mux: make B1 concealment decode-clean on every gap shape

Closes the three residual holes where a concealed/lost gap could still let
a dangling-reference frame reach the muxer (degraded/undecryptable-disc
path only; clean rips are byte-identical and untouched). Root cause: the
discontinuity signal was reconstructed from the 4-bit continuity counter
and applied per-PES, both of which are lossy.

Three coordinated changes:

1. CC-INDEPENDENT marker. fill_null_ts_unit now tags its NULL packets with
   an adaptation-field discontinuity_indicator; the demuxer recognises a
   0x1FFF packet carrying it as a concealed gap and forces a discontinuity
   on every tracked PID (the lost unit's PID is unknowable). This survives
   a loss that is an exact multiple of 16 packets (CC aliases to in-sequence
   — hole 3) and a loss at a PID's very start (no prior CC — hole 4); it
   also drops any open, potentially-truncated partial PES.

2. PUSI ATTRIBUTION. A gap landing on a PES boundary now flags the PES
   STARTING after it, not the one flushed at the boundary (hole 1) —
   stamping the pre-gap frame could arm-then-disarm the gate on a keyframe
   and admit the real post-gap inter frame.

3. PER-FRAME signal. codec::Frame gains `discontinuity`; each parser
   propagates it onto the first post-gap frame. MPEG-2 buffers whole GOPs
   asynchronously, so it associates the gap by ES OFFSET (like PTS/source),
   landing it on the exact post-gap picture mid-GOP (hole 2) — a per-PES
   flag stamped the previous picture. consume_ts (and the EOF flush drain)
   gate on frame.discontinuity.

Tests: CC-independent marker with in-sequence CC + leading-loss; PUSI
attribution flags the post-gap PES; MPEG-2 offset-mark stamps the post-gap
picture through GOP reorder, not the previous one. Existing B1 gate + EOF
tests still green (2270 lib tests).

src/mux/codec/ac3.rs

@@ -115,6 +115,7 @@ impl CodecParser for Ac3Parser {
 
             let duration_ns = frame_duration_ns(remaining, bsid);
             frames.push(Frame {
+                discontinuity: false,
                 coding: None,
                 source: None,
                 pts_ns: frame_pts_ns,
@@ -201,6 +202,7 @@ impl CodecParser for Ac3Parser {
         }
         let duration_ns = frame_duration_ns(frame, bsid);
         vec![Frame {
+            discontinuity: false,
             coding: None,
             source: None,
             pts_ns: self.flush_pts_ns,

src/mux/codec/dts.rs

@@ -243,6 +243,7 @@ impl CodecParser for DtsParser {
             // extensions or the next core.
             let au_pts = self.front_pts();
             frames.push(Frame {
+                discontinuity: false,
                 coding: None,
                 source: None,
                 pts_ns: au_pts,
@@ -299,6 +300,7 @@ impl CodecParser for DtsParser {
         let au = std::mem::take(&mut self.buf);
         self.pts_marks.clear();
         vec![Frame {
+            discontinuity: false,
             coding: None,
             source: None,
             pts_ns,

src/mux/codec/dvdsub.rs

@@ -43,6 +43,7 @@ impl DvdSubParser {
         if force || buf.len() >= *size {
             let (pts_ns, _, data) = self.pending.take().unwrap();
             return Some(Frame {
+                discontinuity: false,
                 coding: None,
                 source: None,
                 pts_ns,
@@ -103,6 +104,7 @@ impl CodecParser for DvdSubParser {
             let d = ((pes.data[0] as usize) << 8) | pes.data[1] as usize;
             if d < 2 {
                 out.push(Frame {
+                    discontinuity: false,
                     coding: None,
                     source: None,
                     pts_ns,
@@ -116,6 +118,7 @@ impl CodecParser for DvdSubParser {
         } else {
             // Too short to carry SPU_size — pass through as a lone frame.
             out.push(Frame {
+                discontinuity: false,
                 coding: None,
                 source: None,
                 pts_ns,

src/mux/codec/h264.rs

@@ -243,6 +243,9 @@ impl CodecParser for H264Parser {
             source: pes.source,
             pts_ns,
             keyframe,
+            // One access unit per PES (BD-TS aligns AUs to PES), so the gap
+            // signal maps straight onto this frame.
+            discontinuity: pes.discontinuity,
             data: frame_data,
             duration_ns: None,
         }]

src/mux/codec/hevc.rs

@@ -661,6 +661,9 @@ impl CodecParser for HevcParser {
             source: pes.source,
             pts_ns,
             keyframe,
+            // One access unit per PES (BD-TS aligns AUs to PES), so the gap
+            // signal maps straight onto this frame.
+            discontinuity: pes.discontinuity,
             data: frame_data,
             duration_ns: None,
         }]

