diff -up chromium-108.0.5359.124/AUTHORS.first_dts chromium-108.0.5359.124/AUTHORS
--- chromium-108.0.5359.124/AUTHORS.first_dts	2022-12-31 15:30:52.923006393 +0100
+++ chromium-108.0.5359.124/AUTHORS	2022-12-31 15:32:50.431215836 +0100
@@ -97,6 +97,7 @@ Andra Paraschiv <andra.paraschiv@intel.c
 Andras Tokodi <a.tokodi@eyeo.com>
 Andreas Nazlidis <andreas221b@gmail.com>
 Andreas Papacharalampous <andreas@apap04.com>
+Andreas Schneider <asn@cryptomilk.org>
 Andrei Borza <andrei.borza@gmail.com>
 Andrei Parvu <andrei.prv@gmail.com>
 Andrei Parvu <parvu@adobe.com>
diff -up chromium-108.0.5359.124/media/filters/ffmpeg_demuxer.cc.first_dts chromium-108.0.5359.124/media/filters/ffmpeg_demuxer.cc
--- chromium-108.0.5359.124/media/filters/ffmpeg_demuxer.cc.first_dts	2022-12-14 01:39:52.000000000 +0100
+++ chromium-108.0.5359.124/media/filters/ffmpeg_demuxer.cc	2022-12-31 15:30:52.923006393 +0100
@@ -58,7 +58,7 @@ namespace media {
 
 namespace {
 
-constexpr int64_t kInvalidPTSMarker = static_cast<int64_t>(0x8000000000000000);
+constexpr int64_t kRelativeTsBase = static_cast<int64_t>(0x7ffeffffffffffff);
 
 void SetAVStreamDiscard(AVStream* stream, AVDiscard discard) {
   DCHECK(stream);
@@ -91,7 +91,7 @@ static base::TimeDelta FramesToTimeDelta
                             sample_rate);
 }
 
-static base::TimeDelta ExtractStartTime(AVStream* stream) {
+static base::TimeDelta ExtractStartTime(AVStream* stream, int64_t first_dts) {
   // The default start time is zero.
   base::TimeDelta start_time;
 
@@ -101,12 +101,12 @@ static base::TimeDelta ExtractStartTime(
 
   // Next try to use the first DTS value, for codecs where we know PTS == DTS
   // (excludes all H26x codecs). The start time must be returned in PTS.
-  if (av_stream_get_first_dts(stream) != kNoFFmpegTimestamp &&
+  if (first_dts != AV_NOPTS_VALUE &&
       stream->codecpar->codec_id != AV_CODEC_ID_HEVC &&
       stream->codecpar->codec_id != AV_CODEC_ID_H264 &&
       stream->codecpar->codec_id != AV_CODEC_ID_MPEG4) {
     const base::TimeDelta first_pts =
-        ConvertFromTimeBase(stream->time_base, av_stream_get_first_dts(stream));
+        ConvertFromTimeBase(stream->time_base, first_dts);
     if (first_pts < start_time)
       start_time = first_pts;
   }
@@ -275,6 +275,7 @@ FFmpegDemuxerStream::FFmpegDemuxerStream
       fixup_negative_timestamps_(false),
       fixup_chained_ogg_(false),
       num_discarded_packet_warnings_(0),
+      first_dts_(AV_NOPTS_VALUE),
       last_packet_pos_(AV_NOPTS_VALUE),
       last_packet_dts_(AV_NOPTS_VALUE) {
   DCHECK(demuxer_);
@@ -341,6 +342,11 @@ void FFmpegDemuxerStream::EnqueuePacket(
   int64_t packet_dts =
       packet->dts == AV_NOPTS_VALUE ? packet->pts : packet->dts;
 
+  if (first_dts_ == AV_NOPTS_VALUE && packet->dts != AV_NOPTS_VALUE &&
+      last_packet_dts_ != AV_NOPTS_VALUE) {
+    first_dts_ = packet->dts - (last_packet_dts_ + kRelativeTsBase);
+  }
+
   // Chained ogg files have non-monotonically increasing position and time stamp
   // values, which prevents us from using them to determine if a packet should
   // be dropped. Since chained ogg is only allowed on single track audio only
@@ -683,6 +689,7 @@ void FFmpegDemuxerStream::FlushBuffers(b
   ResetBitstreamConverter();
 
   if (!preserve_packet_position) {
+    first_dts_ = AV_NOPTS_VALUE;
     last_packet_pos_ = AV_NOPTS_VALUE;
     last_packet_dts_ = AV_NOPTS_VALUE;
   }
@@ -1434,7 +1441,8 @@ void FFmpegDemuxer::OnFindStreamInfoDone
 
     max_duration = std::max(max_duration, streams_[i]->duration());
 
-    base::TimeDelta start_time = ExtractStartTime(stream);
+    base::TimeDelta start_time =
+        ExtractStartTime(stream, streams_[i]->first_dts());
 
     // Note: This value is used for seeking, so we must take the true value and
     // not the one possibly clamped to zero below.
@@ -1591,7 +1599,7 @@ FFmpegDemuxerStream* FFmpegDemuxer::Find
   for (const auto& stream : streams_) {
     if (!stream || stream->IsEnabled() != enabled)
       continue;
-    if (av_stream_get_first_dts(stream->av_stream()) == kInvalidPTSMarker)
+    if (stream->first_dts() == AV_NOPTS_VALUE)
       continue;
     if (!lowest_start_time_stream ||
         stream->start_time() < lowest_start_time_stream->start_time()) {
@@ -1612,7 +1620,7 @@ FFmpegDemuxerStream* FFmpegDemuxer::Find
     if (stream->type() != DemuxerStream::VIDEO)
       continue;
 
-    if (av_stream_get_first_dts(stream->av_stream()) == kInvalidPTSMarker)
+    if (stream->first_dts() == AV_NOPTS_VALUE)
       continue;
 
     if (!stream->IsEnabled())
@@ -1926,4 +1934,4 @@ void FFmpegDemuxer::RunPendingSeekCB(Pip
   std::move(pending_seek_cb_).Run(status);
 }
 
-}  // namespace media
+}  // namespace media
\ Kein Zeilenumbruch am Dateiende.
diff -up chromium-108.0.5359.124/media/filters/ffmpeg_demuxer.h.first_dts chromium-108.0.5359.124/media/filters/ffmpeg_demuxer.h
--- chromium-108.0.5359.124/media/filters/ffmpeg_demuxer.h.first_dts	2022-12-14 01:39:52.000000000 +0100
+++ chromium-108.0.5359.124/media/filters/ffmpeg_demuxer.h	2022-12-31 15:30:52.924006403 +0100
@@ -145,6 +145,8 @@ class MEDIA_EXPORT FFmpegDemuxerStream :
   base::TimeDelta start_time() const { return start_time_; }
   void set_start_time(base::TimeDelta time) { start_time_ = time; }
 
+  int64_t first_dts() const { return first_dts_; }
+
  private:
   friend class FFmpegDemuxerTest;
 
@@ -202,6 +204,7 @@ class MEDIA_EXPORT FFmpegDemuxerStream :
   bool fixup_chained_ogg_;
 
   int num_discarded_packet_warnings_;
+  int64_t first_dts_;
   int64_t last_packet_pos_;
   int64_t last_packet_dts_;
 };