feat(audio): add --strict-audio + app_audio route-status events

With --app, pixelpass mirrors the default-sink monitor (whole desktop) until the
chosen app's streams route, and restores that loopback if the app's audio later
stops. That fallback captures everything playing — including a voice call the
sharer is in — so a caller watching the share can hear themselves echoed back
(peerspeak bug A23: the per-app pick alone is best-effort, not a guarantee).

- New --strict-audio flag (HostOpts.strict_audio): with --app, never load the
  default-sink loopback (not at startup, not on LastRoutedStreamGone). The viewer
  hears only the chosen app, and silence when it's quiet — never the rest of the
  desktop. No effect without --app; standalone best-effort behavior is unchanged.
- New app_audio JSON event ({"event":"app_audio","state":"routed"|"lost"}),
  emitted whenever --app is set, so a front-end (peerspeak) can tell when the
  chosen app's audio is actually live vs. dropped and warn accordingly.
- Banner capture summary shows "(strict)" when active.

Unknown-event-tolerant: pixelpass's own --gui child parser skips lines it can't
deserialize, so app_audio doesn't disturb it. 10 tests (+2: wire-shape + banner),
clippy --all-targets clean.

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

src/cli.rs

@@ -27,6 +27,17 @@ pub struct Cli {
     #[arg(long, value_name = "NAME")]
     pub app: Option<String>,
 
+    /// With `--app`, never fall back to whole-desktop audio. By default an
+    /// app-filtered host mirrors the default sink's monitor until (and again
+    /// after) the chosen app's streams route, so the viewer isn't left in
+    /// silence. That fallback also captures everything else playing — including
+    /// a voice call the sharer is in — so a caller can hear themselves echoed.
+    /// `--strict-audio` suppresses the fallback entirely: the viewer hears only
+    /// the chosen app, and silence when it isn't producing audio. Ignored
+    /// without `--app`.
+    #[arg(long)]
+    pub strict_audio: bool,
+
     /// Override display server autodetection.
     #[arg(long, value_enum)]
     pub display_server: Option<DisplayServerArg>,
@@ -135,6 +146,10 @@ pub enum Quality {
 pub struct HostOpts {
     pub window: bool,
     pub app: Option<String>,
+    /// With `app` set, suppress the whole-desktop loopback fallback so the
+    /// viewer only ever hears the chosen app (silence when it's quiet). No
+    /// effect when `app` is None.
+    pub strict_audio: bool,
     pub display_server: Option<DisplayServerArg>,
     /// Chosen preset (Auto = derive at startup). Defaults to Auto.
     pub quality: Quality,
@@ -164,6 +179,7 @@ impl Cli {
         HostOpts {
             window: self.window,
             app: self.app,
+            strict_audio: self.strict_audio,
             display_server: self.display_server,
             // No `--quality` and nothing picked interactively → the documented
             // default, Auto.

src/common/output.rs

@@ -58,6 +58,11 @@ pub enum Event<'a> {
     ViewerRefused { reason: &'a str },
     /// Viewer-side: the local player URL is ready to open.
     Connected { url: &'a str },
+    /// Per-app audio routing state (only emitted when `--app` is set). `routed`
+    /// = the chosen app's audio is now reaching viewers; `lost` = its last
+    /// stream went away. Under `--strict-audio`, `lost` means viewers currently
+    /// hear silence; without it, viewers fall back to whole-desktop audio.
+    AppAudio { state: AppAudioState },
 }
 
 #[derive(Serialize)]
@@ -67,6 +72,13 @@ pub enum CaptureState {
     Stopped,
 }
 
+#[derive(Serialize, Clone, Copy)]
+#[serde(rename_all = "snake_case")]
+pub enum AppAudioState {
+    Routed,
+    Lost,
+}
+
 /// Emit one event as a JSON line on stdout, flushed. No-op unless JSON
 /// output was enabled with [`set_json`], so call sites can sprinkle these
 /// unconditionally without branching.
@@ -85,3 +97,25 @@ pub fn emit(event: Event) {
         Err(e) => tracing::warn!("failed to serialize event: {e}"),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // The app_audio event is the wire contract peerspeak parses to drive its
+    // echo warning; pin the exact shape so a rename here is caught here.
+    #[test]
+    fn app_audio_event_wire_shape() {
+        let routed = serde_json::to_string(&Event::AppAudio {
+            state: AppAudioState::Routed,
+        })
+        .unwrap();
+        assert_eq!(routed, r#"{"event":"app_audio","state":"routed"}"#);
+
+        let lost = serde_json::to_string(&Event::AppAudio {
+            state: AppAudioState::Lost,
+        })
+        .unwrap();
+        assert_eq!(lost, r#"{"event":"app_audio","state":"lost"}"#);
+    }
+}

src/host/audio.rs

@@ -55,24 +55,38 @@ impl Routing {
         let sink_module = load_module(&["module-null-sink", &format!("sink_name={sink_name}")])
             .context("failed to load module-null-sink")?;
 