src/mux/codec/lpcm.rs

@@ -72,6 +72,7 @@ impl CodecParser for LpcmParser {
         }
         let pts_ns = pes.pts.map(pts_to_ns).unwrap_or(0);
         vec![Frame {
+            discontinuity: false,
             coding: None,
             source: None,
             pts_ns,

src/mux/codec/mod.rs

@@ -44,6 +44,16 @@ pub struct Frame {
     pub pts_ns: i64,
     /// Whether this is a keyframe (used for cue points).
     pub keyframe: bool,
+    /// This frame is the FIRST coded picture after a concealed/lost gap (P3/B1):
+    /// its data begins after packets the demuxer never received (an undecryptable
+    /// unit concealed as NULL-TS upstream, or a continuity break in a damaged
+    /// source). Inter-coded video frames carrying this flag reference data that is
+    /// gone, so the consumer's `ResyncGate` arms here and drops forward to the next
+    /// keyframe. Carried per-FRAME (not per-PES) because buffering parsers — MPEG-2
+    /// emits whole GOPs, H.264/HEVC lag one access unit — decouple the frame from
+    /// the PES that carried the gap signal. Default `false`; only ever set on the
+    /// degraded/conceal path, so a clean rip leaves every frame `false`.
+    pub discontinuity: bool,
     /// Frame data (elementary stream bytes).
     pub data: Vec<u8>,
     /// Optional duration in nanoseconds — only set by parsers that
@@ -123,11 +133,15 @@ impl PassthroughParser {
 impl CodecParser for PassthroughParser {
     fn parse(&mut self, pes: &PesPacket) -> Vec<Frame> {
         let pts_ns = pes.pts.or(pes.dts).map(pts_to_ns).unwrap_or(0);
+        // Passthrough emits exactly one frame per PES with no cross-PES buffering,
+        // so the PES's discontinuity maps directly onto this frame. (Buffering
+        // parsers must instead defer the flag to the next emitted frame.)
         vec![Frame {
             coding: None,
             source: None,
             pts_ns,
             keyframe: self.keyframe,
+            discontinuity: pes.discontinuity,
             data: pes.data.clone(),
             duration_ns: None,
         }]

src/mux/codec/mpeg2.rs

@@ -129,6 +129,15 @@ pub struct Mpeg2Parser {
     /// each GOP's first PES PTS so video stays in sync with the PES-timestamped
     /// audio. None until the first PES timestamp is seen.
     origin_pts_ns: Option<i64>,
+    /// B1: absolute ES offsets at which a concealed/lost-gap PES began, parallel
+    /// to `pts_marks`/`source_marks` and drained by the SAME mark-drain invariant.
+    /// MPEG-2 emits whole GOPs asynchronously, so a per-PES flag can't ride
+    /// through to the right frame (the PES that carries the gap completes the
+    /// PREVIOUS picture); associating by OFFSET instead stamps `discontinuity` on
+    /// the access unit whose own bytes begin after the gap — the first post-gap
+    /// picture — surviving GOP buffering + temporal reorder. The consumer's
+    /// ResyncGate then arms at that exact picture, mid-GOP if need be.
+    disc_marks: VecDeque<u64>,
 }
 
 /// One coded picture buffered awaiting its GOP's completion (see `gop_buf`).
@@ -165,6 +174,7 @@ impl Mpeg2Parser {
             gop_buf: Vec::new(),
             emitted_fields: 0,
             origin_pts_ns: None,
+            disc_marks: VecDeque::new(),
         }
     }
 
