@@ -80,6 +80,76 @@ pub fn concatenate_video_fragments(fragments: &[PathBuf], output: &Path) -> Resu
     result
 }
 
+/// Joins files cut from one recording timeline, such as the output of each
+/// muxer process when one was replaced mid-recording. Each file keeps its own
+/// start time, so video after a gap stays at its capture time instead of
+/// closing the gap; the last frame before a gap is held across it.
+pub fn concatenate_video_fragments_on_timeline(
+    fragments: &[PathBuf],
+    output: &Path,
+) -> Result<(), RemuxError> {
+    if fragments.is_empty() {
+        return Err(RemuxError::NoFragments);
+    }
+
+    for fragment in fragments {
+        if !fragment.exists() {
+            return Err(RemuxError::FragmentNotFound(fragment.clone()));
+        }
+    }
+
+    let spans = fragments
+        .iter()
+        .map(|fragment| media_span(fragment))
+        .collect::<Result<Vec<_>, _>>()?;
+    let concat_list_path = output.with_extension("concat.txt");
+    let entries = fragments
+        .iter()
+        .enumerate()
+        .map(|(index, fragment)| {
+            let entry = concat_fragment_entry(fragment, &concat_list_path)?;
+            Ok(match timeline_duration(&spans, index) {
+                Some(duration) => format!("{entry}duration {duration}us\n"),
+                None => entry,
+            })
+        })
+        .collect::<Result<Vec<_>, RemuxError>>()?;
+    std::fs::write(&concat_list_path, entries.concat())?;
+
+    let result = concatenate_with_concat_demuxer(&concat_list_path, output);
+
+    let _ = std::fs::remove_file(&concat_list_path);
+
+    result
+}
+
+#[derive(Clone, Copy, Debug)]
+struct MediaSpan {
+    start: i64,
+    duration: i64,
+}
+
+fn media_span(path: &Path) -> Result<Option<MediaSpan>, RemuxError> {
+    let input = ffmpeg::format::input(path)?;
+    let (start, duration) = (container_start_time(&input), input.duration());
+    Ok((start != ffmpeg::ffi::AV_NOPTS_VALUE && duration > 0)
+        .then_some(MediaSpan { start, duration }))
+}
+
+fn container_start_time(input: &avformat::context::Input) -> i64 {
+    unsafe { (*input.as_ptr()).start_time }
+}
+
+/// How far into the joined timeline the file after `index` starts, in
+/// microseconds: the gap between the two start times, but never less than the
+/// file's own duration, so overlapping files still play end to end. `None`
+/// leaves the concat demuxer to use the file's probed duration.
+fn timeline_duration(spans: &[Option<MediaSpan>], index: usize) -> Option<i64> {
+    let current = spans.get(index).copied().flatten()?;
+    let next = spans.get(index + 1).copied().flatten()?;
+    Some((next.start - current.start).max(current.duration))
+}
+
 fn concat_fragment_entry(fragment: &Path, concat_list: &Path) -> Result<String, RemuxError> {
     let fragment = std::path::absolute(fragment)?;
     let concat_list = std::path::absolute(concat_list)?;
@@ -1374,7 +1444,8 @@ mod tests {
         concatenate_audio_to_ogg, concatenate_m4s_segments_with_init,
         concatenate_m4s_segments_with_init_validated,
         concatenate_m4s_segments_with_init_validated_with_reader, concatenate_video_fragments,
-        get_video_capture_fps, get_video_fps, probe_video_pts_ladder, write_concat_list,
+        concatenate_video_fragments_on_timeline, get_video_capture_fps, get_video_fps,
+        probe_video_pts_ladder, write_concat_list,
     };
     use std::{
         path::{Path, PathBuf},
@@ -1794,6 +1865,113 @@ mod tests {
         assert!(!output.with_extension("concat.txt").exists());
     }
 