+        // In strict per-app mode we never mirror the default sink: the viewer
+        // must hear *only* the chosen app, never the whole desktop (which would
+        // leak e.g. a voice call the sharer is in back to viewers — the echo
+        // bug A23). Without strict mode (whole-desktop share, or best-effort
+        // app filtering) we load the monitor loopback so the viewer hears
+        // system audio immediately and during any gap before the app routes.
         // 20ms loopback latency keeps the mirrored audio tight; pactl's
         // default of 200ms is enough to be perceptible.
-        let loopback_module = load_module(&[
-            "module-loopback",
-            "source=@DEFAULT_SINK@.monitor",
-            &format!("sink={sink_name}"),
-            "latency_msec=20",
-        ])
-        .context("failed to load module-loopback (null-sink will be cleaned up on Drop)")?;
+        let strict_app = opts.app.is_some() && opts.strict_audio;
+        let loopback_module = if strict_app {
+            None
+        } else {
+            Some(
+                load_module(&[
+                    "module-loopback",
+                    "source=@DEFAULT_SINK@.monitor",
+                    &format!("sink={sink_name}"),
+                    "latency_msec=20",
+                ])
+                .context("failed to load module-loopback (null-sink cleaned up on Drop)")?,
+            )
+        };
 
         tracing::info!(
             sink_module,
-            loopback_module,
+            ?loopback_module,
+            strict_app,
             %sink_name,
-            "audio routing: null-sink + loopback ready"
+            "audio routing: null-sink ready (loopback skipped in strict app mode)"
         );
 
-        let loopback_arc = Arc::new(Mutex::new(Some(loopback_module)));
+        let loopback_arc = Arc::new(Mutex::new(loopback_module));
         let mut routing = Self {
             sink_module: Some(sink_module),
             loopback_module: Arc::clone(&loopback_arc),
@@ -85,7 +99,9 @@ impl Routing {
             let (router, mut event_rx) = StreamRouter::spawn(app.clone(), sink_name.clone())?;
             let loopback_for_task = Arc::clone(&loopback_arc);
             let sink_name_for_task = sink_name.clone();
+            let strict = opts.strict_audio;
             let event_task = tokio::spawn(async move {
+                use crate::common::output::{self, AppAudioState};
                 while let Some(ev) = event_rx.recv().await {
                     match ev {
                         Event::FirstRoutedStream => {
@@ -96,11 +112,30 @@ impl Routing {
                                 );
                                 unload_module(id);
                             }
+                            // Tell the front-end the chosen app's audio is live.
+                            output::emit(output::Event::AppAudio {
+                                state: AppAudioState::Routed,
+                            });
                         }
                         Event::LastRoutedStreamGone => {
-                            // Routed app exited mid-session. Restore the
-                            // default-sink loopback so the viewer hears
-                            // system audio again instead of silence.
+                            // Routed app exited/paused mid-session. Notify the
+                            // front-end either way; the recovery differs by mode.
+                            output::emit(output::Event::AppAudio {
+                                state: AppAudioState::Lost,
+                            });
+                            if strict {
+                                // Strict mode: do NOT restore the whole-desktop
+                                // loopback. Viewers hear silence until the app
+                                // produces audio again — never the rest of the
+                                // desktop (call included).
+                                tracing::info!(
+                                    "audio routing: strict mode — last routed stream gone, leaving viewers silent"
+                                );
+                                continue;
+                            }
+                            // Best-effort mode: restore the default-sink loopback
+                            // so the viewer hears system audio again instead of
+                            // silence.
                             if loopback_for_task.lock().unwrap().is_some() {
                                 continue;
                             }

src/host/mod.rs

@@ -488,9 +488,51 @@ fn copy_to_clipboard(text: &str) -> bool {
 fn capture_summary(opts: &HostOpts) -> String {
     let mut bits = vec![if opts.window { "window" } else { "fullscreen" }.to_string()];
     if let Some(app) = &opts.app {
-        bits.push(format!("app-audio={app}"));
+        if opts.strict_audio {
+            bits.push(format!("app-audio={app} (strict)"));
+        } else {
+            bits.push(format!("app-audio={app}"));
+        }
     } else {
         bits.push("system-audio".to_string());
     }
     bits.join(" + ")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::cli::Quality;
+
+    fn opts(app: Option<&str>, strict_audio: bool) -> HostOpts {
+        HostOpts {
+            window: false,
+            app: app.map(str::to_string),
+            strict_audio,
+            display_server: None,
+            quality: Quality::Auto,
+            bitrate: None,
+            framerate: None,
+            max_height: None,
+            no_hwencode: false,
+            max_viewers: None,
+            interactive: false,
+            relay: None,
+        }
+    }
+
+    #[test]
+    fn capture_summary_reflects_audio_mode() {
+        assert_eq!(capture_summary(&opts(None, false)), "fullscreen + system-audio");
+        assert_eq!(
+            capture_summary(&opts(Some("Firefox"), false)),
+            "fullscreen + app-audio=Firefox"
+        );
+        // strict only shows when an app is selected.
+        assert_eq!(
+            capture_summary(&opts(Some("Firefox"), true)),
+            "fullscreen + app-audio=Firefox (strict)"
+        );
+        assert_eq!(capture_summary(&opts(None, true)), "fullscreen + system-audio");
+    }
+}

src/host/quality.rs

@@ -205,6 +205,7 @@ mod tests {
         HostOpts {
             window: false,
             app: None,
+            strict_audio: false,
             display_server: None::<DisplayServerArg>,
             quality,
             bitrate: None,