@@ -223,6 +233,13 @@ impl Mpeg2Parser {
                             break;
                         }
                     }
+                    while let Some(&off) = self.disc_marks.front() {
+                        if off < cutoff {
+                            self.disc_marks.pop_front();
+                        } else {
+                            break;
+                        }
+                    }
                 }
                 break;
             };
@@ -316,6 +333,11 @@ impl Mpeg2Parser {
             if gop_boundary && !self.gop_buf.is_empty() {
                 self.flush_gop(&mut out);
             }
+            // A concealed-gap mark inside this AU's range [start, end_abs) means
+            // this picture's own bytes begin after the gap — the first post-gap
+            // AU. Same front-mark invariant as PTS/source. Carries through GOP
+            // buffering/reorder to the ResyncGate (which arms at this picture).
+            let discontinuity = self.disc_marks.front().is_some_and(|&off| off < end_abs);
             self.gop_buf.push(BufferedPicture {
                 tr,
                 info,
@@ -323,6 +345,7 @@ impl Mpeg2Parser {
                 frame: Frame {
                     pts_ns: 0,
                     keyframe,
+                    discontinuity,
                     data,
                     duration_ns: None,
                     coding: Some(info),
@@ -352,6 +375,13 @@ impl Mpeg2Parser {
                     break;
                 }
             }
+            while let Some(&off) = self.disc_marks.front() {
+                if off < end_abs {
+                    self.disc_marks.pop_front();
+                } else {
+                    break;
+                }
+            }
         }
         // EOF: emit the final (possibly incomplete) GOP so nothing is dropped.
         if force {
@@ -429,6 +459,12 @@ impl CodecParser for Mpeg2Parser {
         if let Some(src) = pes.source {
             self.source_marks.push_back((off, src));
         }
+        // A concealed/lost gap on this PES marks the access unit its bytes begin —
+        // associated by offset (like PTS/source) so it lands on the first post-gap
+        // picture, not the previous one that completes when this PES arrives.
+        if pes.discontinuity {
+            self.disc_marks.push_back(off);
+        }
         self.buf.extend_from_slice(&pes.data);
         self.drain_complete_aus(false)
     }
@@ -997,6 +1033,48 @@ mod tests {
         assert!(!frames[1].keyframe);
     }
 
+    /// B1 hole-2 regression: MPEG-2 buffers a GOP and emits asynchronously, so a
+    /// concealed gap must be associated by OFFSET (like PTS), landing on the
+    /// picture whose own bytes begin after the gap — NOT the previous picture
+    /// that completes when the discontinuity PES arrives. pic1 (I) is pre-gap;
+    /// pic2 (P), carried by a `discontinuity` PES, is the first post-gap AU.
+    #[test]
+    fn discontinuity_offset_mark_stamps_post_gap_picture_not_previous() {
+        let mut parser = Mpeg2Parser::new();
+        let mut pic1 = make_picture_header(PICTURE_TYPE_I);
+        pic1.extend_from_slice(&[0x11; 100]);
+        let mut pic2 = make_picture_header(2); // P
+        pic2.extend_from_slice(&[0x22; 100]);
+
+        // pic1 on a clean PES; nothing emits (same GOP, buffered).
+        assert!(parser.parse(&make_pes(pic1.clone(), Some(0))).is_empty());
+        // pic2 on a PES flagged discontinuity (a concealed gap preceded it).
+        // parse() of this PES completes pic1's AU (the PREVIOUS picture) — which
+        // must stay clean — while pic2 keeps buffering.
+        let pes2 = PesPacket {
+            source: None,
+            pid: 0x1011,
+            pts: Some(90000),
+            dts: None,
+            data: pic2.clone(),
+            discontinuity: true,
+        };
+        assert!(parser.parse(&pes2).is_empty(), "same GOP — still buffered");
+
+        let frames = parser.flush();
+        assert_eq!(frames.len(), 2);
+        assert_eq!(frames[0].data, pic1);
+        assert!(
+            !frames[0].discontinuity,
+            "the previous (pre-gap) I picture must NOT be flagged"
+        );
+        assert_eq!(frames[1].data, pic2);
+        assert!(
+            frames[1].discontinuity,
+            "the post-gap P picture (the discontinuity PES's own AU) IS flagged"
+        );
+    }
+
     #[test]
     fn picture_coding_extension_stays_with_its_picture() {
         // Regression for `ignoring pic cod ext after 0`: the picture coding

src/mux/codec/pgs.rs

@@ -57,6 +57,7 @@ impl PgsParser {
         let (start_pts, data) = self.pending.take()?;
         let duration = end_pts_ns.saturating_sub(start_pts).max(0) as u64;
         Some(Frame {
+            discontinuity: false,
             coding: None,
             source: None,
             pts_ns: start_pts,
@@ -92,6 +93,7 @@ impl CodecParser for PgsParser {
                 .take()
                 .map(|(start_pts, data)| {
                     vec![Frame {
+                        discontinuity: false,
                         coding: None,
                         source: None,
                         pts_ns: start_pts,
@@ -119,6 +121,7 @@ impl CodecParser for PgsParser {
                 let frame = match pts {
                     Some(end) => self.emit_pending(end),
                     None => self.pending.take().map(|(start_pts, data)| Frame {
+                        discontinuity: false,
                         coding: None,
                         source: None,
                         pts_ns: start_pts,
@@ -142,6 +145,7 @@ impl CodecParser for PgsParser {
                 // Flush any prior pending undurated and skip storing this one.
                 None => {
                     out.extend(self.pending.take().map(|(start_pts, data)| Frame {
+                        discontinuity: false,
                         coding: None,
                         source: None,
                         pts_ns: start_pts,
@@ -167,6 +171,7 @@ impl CodecParser for PgsParser {
                     // (A missing PTS falls through to the drop path below: a
                     // bitmap with no timing reference would land at 00:00:00.)
                     out.push(Frame {
+                        discontinuity: false,
                         coding: None,
                         source: None,
                         pts_ns: pts.unwrap_or(0),
@@ -195,6 +200,7 @@ impl CodecParser for PgsParser {
         // the final on-screen subtitle (see the module doc).
         match self.pending.take() {
             Some((start_pts, data)) => vec![Frame {
+                discontinuity: false,
                 coding: None,
                 source: None,
                 pts_ns: start_pts,

