Add experimental stereo-effects toggle; theme the system nav bar

Stereo effects (experimental), off by default and persisted:
- When on, each mono insert effect runs a second instance for the right
  channel so upstream stereo (e.g. a delay's width) carries into later
  effects like the reverb; the delay stays single-instance (nativeStereo).
- Settings toggle under Audio Devices; rebuilds the FX chains on change.

Seamless system bars:
- SizzleTheme tints the system navigation bar to the palette's surface
  (matching the bottom tab bar) and the status bar to the background, with
  light/dark icon appearance by luminance, re-applied on theme change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/AudioEngine.kt

@@ -104,14 +104,29 @@ class AudioEngine(
         }
     }
 
-    /** One insert-effect instance bound to the toolbox slot that configures it. */
+    /** One insert-effect instance bound to the toolbox slot that configures it.
-    private class FxUnit(val slot: ToolboxSlot, val effect: AudioEffect) {
+     *  [effectR] is a second copy driving the RIGHT channel in experimental stereo-FX
+     *  mode (null otherwise, and always null for natively-stereo effects like the
+     *  delay); [effect] then drives the left. */
+    private class FxUnit(val slot: ToolboxSlot, val effect: AudioEffect, val effectR: AudioEffect? = null) {
         // Last slot version / tempo pushed into the effect, so [updateFxParams] can
         // skip re-parsing params (which allocates) when nothing changed.
         var lastVersion = -1
         var lastTempo = Float.NaN
     }
 
+    /** Experimental: process every insert in true stereo (a mono effect runs one copy
+     *  per channel) so upstream stereo — e.g. a delay's width — survives into later
+     *  effects like the reverb. Higher CPU; off by default. */
+    @Volatile var stereoFx = false; private set
+
+    /** Toggle stereo-FX mode and rebuild the chains (adds/removes the R-instances). */
+    fun setStereoFx(enabled: Boolean) {
+        if (stereoFx == enabled) return
+        stereoFx = enabled
+        rebuildFxChains()
+    }
+
     /** Per-channel effect chains (mixer FX slots resolved to real processors).
      *  Rebuilt when routing changes; parameters refresh live every block. Stored as
      *  arrays (not lists) because the audio thread iterates them per sample, and a
@@ -257,7 +272,12 @@ class AudioEngine(
                 if (slotIndex < 0) continue
                 val slot = proj.toolbox.getOrNull(slotIndex) ?: continue
                 val type = slot.type ?: continue
-                AudioEffect.create(type, sampleRate)?.let { units.add(FxUnit(slot, it)) }
+                AudioEffect.create(type, sampleRate)?.let { fx ->
+                    // In stereo-FX mode a mono effect gets a second instance for the
+                    // right channel; natively-stereo effects stay single-instance.
+                    val right = if (stereoFx && !fx.nativeStereo) AudioEffect.create(type, sampleRate) else null
+                    units.add(FxUnit(slot, fx, right))
+                }
             }
             next[ch] = units.toTypedArray()
         }
@@ -520,7 +540,16 @@ class AudioEngine(
                     val mono = if (mc.audible(anySolo)) channelSum[ch] * mc.volume else 0f
                     io[0] = mono; io[1] = mono
                     val chain = chains[ch]
-                    for (u in chain) u.effect.processStereo(io)
+                    for (u in chain) {
+                        val er = u.effectR
+                        if (er != null) {
+                            // Stereo-FX mode, mono effect: a copy per channel keeps width.
+                            io[0] = u.effect.process(io[0])
+                            io[1] = er.process(io[1])
+                        } else {
+                            u.effect.processStereo(io)
+                        }
+                    }
                     // Isolate a misbehaving effect: a non-finite sample from one
                     // channel's FX must not poison the whole master sum.
                     var l = io[0]; var r = io[1]
@@ -553,6 +582,7 @@ class AudioEngine(
                 val v = u.slot.version
                 if (v != u.lastVersion || tempo != u.lastTempo) {
                     u.effect.update(u.slot, tempo)
+                    u.effectR?.update(u.slot, tempo) // keep the right-channel copy in sync
                     u.lastVersion = v
                     u.lastTempo = tempo
                 }

app/src/main/java/com/reactorcoremeltdown/sizzletracker/audio/Effects.kt

@@ -29,13 +29,21 @@ interface AudioEffect {
     /** Process one mono sample. */
     fun process(x: Float): Float
 