+    fn first_video_pts_after(path: &Path, frames_before: usize) -> f64 {
+        let (time_base, packets) = video_packet_contents(path);
+        let mut pts: Vec<i64> = packets.iter().filter_map(|packet| packet.pts).collect();
+        pts.sort_unstable();
+        pts[frames_before] as f64 * f64::from(time_base)
+    }
+
+    fn timeline_inputs(parent: &Path, second_start_frame: u64) -> [PathBuf; 2] {
+        let frames = |range: std::ops::Range<u64>| -> Vec<Duration> {
+            range
+                .map(|frame| Duration::from_nanos(frame * 1_000_000_000 / 30))
+                .collect()
+        };
+        let first_dir = parent.join("first");
+        let second_dir = parent.join("second");
+        std::fs::create_dir_all(&first_dir).unwrap();
+        std::fs::create_dir_all(&second_dir).unwrap();
+        let second = second_dir.join("shifted.mp4");
+        shift_test_mp4(
+            &encode_test_mp4(&second_dir, &frames(0..12)),
+            &second,
+            second_start_frame as f64 / 30.0,
+        );
+        [encode_test_mp4(&first_dir, &frames(0..12)), second]
+    }
+
+    /// A copy of `input` whose packets start `offset` seconds later, like a
+    /// replacement muxer's output on the recording's shared timeline.
+    fn shift_test_mp4(input: &Path, output: &Path, offset: f64) {
+        let mut ictx = ffmpeg::format::input(input).unwrap();
+        let mut octx = ffmpeg::format::output(output).unwrap();
+        let stream = ictx.streams().best(ffmpeg::media::Type::Video).unwrap();
+        let (index, time_base) = (stream.index(), stream.time_base());
+        let mut output_stream = octx.add_stream(None).unwrap();
+        output_stream.set_parameters(stream.parameters());
+        unsafe {
+            (*output_stream.as_mut_ptr())
+                .codecpar
+                .as_mut()
+                .unwrap()
+                .codec_tag = 0;
+        }
+        octx.write_header().unwrap();
+        let output_time_base = octx.stream(0).unwrap().time_base();
+        let shift = (offset * f64::from(time_base.denominator()) / f64::from(time_base.numerator()))
+            .round() as i64;
+        for (stream, mut packet) in ictx.packets() {
+            if stream.index() != index {
+                continue;
+            }
+            packet.set_pts(packet.pts().map(|pts| pts + shift));
+            packet.set_dts(packet.dts().map(|dts| dts + shift));
+            packet.rescale_ts(time_base, output_time_base);
+            packet.set_stream(0);
+            packet.set_position(-1);
+            packet.write_interleaved(&mut octx).unwrap();
+        }
+        octx.write_trailer().unwrap();
+    }
+
+    #[test]
+    fn timeline_concat_keeps_a_later_file_at_its_start_time() {
+        let directory = tempfile::tempdir().unwrap();
+        let inputs = timeline_inputs(directory.path(), 30);
+        let start = |path: &Path| {
+            super::container_start_time(&ffmpeg::format::input(path).unwrap()) as f64
+                / f64::from(ffmpeg::ffi::AV_TIME_BASE)
+        };
+        assert!(
+            (start(&inputs[1]) - 1.0).abs() < 0.002,
+            "{}",
+            start(&inputs[1])
+        );
+
+        let joined = directory.path().join("timeline.mp4");
+        concatenate_video_fragments_on_timeline(&inputs, &joined).unwrap();
+        let second_at = first_video_pts_after(&joined, 12);
+        assert!(
+            (second_at - 1.0).abs() < 0.002,
+            "second file starts at {second_at}"
+        );
+        assert_eq!(video_packet_contents(&joined).1.len(), 24);
+        assert!(!joined.with_extension("concat.txt").exists());
+
+        let end_to_end = directory.path().join("end-to-end.mp4");
+        concatenate_video_fragments(&inputs, &end_to_end).unwrap();
+        assert!(first_video_pts_after(&end_to_end, 12) < 0.5);
+    }
+
+    #[test]
+    fn timeline_concat_plays_overlapping_files_end_to_end() {
+        let directory = tempfile::tempdir().unwrap();
+        let inputs = timeline_inputs(directory.path(), 6);
+
+        let joined = directory.path().join("timeline.mp4");
+        concatenate_video_fragments_on_timeline(&inputs, &joined).unwrap();
+        let packets = video_packet_contents(&joined).1;
+        assert_eq!(packets.len(), 24);
+        let dts: Vec<i64> = packets.iter().filter_map(|packet| packet.dts).collect();
+        assert!(dts.windows(2).all(|pair| pair[0] < pair[1]), "{dts:?}");
+        let second_at = first_video_pts_after(&joined, 12);
+        assert!(
+            second_at >= 0.39,
+            "overlap played over the first file: {second_at}"
+        );
+    }
+
     #[test]
     fn concat_audio_and_video_with_quoted_parent_directories() {
         assert_concat_roundtrip("recording 'quoted' λ");
 