src/mux/codec/truehd.rs

@@ -255,6 +255,7 @@ impl CodecParser for TrueHdParser {
             }
 
             frames.push(Frame {
+                discontinuity: false,
                 coding: None,
                 source: None,
                 pts_ns: self.next_pts_ns,

src/mux/codec/vc1.rs

@@ -328,6 +328,9 @@ impl CodecParser for Vc1Parser {
             source: pes.source,
             pts_ns: ts_ns,
             keyframe,
+            // One frame per PES (BD-TS aligns frames to PES), so the gap signal
+            // maps straight onto this frame.
+            discontinuity: pes.discontinuity,
             data: frame_data,
             duration_ns: None,
         }]

src/mux/pipelined_stream.rs

@@ -182,17 +182,21 @@ impl PipelinedPesStream {
                     self.parsers.iter_mut().find(|(pid, _)| *pid == pes.pid)
                 {
                     let is_video = self.is_video.get(track).copied().unwrap_or(false);
-                    let discontinuity = pes.discontinuity;
                     for frame in parser.parse(&pes) {
-                        // B1: after a TS gap on a video track, drop forward to the
-                        // next keyframe so no frame with a dangling reference is
-                        // emitted. Audio/subtitle always admit (independent frames).
-                        // A track with no gate (out-of-range index) emits as-is.
+                        // B1: after a concealed/lost gap, drop forward to the next
+                        // keyframe on a video track so no frame with a dangling
+                        // reference is emitted. The signal is read PER-FRAME
+                        // (`frame.discontinuity`), not per-PES: buffering parsers
+                        // (MPEG-2 GOPs, H.264/HEVC AU lag) stamp the exact post-gap
+                        // picture, so only it arms the gate — not a whole PES of
+                        // frames. Audio/subtitle always admit (independent frames);
+                        // a track with no gate (out-of-range index) emits as-is.
                         let emit = match self.resync.get_mut(track) {
                             Some(gate) => {
                                 let was_armed = gate.is_armed();
                                 let dropped = gate.dropped_in_run();
-                                let admit = gate.admit(is_video, discontinuity, frame.keyframe);
+                                let admit =
+                                    gate.admit(is_video, frame.discontinuity, frame.keyframe);
                                 if admit && was_armed {
                                     tracing::warn!(
                                         target: "mux",
@@ -299,15 +303,47 @@ impl Stream for PipelinedPesStream {
                         );
                     }
                     // Drain any access unit a parser buffered past the last
-                    // PES (e.g. DTS-HD's final core+extension unit).
+                    // PES (e.g. DTS-HD's final core+extension unit, or MPEG-2's
+                    // final GOP). These flush frames carry their own per-frame
+                    // `discontinuity` (a post-gap picture buffered at EOF was
+                    // stamped by the parser), so route them through the SAME B1
+                    // gate the in-stream path uses — otherwise a trailing
+                    // dangling-reference frame (MPEG-2 final-GOP corner) would
+                    // bypass the resync. Disjoint field borrows so the gate +
+                    // is_video reads coexist with the mutable parser drain.
                     let pid_to_track = &self.pid_to_track;
                     let pending = &mut self.pending_frames;
+                    let resync = &mut self.resync;
+                    let is_video = &self.is_video;
                     for (pid, parser) in self.parsers.iter_mut() {
                         let Some(&(_, track)) = pid_to_track.iter().find(|(p, _)| p == pid) else {
                             continue;
                         };
                         for frame in parser.flush() {
-                            pending.push_back(PesFrame::from_codec_frame(track, frame));
+                            let emit = match resync.get_mut(track) {
+                                Some(gate) => gate.admit(
+                                    is_video.get(track).copied().unwrap_or(false),
+                                    frame.discontinuity,
+                                    frame.keyframe,
+                                ),
+                                None => true,
+                            };
+                            if emit {
+                                pending.push_back(PesFrame::from_codec_frame(track, frame));
+                            }
+                        }
+                    }
+                    // A gate still armed at EOF dropped post-gap frames that never
+                    // reached a keyframe (e.g. a concealed gap in the final GOP).
+                    // Surface it once so the loss is visible, not silent.
+                    for (track, gate) in self.resync.iter().enumerate() {
+                        if gate.is_armed() {
+                            tracing::warn!(
+                                target: "mux",
+                                track,
+                                dropped = gate.dropped_in_run(),
+                                "B1: stream ended while dropping to a keyframe after a concealed gap (no trailing keyframe)"
+                            );
                         }
                     }
                     return Ok(self.pending_frames.pop_front());
@@ -427,6 +463,7 @@ mod tests {
                     source: None,
                     pts_ns: pes.pts.unwrap_or(0) + i as i64,
                     keyframe: i == 0,
+                    discontinuity: false,
                     data: pes.data.clone(),
                     duration_ns: None,
                 })
@@ -439,6 +476,7 @@ mod tests {
                     source: None,
                     pts_ns: 0,
                     keyframe: false,
+                    discontinuity: false,
                     data: vec![0xEE],
                     duration_ns: None,
                 })
@@ -555,6 +593,8 @@ mod tests {
                 source: None,
                 pts_ns: pes.pts.unwrap_or(0),
                 keyframe: pes.data.first() == Some(&b'K'),
+                // Propagate so the B1 gate can be driven end-to-end in tests.
+                discontinuity: pes.discontinuity,
                 data: pes.data.clone(),
                 duration_ns: None,
             }]