+    /**
+     * True if this effect produces its own stereo width and must run as a SINGLE
+     * instance via [processStereo] (the tape delay's per-head panning). Effects that
+     * are inherently mono return false: in the experimental stereo-FX mode the engine
+     * runs two copies of them (one per channel) to keep upstream stereo intact.
+     */
+    val nativeStereo: Boolean get() = false
+
     /**
      * Process one STEREO frame in place ([io] is a 2-element scratch: io[0]=L,
      * io[1]=R). The default runs the mono [process] on the average of the two
      * channels and writes it to both — so a mono effect passes audio through as
-     * mono, collapsing any upstream stereo width. Only effects that genuinely
+     * mono, collapsing any upstream stereo width. It's used when stereo-FX mode is
-     * produce width (the tape delay's per-head panning) override this, so to keep
+     * off; when on, the engine calls [process] per channel on separate instances
-     * a delay's stereo place it last in the chain.
+     * instead. Only [nativeStereo] effects override this to produce real width.
      */
     fun processStereo(io: FloatArray) {
         val m = process((io[0] + io[1]) * 0.5f)
@@ -101,6 +109,8 @@ private class TapeDelay(private val sampleRate: Int) : AudioEffect {
         flutterDepth = tape * sampleRate * 0.0015f   // up to ~1.5 ms wow/flutter
     }
 
+    override val nativeStereo: Boolean get() = true // produces width via per-head panning
+
     // Mono path delegates to the stereo one (the chain always calls processStereo).
     override fun process(x: Float): Float {
         stereoScratch[0] = x; stereoScratch[1] = x

app/src/main/java/com/reactorcoremeltdown/sizzletracker/io/SettingsStore.kt

@@ -3,6 +3,7 @@ package com.reactorcoremeltdown.sizzletracker.io
 import android.content.Context
 import androidx.datastore.core.DataStore
 import androidx.datastore.preferences.core.Preferences
+import androidx.datastore.preferences.core.booleanPreferencesKey
 import androidx.datastore.preferences.core.edit
 import androidx.datastore.preferences.core.intPreferencesKey
 import androidx.datastore.preferences.core.stringPreferencesKey
@@ -23,6 +24,8 @@ data class AppSettings(
     val kbdChannel: Int = 0,
     /** Base octave for the on-screen keyboards. */
     val kbdOctave: Int = 3,
+    /** Experimental: process every insert effect in true stereo (higher CPU). */
+    val stereoFx: Boolean = false,
 )
 
 /**
@@ -44,6 +47,7 @@ class SettingsStore(private val context: Context) {
             recordTailBars = prefs[RECORD_TAIL] ?: 2,
             kbdChannel = prefs[KBD_CHANNEL] ?: 0,
             kbdOctave = prefs[KBD_OCTAVE] ?: 3,
+            stereoFx = prefs[STEREO_FX] ?: false,
         )
     }
 
@@ -70,11 +74,17 @@ class SettingsStore(private val context: Context) {
         }
     }
 
+    /** Persist the experimental stereo-effects toggle. */
+    suspend fun saveStereoFx(enabled: Boolean) {
+        context.settingsDataStore.edit { prefs -> prefs[STEREO_FX] = enabled }
+    }
+
     private companion object {
         val PALETTE = stringPreferencesKey("palette")
         val RECORD_DIR = stringPreferencesKey("recordDirUri")
         val RECORD_TAIL = intPreferencesKey("recordTailBars")
         val KBD_CHANNEL = intPreferencesKey("kbdChannel")
         val KBD_OCTAVE = intPreferencesKey("kbdOctave")
+        val STEREO_FX = booleanPreferencesKey("stereoFx")
     }
 }

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/AppViewModel.kt

@@ -660,6 +660,15 @@ class AppViewModel(
     var nativeAudioError by mutableStateOf<String?>(null); private set
     fun dismissNativeAudioError() { nativeAudioError = null }
 
