MPV Fails to launch if one specific font is muxed in

[Asinin3]

mpv Information

mpv v0.41.0-39-ga58dd8ac4 Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
 built on Dec 28 2025 00:05:50
libplacebo version: v7.358.0 (v7.351.0-106-gbc90ef9-dirty)
FFmpeg version: N-122272-g224b3ff82
FFmpeg library versions:
   libavcodec      62.23.102
   libavdevice     62.2.100
   libavfilter     11.10.101
   libavformat     62.8.101
   libavutil       60.20.100
   libswresample   6.2.100
   libswscale      9.3.100

Other Information

- Windows version: 11 23H2
- GPU model, driver and version: RTX 4090, 581.89
- Source of mpv: https://github.com/shinchiro/mpv-winbuild-cmake/
- Latest known working version: N/A - Issue occurred on a earlier version I updated and same issue
- Issue started after the following happened: Muxing a font "Script Demolition.ttf" into a video (all other fonts work)

Reproduction Steps

Mux this specific font into a mkv (with mkvtoolnix): https://www.dafont.com/script-demolition.font
The file must also have a sub file e.g .ass but it doesn't matter which video I mux it into, even if the sub file is empty and the video has no other attachments, it will always hang.

Note: I already tried sourcing the font file elsewhere, the hash matched 1:1, and it appears to be a MPV issue as other media players work with it, and windows can also install the font with no errors

Expected Behavior

Opens without hanging

Actual Behavior

Command: mpv "Test.mkv" --msg-level=all=debug --no-config

[sub/ass] ASS library version: 0x1704000 (runtime 0x1704000)
[sub/ass] libass API version: 0x1704000
[sub/ass] libass source: meson, commit: 0.17.4-20-gcbb743215bad0fe202477e569fe8005545258ccd
[sub/ass] Shaper: FriBidi 1.0.16 (SIMPLE) HarfBuzz-ng 12.3.0 (COMPLEX)
[sub/ass] Setting up fonts...

MPV will always hang like this, if the mkv has the font and any sub file (Does MPV only load fonts a sub file?)

If I use "--embeddedfonts=no" the error doesn't occur, however the font is not installed on my system, and this issue happens regardless if I install it or install than uninstall. As this only happens with this one font, and I cannot reproduce the issue with VLC, this appears to be a bug with MPV or something else on my system causing weird behavior even though mpv is launching with no config.

Log File

output.txt

Sample Files

files.zip
Includes the broken muxed mkv file as well as the files I used to mux.

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.

[sfan5]

This does not reproduce with the 'mpv-x86_64-w64-mingw32' binary from CI for me.

Aside from toolchain differences/build settings (which has been the cause of problems in the past!) a noticeable difference is me using Harfbuzz 12.2.0 and you have 12.3.0.

[JanitorialMess]

I was able to replicate the issue across multiple Shinchiro v3 builds:

• mpv-x86_64-v3-20251231-git-5cc1094 (latest)
• mpv-x86_64-v3-20251228-git-a58dd8a (Asinin3's version)
• mpv-x86_64-v3-20251117-git-9379c07 (earliest available in repo)

Latest and Asinin3's builds use HarfBuzz 12.3.0 while the earliest uses 12.2.0, but the problem persists across all versions.
The non-v3 builds seem to work fine, so it's probably tied to how HarfBuzz / libass are compiled with x86_64-v3.

[Asinin3]

Thank you for testing, I will raise a issue over at the Shinchiro github. Feel free to close this, or let me know if you can't - I wasnt sure if you wanted to leave open for other people to find