Embedded subtitles out of sync in time-cropped video

mpv Information

mpv   : git-2025-06-06-v0.40.0-dev-g6b178e4f5
FFmpeg: git 2025-06-05 18:24:07 17729aa8
System: macOS 15.5 (24F74)
Model : MacBookAir10,1

Sample Files

xy.zip

Reproduction Steps

#!/bin/bash

mpv --config=no --sub-auto=no /tmp/x.m4v # RIGHT
mpv --config=no --sub-auto=no /tmp/y.m4v # WRONG: shows subtitle for 10s at 8s
open -a 'QuickTime Player' /tmp/y.m4v # RIGHT

Expected Behavior

Subtitles in sync

Actual Behavior

Subtitles out of sync

(Blocky screenshot of subtitle might be a separate issue with Retina displays.)

Log File

Issue might be related to keyframes.

#!/usr/bin/env ruby

%w[x y].each do |f|
  base = { 'video' => 16_384, 'subtitle' => 1_000_000 }
  info = base.transform_values { [] }
  `ffprobe -v error -show_entries packet=codec_type,pts,flags -of csv /tmp/#{f}.m4v | grep K`.lines(chomp: true).each do |item|
    _, type, pts = item.split(',')
    info[type] << (pts.to_f / base[type])
  end
  pp info
end

{"video" => [0.0, 10.0], "subtitle" => [0.0, 5.0, 10.0]}
{"video" => [0.0, 7.0], "subtitle" => [-5.0, 0.0, 5.0]}

Other Information

To create variants with different timings for debugging/testing:

#!/usr/bin/env ruby

def ts(sec)
  ms = sec * 1000
  format '%02d:%02d:%02d,%03d', *([1000, 60, 60, 100].map { |f| ms.tap { ms /= f } % f }).reverse
end

(0..15).step(5).each_cons(2).with_index do |(a, b), i|
  printf "%s\n%s --> %s\n%s / %s\n\n", i + 1, ts(a), ts(b), ts(a), a
end

#!/bin/bash

for t in $(seq 0 12); do
  magick null: -background '#8c8' -extent 320x240 -pointsize 40 -gravity center -annotate +0+0 "$t" "/tmp/$t.png"
done

ffmpeg -r 1 -i /tmp/%d.png -i /tmp/sub.srt -pix_fmt yuv420p -c:s mov_text -y /tmp/x.m4v
ffmpeg -ss 3 -i /tmp/x.m4v -c:s mov_text -y /tmp/y.m4v

I carefully read all instruction and confirm that I did the following:

• I tested with the latest mpv version to validate that the issue is not already fixed.
• I provided all required information including system and mpv version.
• I produced the log file with the exact same set of files, parameters, and conditions used in "Reproduction Steps", with the addition of --log-file=output.txt.
• I produced the log file while the behaviors described in "Actual Behavior" were actively observed.
• I attached the full, untruncated log file.
• I attached the backtrace in the case of a crash.

Start by reproducing the issue with the attached xy.zip files and the provided mpv commands, then compare the ffprobe keyframe timing output for x.m4v and y.m4v. Investigate subtitle timing during the time-cropped playback path; done means embedded subtitles remain synchronized in y.m4v while the existing reproduction still distinguishes the expected and actual behavior.