src/mux/ts.rs

@@ -12,6 +12,10 @@ use crate::consts::TS_PACKET_BYTES;
 
 /// TS sync byte.
 const SYNC_BYTE: u8 = 0x47;
+/// MPEG-TS null-packet PID (0x1FFF). Carries no elementary stream; the P3
+/// concealment fill emits null packets on this PID, tagged with an
+/// adaptation-field discontinuity_indicator to signal a concealed gap.
+const NULL_PID: u16 = 0x1FFF;
 
 /// A reassembled PES packet with timestamp info.
 #[derive(Debug)]
@@ -28,14 +32,17 @@ pub struct PesPacket {
     /// from the producer's known stream offset. `None` when the demuxer was fed
     /// without a base offset (callers that don't need provenance).
     pub source: Option<crate::pes::SourcePos>,
-    /// True when a TS continuity gap (a CC discontinuity, or an adaptation-field
-    /// discontinuity_indicator) was seen on this PID since the previous PES
-    /// completed — i.e. one or more packets for this stream were lost (e.g. the
-    /// mux replaced an undecryptable unit with NULL TS packets, P3/A2). This is
-    /// the FIRST surviving PES after the gap, so for inter-coded video it (and
-    /// every later frame up to the next IRAP/IDR) may reference data that is now
-    /// gone. The codec-parse consumer uses it to drop forward to the next
-    /// keyframe (B1) instead of emitting frames with dangling references.
+    /// True when one or more packets for this stream were lost before this PES —
+    /// a continuity break (CC gap or adaptation-field discontinuity_indicator) on
+    /// a tracked PID, or the CC-independent concealment marker the mux emits when
+    /// it replaces an undecryptable unit with NULL-TS packets (P3/A2). This PES is
+    /// the FIRST whose data is entirely after the gap: a mid-frame loss drops the
+    /// truncated partial and flags the next complete PES; a loss landing on a PES
+    /// boundary flags the PES STARTING after it (never the one just flushed). So
+    /// for inter-coded video this PES — and every later frame up to the next
+    /// IRAP/IDR — may reference data that is now gone. The codec-parse consumer
+    /// (via the per-frame `Frame::discontinuity` its parser propagates) drops
+    /// forward to the next keyframe (B1) instead of emitting dangling references.
     pub discontinuity: bool,
 }
 
@@ -67,10 +74,11 @@ struct PesAssembler {
     /// Stamped at PES start, emitted on the completed packet — provenance is
     /// carried, never reconstructed downstream.
     pes_source: Option<crate::pes::SourcePos>,
-    /// Sticky "a continuity gap occurred on this PID" flag. Set whenever a CC
-    /// gap or an explicit discontinuity_indicator is seen; carried onto the NEXT
-    /// completed PES (which is the first surviving frame after the loss) and then
-    /// cleared. Drives B1 drop-to-keyframe in the codec consumer.
+    /// Sticky "a gap occurred on this PID" flag. Set by a CC gap, an explicit
+    /// discontinuity_indicator, or the concealment marker; consumed by the NEXT
+    /// PES this assembler completes — the first whose data is entirely post-gap —
+    /// then cleared. A gap detected on a PUSI sets it AFTER `start()` so it rides
+    /// the new PES, not the one just flushed. Drives B1 drop-to-keyframe.
     pending_discontinuity: bool,
 }
 
