Letterboxing renders brighter than absolute black with `target-colorspace-hint-mode=source`

mpv Information

mpv v0.40.0-402-gaa2dad035 Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
 built on Oct 25 2025 00:27:42
libplacebo version: v7.357.0 (v7.351.0-87-g9bffcaf-dirty)
FFmpeg version: N-121522-gd01608e02
FFmpeg library versions:
   libavcodec      62.16.100
   libavdevice     62.2.100
   libavfilter     11.9.100
   libavformat     62.6.101
   libavutil       60.14.100
   libswresample   6.2.100
   libswscale      9.3.100

Other Information

- Windows version: [Version 10.0.26100.6584]
- GPU model, driver and version:
- Source of mpv: github
- Latest known working version: built on Apr 3 2025 05:11:10
libplacebo version: v7.350.0 (v7.349.0-64-gdd16ddf-dirty)
- Issue started after the following happened: target-colorspace-hint-mode=source became necessary to achieve the same behavior as before with target-colorspace-hint=yes, particularly making the display and mpv instance run native PQ videos as HDR.

Reproduction Steps

gpu-api=vulkan
target-colorspace-hint=yes
target-colorspace-hint-mode=source

run HDR content.

Known fix: add target-contrast=inf

Expected Behavior

Blank area around content should appear completely black.

Actual Behavior

When using target-colorspace-hint-mode=source and running HDR content, area around content renders as near-black, not absolute black.
target-contrast=inf remedies this issue.

Log File

output.txt

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.

Reproduce the issue with gpu-api=vulkan, target-colorspace-hint=yes, and target-colorspace-hint-mode=source while running HDR content, then inspect output.txt. Compare the letterboxing with and without target-contrast=inf; done means the blank area is absolute black without requiring that workaround.