fix(screenshare): keep low-latency playback live

CHANGELOG.md

@@ -4,6 +4,13 @@ All notable changes to PeerSpeak are documented here.
 
 ## [Unreleased]
 
+### Fixed
+- **Low-latency screen sharing stays near the live edge again.** mpv's
+  timestamp pacing could let stale frames accumulate across the reliable
+  PixelPass transport until a share was 7–10 seconds behind. Low-latency mode
+  now presents decoded frames immediately; Smooth mode retains timestamp pacing
+  when keeping shared-video audio and video synchronized matters more.
+
 ## [0.6.4] — 2026-07-18
 
 ### Added

src/screenshare/mod.rs

@@ -607,14 +607,13 @@ fn event_for_log(ev: &PixelpassEvent) -> String {
 /// player is reaped in a background task so it doesn't linger as a zombie when
 /// its window closes.
 ///
-/// The flags keep latency low while preserving A/V sync. We deliberately do
-/// NOT pass mpv's `--untimed`: that displays each video frame the instant it
-/// decodes, ignoring audio timestamps, which makes a shared *video* drift
-/// progressively out of sync with its audio. Pacing to the audio clock costs a
-/// little latency (negligible for pointing at a desktop) and keeps a shared
-/// video in sync. We also leave hwdec at the `low-latency` default (software
-/// decode): forcing `--hwdec=auto` froze some viewers on frame 1 while audio
-/// kept playing.
+/// The buffering posture chooses the latency/A/V-sync tradeoff. Low latency
+/// passes mpv's `--untimed`, displaying frames as soon as they decode so a live
+/// share cannot accumulate several seconds in the reliable QUIC/TCP/player
+/// buffer chain. Smooth leaves timestamp pacing enabled, which keeps a shared
+/// video's audio and video synchronized at the cost of extra live latency.
+/// Hardware decoding remains opt-in: forcing `--hwdec=auto` froze some viewers
+/// on frame 1 while audio kept playing.
 fn launch_player(url: &str, settings: &ScreenShareSettings) -> std::io::Result<()> {
     let mpv_args = mpv_args(settings);
     let vlc_args = vlc_args(settings);
@@ -648,6 +647,12 @@ pub fn mpv_args(settings: &ScreenShareSettings) -> Vec<String> {
     match settings.buffering {
         ShareBuffering::LowLatency => {
             args.push("--profile=low-latency".to_string());
+            // Pixelpass carries MPEG-TS through reliable ordered QUIC/TCP. If
+            // mpv paces a live stream even slightly slower than capture, those
+            // downstream buffers retain stale frames and latency grows into
+            // seconds. Immediate presentation keeps this posture at the live
+            // edge; Smooth deliberately retains timestamp pacing for A/V sync.
+            args.push("--untimed".to_string());
             args.push("--audio-buffer=0.2".to_string());
             args.push("--demuxer-readahead-secs=0.5".to_string());
         }
@@ -828,6 +833,7 @@ mod tests {
             mpv_args(&ScreenShareSettings::default()),
             vec![
                 "--profile=low-latency",
+                "--untimed",
                 "--audio-buffer=0.2",
                 "--demuxer-readahead-secs=0.5",
                 "--demuxer-max-bytes=2M",