@@ -365,7 +365,9 @@ impl MuxerSubprocess {
             data,
         };
 
-        match write_packet_frame(stdin, &packet) {
+        // Flush every packet: anything left in the buffer is lost if the
+        // recorder dies, and a dead child only shows up as a write error.
+        match write_packet_frame(stdin, &packet).and_then(|()| Ok(stdin.flush()?)) {
             Ok(()) => {
                 self.packets_written.fetch_add(1, Ordering::Relaxed);
                 Ok(())
@@ -871,6 +873,61 @@ mod tests {
         (directory, subprocess)
     }
 
+    fn script_subprocess(script: &str) -> (tempfile::TempDir, MuxerSubprocess, PathBuf) {
+        let directory = tempfile::tempdir().unwrap();
+        let binary = directory.path().join("muxer");
+        std::fs::write(&binary, format!("#!/bin/sh\n{script}\n")).unwrap();
+        std::fs::set_permissions(&binary, std::fs::Permissions::from_mode(0o700)).unwrap();
+        let config = MuxerSubprocessConfig {
+            output_directory: directory.path().join("video"),
+            init_segment_name: "init.mp4".into(),
+            media_segment_pattern: "segment_$Number%03d$.m4s".into(),
+            video_init: None,
+            audio_init: None,
+        };
+        let received = directory.path().join("received");
+        let subprocess = MuxerSubprocess::spawn(binary, config, None).unwrap();
+        (directory, subprocess, received)
+    }
+
+    #[test]
+    fn every_packet_reaches_the_child_before_the_next_one_is_written() {
+        let (directory, mut subprocess, received) =
+            script_subprocess("exec cat > \"$(dirname \"$0\")/received\"");
+        let size = |path: &Path| std::fs::metadata(path).map(|m| m.len()).unwrap_or(0);
+        let deadline = Instant::now() + Duration::from_secs(5);
+        while size(&received) == 0 && Instant::now() < deadline {
+            std::thread::sleep(Duration::from_millis(5));
+        }
+        let after_start = size(&received);
+        assert!(after_start > 0, "init and start frames never arrived");
+        subprocess
+            .write_video_packet(0, 0, 1, true, &[7; 1000])
+            .unwrap();
+        let deadline = Instant::now() + Duration::from_secs(5);
+        while size(&received) < after_start + 1000 && Instant::now() < deadline {
+            std::thread::sleep(Duration::from_millis(5));
+        }
+        assert!(
+            size(&received) >= after_start + 1000,
+            "a written packet stayed in the recorder's buffer"
+        );
+        drop(subprocess);
+        drop(directory);
+    }
+
+    #[test]
+    fn a_dead_child_is_reported_on_the_next_packet() {
+        let (_directory, mut subprocess, _) = script_subprocess("sleep 0.3; exit 3");
+        let child = subprocess.child.as_mut().unwrap();
+        let status = child.wait().unwrap();
+        assert_eq!(status.code(), Some(3));
+        assert!(matches!(
+            subprocess.write_video_packet(0, 0, 1, true, &[7; 1000]),
+            Err(MuxerSubprocessError::Crashed(_))
+        ));
+    }
+
     #[test]
     fn disk_full_stays_terminal_and_finish_confirms_shutdown() {
         let (_directory, mut subprocess) = exited_subprocess(EXIT_DISK_FULL, 2);
 