@@ -356,6 +364,39 @@ impl TsDemuxer {
         let pusi = ts[1] & 0x40 != 0; // Payload Unit Start Indicator
         let adaptation = (ts[3] >> 4) & 0x03;
 
+        // P3/B1 CONCEALMENT MARKER. The decrypt layer fills an undecryptable
+        // aligned unit with NULL-TS packets (PID 0x1FFF) that carry an
+        // adaptation-field discontinuity_indicator (see `aacs::fill_null_ts_unit`).
+        // This is the authoritative loss signal — unlike a tracked PID's 4-bit
+        // continuity_counter it is CC-INDEPENDENT, so it survives a loss that is
+        // an exact multiple of 16 packets and a loss at the very start of a PID
+        // (no prior CC to diff against). The decrypt layer cannot know which
+        // elementary PID(s) the lost unit carried (the data was undecryptable),
+        // so force a pending discontinuity on EVERY tracked assembler: the next
+        // completed PES of each resyncs. Harmless for audio/subtitle (the codec
+        // gate is a no-op there) and at most one extra GOP on a video track that
+        // did not actually lose packets — bounded, and only on a degraded disc.
+        if pid == NULL_PID
+            && (adaptation == 0x02 || adaptation == 0x03)
+            && (ts[4] as usize) > 0
+            && (ts[5] & 0x80) != 0
+        {
+            for a in &mut self.assemblers {
+                // A concealed unit may have dropped packets belonging to a PES
+                // currently open on any PID — so that partial is potentially
+                // TRUNCATED (a hole in the middle of its access unit). Drop it
+                // like a mid-PES continuity break, and flag pending so the NEXT
+                // completed PES (the first frame whose data is entirely post-gap)
+                // resyncs. Mirrors the non-PUSI cc_gap path, applied to every PID
+                // because the lost unit's PID(s) are unknowable (undecryptable).
+                a.buffer.clear();
+                a.active = false;
+                a.header_remaining = 0;
+                a.pending_discontinuity = true;
+            }
+            return;
+        }
+
         let idx = if (pid as usize) < self.pid_index.len() {
             self.pid_index[pid as usize]
         } else {
@@ -401,8 +442,9 @@ impl TsDemuxer {
         // — splicing the new payload would corrupt the elementary stream — so
         // drop the partial and resync on the next PUSI.
         let cc = ts[3] & 0x0f;
-        let discontinuity_flag =
-            (adaptation == 0x03 || adaptation == 0x02) && ts[4] > 0 && (ts[5] & 0x80) != 0;
+        // adaptation == 0x02 (AF only) already returned above, so only 0x03
+        // (AF + payload) can carry an adaptation field here.
+        let discontinuity_flag = adaptation == 0x03 && ts[4] > 0 && (ts[5] & 0x80) != 0;
         // A gap is a CC that is neither the expected `(prev + 1) & 0xf` nor a
         // duplicate `prev` (ISO 13818-1 permits a packet to repeat its CC; a
         // duplicate is not a loss). Anything else means one or more packets for
@@ -412,35 +454,30 @@ impl TsDemuxer {
             None => false,
         };
         asm.last_cc = Some(cc);
-        // Any continuity gap — at a PUSI boundary or mid-PES — means packets for
-        // this stream were lost (an upstream NULL-TS conceal, a damaged source).
-        // Mark it sticky so the NEXT completed PES carries `discontinuity` and the
-        // codec consumer can drop forward to the next keyframe (B1). The partial
-        // PES is still dropped below only for a NON-PUSI continuation (a hole in
-        // the middle of the current frame); a gap landing exactly on a PUSI starts
-        // a clean new frame, but it is still the first frame after the loss.
-        if discontinuity_flag || cc_gap {
-            asm.pending_discontinuity = true;
-        }
-        if !pusi && (discontinuity_flag || cc_gap) && asm.active {
-            tracing::trace!(
-                target: "mux",
-                pid = asm.pid,
-                "TS continuity break on non-PUSI continuation; dropping partial PES",
-            );
-            asm.buffer.clear();
-            asm.active = false;
-            asm.header_remaining = 0;
-            return;
-        }
+        // A continuity gap means packets for THIS PID were lost (a damaged source,
+        // or — for the conceal path — a loss that the CC-independent NULL-TS marker
+        // above did not already flag). The flag is sticky and rides to the FIRST
+        // post-gap PES so the codec consumer drops forward to the next keyframe
+        // (B1). Attribution differs by where the gap lands (see below).
+        let gap = discontinuity_flag || cc_gap;
 
