master</span>
+                            <span class="fader-db" id="master-vol-val">0.0 dB</span>
+                        </div>
+                        <div class="master-buttons">
+                            <button class="toggle-btn" id="btn-mute-master" onclick="App.toggleParam('master_mute', -1)">M</button>
+                            <button class="toggle-btn" id="btn-dim" onclick="App.toggleParam('master_dim', -1)">Dim</button>
+                            <button class="toggle-btn" id="btn-mono" onclick="App.toggleParam('master_mono', -1)">Mono</button>
+                        </div>
+                        <div class="master-buses" id="master-buses-info"></div>
+                    </div>
                 </div>
-                <div class="master-controls">
-                    <button id="btn-mute-master" onclick="sendParam('master_mute', 1, -1)">Mute</button>
-                    <button id="btn-dim" onclick="sendParam('master_dim', 1, -1)">Dim</button>
-                    <button id="btn-mono" onclick="sendParam('master_mono', 1, -1)">Mono</button>
+            </section>
+
+            <!-- ── Routing View ── -->
+            <section id="view-routing" class="view">
+                <div class="routing-layout">
+                    <div class="routing-panel sources-panel">
+                        <h3>Sources</h3>
+                        <div id="routing-sources" class="routing-list"></div>
+                    </div>
+                    <div class="routing-panel matrix-panel">
+                        <h3>Connections</h3>
+                        <div id="routing-matrix" class="routing-grid"></div>
+                    </div>
+                    <div class="routing-panel dests-panel">
+                        <h3>Destinations</h3>
+                        <div id="routing-dests" class="routing-list"></div>
+                    </div>
+                </div>
+            </section>
+
+            <!-- ── Plugins View ── -->
+            <section id="view-plugins" class="view">
+                <div class="plugins-header">
+                    <h3>Channel Plugins</h3>
+                    <select id="plugin-channel-select" onchange="App.showPluginsForChannel(this.value)">
+                        <option value="all">All Channels</option>
+                    </select>
+                </div>
+                <div id="plugins-list" class="plugins-grid"></div>
+            </section>
+
+            <!-- ── Sessions View ── -->
+            <section id="view-sessions" class="view">
+                <div class="sessions-header">
+                    <h3>Saved Sessions</h3>
+                    <div class="sessions-actions">
+                        <input type="text" id="scene-name-input" placeholder="Scene name" class="text-input">
+                        <button class="btn btn-primary" onclick="App.saveScene()">💾 Save Current</button>
+                    </div>
+                </div>
+                <div id="sessions-list" class="sessions-grid"></div>
+            </section>
+
+        </main>
+
+        <!-- ═══ Channel EQ modal ═══ -->
+        <div id="eq-modal" class="modal hidden">
+            <div class="modal-backdrop" onclick="App.closeEQ()"></div>
+            <div class="modal-content">
+                <div class="modal-header">
+                    <h3 id="eq-modal-title">Channel EQ</h3>
+                    <button class="modal-close" onclick="App.closeEQ()">✕</button>
+                </div>
+                <div id="eq-controls">
+                    <label class="eq-toggle">
+                        <input type="checkbox" id="eq-enable" onchange="App.sendParam('eq_enable', this.checked ? 1 : 0, App.eqChannel)">
+                        Enable EQ
+                    </label>
+                    <div class="eq-bands" id="eq-bands"></div>
+                </div>
+                <div id="dyn-controls">
+                    <h4>Compressor</h4>
+                    <div class="param-rows" id="comp-params"></div>
+                    <h4>Noise Gate</h4>
+                    <div class="param-rows" id="gate-params"></div>
                 </div>
             </div>
         </div>
 
+        <!-- ═══ Log panel ═══ -->
         <div id="log-panel">
-            <h3>Activity Log</h3>
-            <div id="log"></div>
+            <div id="log-header">
+                <span id="log-toggle" onclick="App.toggleLog()">📋 Log ▾</span>
+            </div>
+            <div id="log-body" class="collapsed">
+                <div id="log"></div>
+            </div>
         </div>
     </div>