AVFoundation causes frame drops when resuming from an underrun state.

mpv Information

mpv v0.40.0-dev-gf9ec3d2c2 Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
 built on May 14 2025 13:53:42
libplacebo version: v7.349.0
FFmpeg version: 7.1.1
FFmpeg library versions:
   libavcodec      61.19.101
   libavdevice     61.3.100
   libavfilter     10.4.100
   libavformat     61.7.100
   libavutil       59.39.100
   libswresample   5.3.100
   libswscale      8.3.100

Other Information

- macOS version: macOS 15.1.1 
- Source of mpv: https://nightly.link/mpv-player/mpv/workflows/build/master
- Latest known working version: -
- Issue started after the following happened: -

Reproduction Steps

mpv --no-config  --input-commands='script-message display-stats-toggle'  --hwdec=yes  --ao=avfoundation [slow_url]

Expected Behavior

Don't drop frames when resuming from an underrun state.

Actual Behavior

When the playback URL is slow, the player pauses to buffer after underrun. However, resuming playback causes a drop of approximately 30 or more frames.

Log File

● Video  --vid=1  (h264 720x540 25 fps) [default]
● Audio  --aid=1  (aac 1ch 48000 Hz 16 kbps) [default]
Using hardware decoding (videotoolbox).
AO: [avfoundation] 48000Hz mono 1ch float
VO: [gpu] 720x540 videotoolbox[nv12]
AV: 00:00:00 / 00:23:43 (0%) A-V:  0.000 Cache: 0.0s/196KB2025-05-15 11:51:36.081 mpv[94746:20250838] +[IMKClient subclass]: chose IMKClient_Modern
2025-05-15 11:51:36.081 mpv[94746:20250838] +[IMKInputSession subclass]: chose IMKInputSession_Modern
Audio device underrun detected.

Audio/Video desynchronisation detected! Possible reasons include too slow
hardware, temporary CPU spikes, broken drivers, and broken files. Audio
position will not match to the video (see A-V status field).
Consider trying `--profile=fast` and/or `--hwdec=auto` as they may help.

Audio device underrun detected.
Audio device underrun detected.
Audio device underrun detected.
Audio device underrun detected.
Audio device underrun detected.
[ffmpeg/demuxer] mov,mp4,m4a,3gp,3g2,mj2: stream 1, offset 0x3e6c7b: partial file
AV: 00:00:34 / 00:23:43 (2%) A-V:  0.000 Dropped: 184 Cache: 3.1s/759KB
Exiting... (Quit)

mpv_debug.log

Sample Files

No response

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 tracing the AVFoundation audio-output path used by --ao=avfoundation, focusing on underrun recovery and its interaction with video timing. Reproduce with the supplied mpv --no-config command and attached debug log; done means playback resumes without the reported burst of dropped frames.