         if pusi {
             // `header_len` is the FULL (uncapped) PES-header length:
             // 0 = malformed (payload is not a PES start), else 6/9+N.
             let (pts, dts, header_len) = parse_pes_header(payload);
+            // Flush the previous PES FIRST — a gap detected on this PUSI packet
+            // belongs to the PES STARTING now (its data begins after the lost
+            // packets), NOT the one just completing. So set `pending_discontinuity`
+            // AFTER start(): it rides the new PES to its own completion. (Setting
+            // it before would stamp the pre-gap frame; if that frame were a
+            // keyframe the gate would arm-then-disarm on it and admit the real
+            // post-gap inter frame with a dangling reference.)
             if let Some(prev) = asm.start(pts, dts, source) {
                 completed.push(prev);
             }
+            if gap {
+                asm.pending_discontinuity = true;
+            }
             if header_len == 0 {
                 // PUSI packet whose payload is not a valid PES start. Do
                 // NOT push it — those bytes are not elementary-stream data
@@ -457,16 +494,37 @@ impl TsDemuxer {
                 // the following continuation packet(s).
                 asm.header_remaining = header_len - payload.len();
             }
-        } else if asm.header_remaining > 0 {
-            // Continuation packet still inside a PES header that spanned
-            // the boundary — consume header bytes before any ES data.
-            let skip = asm.header_remaining.min(payload.len());
-            asm.header_remaining -= skip;
-            if skip < payload.len() {
-                asm.push(&payload[skip..]);
-            }
         } else {
-            asm.push(payload);
+            // Non-PUSI continuation.
+            if gap {
+                // Mid-PES hole: the open partial has a gap, so splicing this
+                // payload would corrupt the ES. Flag pending (consumed at the
+                // NEXT completed PES — the first post-gap frame) and drop the
+                // open partial; resync on the next PUSI.
+                asm.pending_discontinuity = true;
+                if asm.active {
+                    tracing::trace!(
+                        target: "mux",
+                        pid = asm.pid,
+                        "TS continuity break on non-PUSI continuation; dropping partial PES",
+                    );
+                    asm.buffer.clear();
+                    asm.active = false;
+                    asm.header_remaining = 0;
+                    return;
+                }
+            }
+            if asm.header_remaining > 0 {
+                // Continuation packet still inside a PES header that spanned
+                // the boundary — consume header bytes before any ES data.
+                let skip = asm.header_remaining.min(payload.len());
+                asm.header_remaining -= skip;
+                if skip < payload.len() {
+                    asm.push(&payload[skip..]);
+                }
+            } else {
+                asm.push(payload);
+            }
         }
     }
 
@@ -947,12 +1005,13 @@ mod tests {
         );
     }
 
-    /// B1 plumbing: a continuity gap must STAMP `discontinuity = true` on the
-    /// next completed PES so the codec consumer can drop forward to the next
-    /// keyframe. A clean in-sequence PES carries `discontinuity = false`. We
-    /// open A (cc=0), flush it cleanly via B's PUSI (cc=1), then jump the CC
-    /// (cc 1 -> 5) on C's PUSI: the gap is sticky and lands on the PES flushed
-    /// at that boundary (B — the frame whose tail packets were the lost ones).
+    /// B1 plumbing: a continuity gap detected on a PUSI must stamp
+    /// `discontinuity = true` on the PES STARTING after the gap, NOT the one
+    /// flushed at the boundary — the post-gap PES is the one whose data begins
+    /// after the lost packets and references them. (Attribution fix: stamping the
+    /// pre-gap PES would, if it were a keyframe, arm-then-disarm the gate and let
+    /// the real post-gap inter frame through with a dangling reference.) A clean
+    /// in-sequence PES carries `discontinuity = false`.
     #[test]
     fn continuity_gap_stamps_discontinuity_on_next_pes() {
         let pid = 0x1011;
@@ -971,23 +1030,100 @@ mod tests {
             "in-sequence PES is not a discontinuity"
         );
 
-        // C's PUSI jumps cc 1 -> 5: packets were lost. The gap is sticky and is
-        // attributed to the PES flushed here (B), which lost its tail packets.
+        // C's PUSI jumps cc 1 -> 5: packets were lost between B and C. B (flushed
+        // here) is PRE-gap and stays clean — the gap belongs to C, which starts
+        // after the lost packets.
         let out = demux.feed(&ts_payload_packet(pid, true, 5, &pes_start(b"CCCC")));
         assert_eq!(out.len(), 1, "B completes");
         assert_eq!(&out[0].data[..4], b"BBBB");
         assert!(
-            out[0].discontinuity,
-            "the PES at the continuity gap must be flagged so B1 can resync"
+            !out[0].discontinuity,
+            "the pre-gap PES flushed at the boundary must NOT be flagged"
         );
 