@@ -8,9 +8,10 @@ use cap_enc_ffmpeg::RelocatableSource;
 use cap_enc_ffmpeg::fragmented_mp4::{read_fragment_prefix, tail_is_complete};
 use cap_enc_ffmpeg::remux::{
     concatenate_audio_to_ogg, concatenate_m4s_segments_with_init,
-    concatenate_m4s_segments_with_init_validated, concatenate_video_fragments, get_media_duration,
-    get_video_base_fps, merge_video_audio, probe_media_valid, probe_video_can_decode,
-    probe_video_seek_points, remux_file,
+    concatenate_m4s_segments_with_init_validated, concatenate_video_fragments,
+    concatenate_video_fragments_on_timeline, get_media_duration, get_video_base_fps,
+    merge_video_audio, probe_media_valid, probe_video_can_decode, probe_video_seek_points,
+    remux_file,
 };
 use cap_project::{
     AudioMeta, Cursors, MultipleSegment, MultipleSegments, ProjectConfiguration, RecordingMeta,
@@ -2336,7 +2337,8 @@ impl RecoveryManager {
             std::fs::rename(&group_outputs[0], output)
                 .map_err(|e| RecoveryError::VideoConcat(cap_enc_ffmpeg::remux::RemuxError::Io(e)))
         } else {
-            concatenate_video_fragments(&group_outputs, output).map_err(RecoveryError::VideoConcat)
+            concatenate_video_fragments_on_timeline(&group_outputs, output)
+                .map_err(RecoveryError::VideoConcat)
         };
 
         for tmp in &temp_paths {
 
@@ -217,6 +217,43 @@ fn subprocess_exits_after_finish_or_abort_without_waiting_for_stdin_eof() {
     }
 }
 
+#[cfg(unix)]
+#[test]
+fn finish_waits_for_a_muxer_that_takes_more_than_five_seconds_to_exit() {
+    use cap_muxer_protocol::{Frame, read_frame};
+    use std::io::Cursor;
+    use std::os::unix::fs::PermissionsExt;
+    use std::time::{Duration, Instant};
+
+    let directory = TempDir::new().unwrap();
+    let binary = directory.path().join("slow-exit-muxer");
+    std::fs::write(&binary, "#!/bin/sh\ncat > \"$0.received\"\nsleep 6\n").unwrap();
+    std::fs::set_permissions(&binary, std::fs::Permissions::from_mode(0o700)).unwrap();
+    let config = minimal_video_config(&directory.path().join("video"), Vec::new());
+    let mut subprocess = MuxerSubprocess::spawn(binary, config, None).unwrap();
+    let payload = vec![42; 128 * 1024];
+    subprocess
+        .write_video_packet(0, 0, 1, true, &payload)
+        .unwrap();
+    let started = Instant::now();
+    let report = subprocess.finish().unwrap();
+    assert_eq!(report.exit_code, Some(0));
+    assert!(started.elapsed() >= Duration::from_secs(5));
+
+    let received = std::fs::read(directory.path().join("slow-exit-muxer.received")).unwrap();
+    let mut reader = Cursor::new(received.as_slice());
+    assert!(matches!(
+        read_frame(&mut reader).unwrap(),
+        Frame::InitVideo(_)
+    ));
+    assert!(matches!(read_frame(&mut reader).unwrap(), Frame::Start(_)));
+    let Frame::Packet(packet) = read_frame(&mut reader).unwrap() else {
+        panic!("expected the video packet");
+    };
+    assert_eq!(packet.data, payload);
+    assert!(matches!(read_frame(&mut reader).unwrap(), Frame::Finish));
+}
+
 #[cfg(unix)]
 #[test]
 fn finish_preserves_packets_when_a_slow_muxer_takes_more_than_five_seconds() {
@@ -232,10 +269,10 @@ fn finish_preserves_packets_when_a_slow_muxer_takes_more_than_five_seconds() {
     let config = minimal_video_config(&directory.path().join("video"), Vec::new());
     let mut subprocess = MuxerSubprocess::spawn(binary, config, None).unwrap();
     let payload = vec![42; 128 * 1024];
+    let started = Instant::now();
     subprocess
         .write_video_packet(0, 0, 1, true, &payload)
         .unwrap();
-    let started = Instant::now();
     let report = subprocess.finish().unwrap();
     assert_eq!(report.exit_code, Some(0));
     assert!(started.elapsed() >= Duration::from_secs(5));