+    /** Experimental: process every insert effect in true stereo (higher CPU). */
+    var stereoFx by mutableStateOf(false); private set
+    fun applyStereoFx(enabled: Boolean) {
+        stereoFx = enabled
+        engine.setStereoFx(enabled)
+        viewModelScope.launch { settingsStore.saveStereoFx(enabled) }
+        touched()
+    }
+
     /** Choose the colour theme and persist the choice. */
     fun selectPalette(p: com.reactorcoremeltdown.sizzletracker.ui.theme.RetroPalette) {
         palette = p
@@ -738,6 +747,9 @@ class AppViewModel(
             // Restore the on-screen keyboard's channel + octave.
             kbdChannel = s.kbdChannel.coerceIn(0, Cell.MAX_CHANNEL)
             kbdOctave = s.kbdOctave.coerceIn(0, 8)
+            // Restore the experimental stereo-FX mode (rebuilds the FX chains).
+            stereoFx = s.stereoFx
+            engine.setStereoFx(s.stereoFx)
             engine.recTempDir = appContext.cacheDir
             engine.setRecordTailBars(recordTailBars)
             engine.onRecordingStarted = {

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/settings/SettingsScreen.kt

@@ -44,6 +44,7 @@ import androidx.compose.material3.ListItemDefaults
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.OutlinedButton
 import androidx.compose.material3.OutlinedTextField
+import androidx.compose.material3.Switch
 import androidx.compose.material3.Text
 import androidx.compose.material3.TextButton
 import androidx.compose.runtime.Composable
@@ -362,6 +363,24 @@ private fun AudioDevicesCard(vm: AppViewModel) {
         DeviceRow("Output", outputs, vm.outputDeviceId) { vm.setAudioOutput(it) }
         HorizontalDivider()
         DeviceRow("Input", inputs, vm.inputDeviceId) { vm.setAudioInput(it) }
+        HorizontalDivider()
+        Row(
+            Modifier.fillMaxWidth(),
+            horizontalArrangement = Arrangement.SpaceBetween,
+            verticalAlignment = Alignment.CenterVertically,
+        ) {
+            Column(Modifier.weight(1f)) {
+                Text("Stereo effects (experimental)", style = MaterialTheme.typography.bodyMedium)
+                Text(
+                    "Process every insert in true stereo, so a delay's width carries " +
+                        "into later effects (e.g. delay → reverb). Higher CPU — may cause " +
+                        "dropouts on some devices.",
+                    style = MaterialTheme.typography.bodySmall,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                )
+            }
+            Switch(checked = vm.stereoFx, onCheckedChange = { vm.applyStereoFx(it) })
+        }
     }
 }
 

app/src/main/java/com/reactorcoremeltdown/sizzletracker/ui/theme/Theme.kt

@@ -1,17 +1,23 @@
 package com.reactorcoremeltdown.sizzletracker.ui.theme
 
+import android.app.Activity
 import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Typography
 import androidx.compose.material3.darkColorScheme
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.CompositionLocalProvider
+import androidx.compose.runtime.SideEffect
 import androidx.compose.runtime.staticCompositionLocalOf
 import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.luminance
+import androidx.compose.ui.graphics.toArgb
+import androidx.compose.ui.platform.LocalView
 import androidx.compose.ui.text.TextStyle
 import androidx.compose.ui.text.font.FontFamily
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.sp
+import androidx.core.view.WindowCompat
 
 /**
  * The retro look-and-feel. Two ideas drive everything:
@@ -98,6 +104,24 @@ fun SizzleTheme(
         onSurface = palette.text,
         error = palette.danger,
     )
+
+    // Tint the system navigation bar to match the app's bottom tab bar (also
+    // `surface`) so the gesture handle below it looks seamless — and re-tint when the
+    // palette changes. The status bar keeps the window background for the same reason.
+    val view = LocalView.current
+    if (!view.isInEditMode) {
+        SideEffect {
+            val window = (view.context as Activity).window
+            window.navigationBarColor = palette.surface.toArgb()
+            window.statusBarColor = palette.background.toArgb()
+            val lightSurface = palette.surface.luminance() > 0.5f
+            WindowCompat.getInsetsController(window, view).apply {
+                isAppearanceLightNavigationBars = lightSurface
+                isAppearanceLightStatusBars = palette.background.luminance() > 0.5f
+            }
+        }
+    }
+
     CompositionLocalProvider(LocalRetro provides palette) {
         MaterialTheme(colorScheme = scheme, typography = retroTypography, content = content)
     }