-        // C itself was opened clean (after the gap) and carries no new gap.
+        // C carries the discontinuity — it is the first post-gap PES.
         let out = demux.flush();
         assert_eq!(out.len(), 1);
         assert_eq!(&out[0].data[..4], b"CCCC");
         assert!(
-            !out[0].discontinuity,
-            "post-gap PES with no further gap is clean"
+            out[0].discontinuity,
+            "the post-gap PES must be flagged so B1 resyncs at/after it"
+        );
+    }
+
+    /// One 192-byte BD source packet that is a B1 concealment marker: a PID-0x1FFF
+    /// null packet carrying the adaptation-field discontinuity_indicator (the byte
+    /// shape `fill_null_ts_unit` writes for every packet of a concealed unit).
+    fn null_marker_packet() -> Vec<u8> {
+        let mut pkt = vec![0u8; BD_SOURCE_PACKET_BYTES];
+        pkt[4] = SYNC_BYTE; // 0x47
+        pkt[5] = 0x1F; // PID 0x1FFF
+        pkt[6] = 0xFF;
+        pkt[7] = 0x20; // adaptation-field only
+        pkt[8] = 0xB7; // af_len 183
+        pkt[9] = 0x80; // discontinuity_indicator
+        for b in &mut pkt[10..] {
+            *b = 0xFF;
+        }
+        pkt
+    }
+
+    /// HOLE 3 (16-multiple CC blind spot) + the truncated-partial drop. A concealed
+    /// unit can drop an exact multiple of 16 packets on a PID, leaving its 4-bit
+    /// continuity_counter looking IN-SEQUENCE — so CC-based detection is blind. The
+    /// CC-INDEPENDENT marker flags the loss anyway, drops the (potentially
+    /// truncated) open PES, and stamps the first post-gap PES.
+    #[test]
+    fn conceal_marker_forces_discontinuity_with_in_sequence_cc() {
+        let pid = 0x1011;
+        let mut demux = TsDemuxer::new(&[pid]);
+        // A (cc=0) opens; B's PUSI (cc=1) flushes A clean.
+        demux.feed(&ts_payload_packet(pid, true, 0, &pes_start(b"AAAA")));
+        let out = demux.feed(&ts_payload_packet(pid, true, 1, &pes_start(b"BBBB")));
+        assert_eq!(&out[0].data[..4], b"AAAA");
+        assert!(!out[0].discontinuity);
+
+        // Concealment marker: a unit was dropped. B is open → potentially truncated
+        // → dropped. CC is NOT consulted.
+        assert!(
+            demux.feed(&null_marker_packet()).is_empty(),
+            "marker emits nothing itself"
+        );
+
+        // C (cc=2) is EXACTLY in-sequence after B's cc=1 — as if a multiple of 16
+        // packets were lost, so `cc_gap` is false. The marker is the only signal.
+        let out = demux.feed(&ts_payload_packet(pid, true, 2, &pes_start(b"CCCC")));
+        assert!(
+            out.is_empty(),
+            "the truncated open PES (B) is dropped, not emitted"
+        );
+        let out = demux.flush();
+        assert_eq!(out.len(), 1);
+        assert_eq!(&out[0].data[..4], b"CCCC");
+        assert!(
+            out[0].discontinuity,
+            "marker flags the post-gap PES despite in-sequence CC (16-aligned blind spot)"
+        );
+    }
+
+    /// HOLE 4 (leading loss). If the disc's very first unit is undecryptable the
+    /// first surviving packet has no predecessor CC (`last_cc == None`), so CC
+    /// detection is blind. The marker still flags the first PES.
+    #[test]
+    fn conceal_marker_at_stream_start_flags_first_pes() {
+        let pid = 0x1011;
+        let mut demux = TsDemuxer::new(&[pid]);
+        // Marker FIRST — no prior CC exists for this PID.
+        assert!(demux.feed(&null_marker_packet()).is_empty());
+        // The first real PES of the PID.
+        let out = demux.feed(&ts_payload_packet(pid, true, 0, &pes_start(b"AAAA")));
+        assert!(out.is_empty());
+        let out = demux.flush();
+        assert_eq!(out.len(), 1);
+        assert_eq!(&out[0].data[..4], b"AAAA");
+        assert!(
+            out[0].discontinuity,
+            "leading concealed loss flags the first PES (last_cc == None case)"
         );
     }